From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3FDAAC47094 for ; Thu, 10 Jun 2021 17:06:56 +0000 (UTC) Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 113A1613CB for ; Thu, 10 Jun 2021 17:06:56 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 113A1613CB Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ffwll.ch Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=intel-gfx-bounces@lists.freedesktop.org Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id A8A976EDBB; Thu, 10 Jun 2021 17:06:55 +0000 (UTC) Received: from mail-wm1-x335.google.com (mail-wm1-x335.google.com [IPv6:2a00:1450:4864:20::335]) by gabe.freedesktop.org (Postfix) with ESMTPS id 519B46EDBE for ; Thu, 10 Jun 2021 17:06:55 +0000 (UTC) Received: by mail-wm1-x335.google.com with SMTP id l11-20020a05600c4f0bb029017a7cd488f5so6909493wmq.0 for ; Thu, 10 Jun 2021 10:06:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ffwll.ch; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:content-transfer-encoding:in-reply-to; bh=uD68Nj+xP/v1TEtGCzRHpn+XiABi87PTkexV36y+uvk=; b=QjZ7pvuh8MdaUcP1DhxofOY5HR9DKfyTZl/pS4+sFCxQHNBVtClPgBv6T+JJMQyoG8 S9043tmfF7hQto5CD1uPn7mqQzY6NtVFx3zV7IkyGzA/ObQKabm0sG5mQtZrZRO0w3JG rKBRGqjCZDVKsoXhQz0Ji7AY0fUZQagKYLj9I= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:content-transfer-encoding :in-reply-to; bh=uD68Nj+xP/v1TEtGCzRHpn+XiABi87PTkexV36y+uvk=; b=XttdLcv1YA4FPTgwAlPZ98l4lCFMKfaVT0Eg2VbZ/xCYTEXQoySgOM/czQe1MRU0NM ivm/Q5RVyi8HOmunYtKnkKLWmZRjom+DrbeWGGDlF3oZq2a8iyxsqL5q6UDe1M6L/ikA A7DYtQpzVI/5Wd8wNFpO7y1aIqqWihhaJ0uGo7FEdIfRoq6AFwMSfmltto0h8c926WlI GgSxq2ETNLQPlFA9x9TTqjkimYk612q2+Xtq03bOcJIr9XJ/YZw/BhE8+0JNEzvyUhne 1ShNCm6XHnOASrCncEeq8Xt7gGQhnPoseMA4Dbi34/iEbbQvwCUHYmlq8Hcdt5i8ZYaA 6RuQ== X-Gm-Message-State: AOAM533jzx81KwPo7bkEjm1zRgc5p9hz07KcBL/YrujfoySsaoqUwG26 1dXigXrwkeMuyRUH8N21U3p5Wg== X-Google-Smtp-Source: ABdhPJwNitQ/PfoqG4PXUVoxcMyh/WazePR2VtmSfm9JppsEPK0j6TLQ6Or3MYEylqahA+XBk1n5FQ== X-Received: by 2002:a1c:e907:: with SMTP id q7mr16368821wmc.1.1623344813486; Thu, 10 Jun 2021 10:06:53 -0700 (PDT) Received: from phenom.ffwll.local ([2a02:168:57f4:0:efd0:b9e5:5ae6:c2fa]) by smtp.gmail.com with ESMTPSA id q5sm3494635wmc.0.2021.06.10.10.06.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Jun 2021 10:06:52 -0700 (PDT) Date: Thu, 10 Jun 2021 19:06:51 +0200 From: Daniel Vetter To: Jason Ekstrand Message-ID: References: <20210609212959.471209-1-jason@jlekstrand.net> <20210609212959.471209-6-jason@jlekstrand.net> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-Operating-System: Linux phenom 5.10.32scarlett+ Subject: Re: [Intel-gfx] [PATCH 5/5] DONOTMERGE: dma-buf: Get rid of dma_fence_get_rcu_safe X-BeenThere: intel-gfx@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Intel graphics driver community testing & development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Daniel Vetter , Intel GFX , Maling list - DRI developers , Matthew Auld , Christian =?iso-8859-1?Q?K=F6nig?= Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Errors-To: intel-gfx-bounces@lists.freedesktop.org Sender: "Intel-gfx" On Thu, Jun 10, 2021 at 11:52:23AM -0500, Jason Ekstrand wrote: > On Thu, Jun 10, 2021 at 11:38 AM Daniel Vetter w= rote: > > > > On Thu, Jun 10, 2021 at 6:24 PM Jason Ekstrand w= rote: > > > > > > On Thu, Jun 10, 2021 at 10:13 AM Daniel Vetter wrote: > > > > > > > > On Thu, Jun 10, 2021 at 3:59 PM Jason Ekstrand wrote: > > > > > > > > > > On Thu, Jun 10, 2021 at 1:51 AM Christian K=F6nig > > > > > wrote: > > > > > > > > > > > > Am 09.06.21 um 23:29 schrieb Jason Ekstrand: > > > > > > > This helper existed to handle the weird corner-cases caused b= y using > > > > > > > SLAB_TYPESAFE_BY_RCU for backing dma_fence. Now that no one = is using > > > > > > > that anymore (i915 was the only real user), dma_fence_get_rcu= is > > > > > > > sufficient. The one slightly annoying thing we have to deal = with here > > > > > > > is that dma_fence_get_rcu_safe did an rcu_dereference as well= as a > > > > > > > SLAB_TYPESAFE_BY_RCU-safe dma_fence_get_rcu. This means each= call site > > > > > > > ends up being 3 lines instead of 1. > > > > > > > > > > > > That's an outright NAK. > > > > > > > > > > > > The loop in dma_fence_get_rcu_safe is necessary because the und= erlying > > > > > > fence object can be replaced while taking the reference. > > > > > > > > > > Right. I had missed a bit of that when I first read through it. = I > > > > > see the need for the loop now. But there are some other tricky b= its > > > > > in there besides just the loop. > > > > > > > > I thought that's what the kref_get_unless_zero was for in > > > > dma_fence_get_rcu? Otherwise I guess I'm not seeing why still have > > > > dma_fence_get_rcu around, since that should either be a kref_get or > > > > it's just unsafe to call it ... > > > > > > AFAICT, dma_fence_get_rcu is unsafe unless you somehow know that it's > > > your fence and it's never recycled. > > > > > > Where the loop comes in is if you have someone come along, under the > > > RCU write lock or not, and swap out the pointer and unref it while > > > you're trying to fetch it. In this case, if you just write the three > > > lines I duplicated throughout this patch, you'll end up with NULL if > > > you (partially) lose the race. The loop exists to ensure that you get > > > either the old pointer or the new pointer and you only ever get NULL > > > if somewhere during the mess, the pointer actually gets set to NULL. > > > > It's not that easy. At least not for dma_resv. > > > > The thing is, you can't just go in and replace the write fence with > > something else. There's supposed to be some ordering here (how much we > > actually still follow that or not is a bit another question, that I'm > > trying to answer with an audit of lots of drivers), which means if you > > replace e.g. the exclusive fence, the previous fence will _not_ just > > get freed. Because the next exclusive fence needs to wait for that to > > finish first. > > > > Conceptually the refcount will _only_ go to 0 once all later > > dependencies have seen it get signalled, and once the fence itself has > > been signalled. A signalled fence might as well not exist, so if > > that's what happened in that tiny window, then yes a legal scenario > > is the following: > > > > thread A: > > - rcu_dereference(resv->exclusive_fence); > > > > thread B: > > - dma_fence signals, retires, drops refcount to 0 > > - sets the exclusive fence to NULL > > - creates a new dma_fence > > - sets the exclusive fence to that new fence > > > > thread A: > > - kref_get_unless_zero fails, we report that the exclusive fence slot i= s NULL > > > > Ofc normally we're fully pipeline, and we lazily clear slots, so no > > one ever writes the fence ptr to NULL. But conceptually it's totally > > fine, and an indistinguishable sequence of events from the point of > > view of thread A. > = > How is reporting that the exclusive fence is NULL ok in that scenario? > If someone comes along and calls dma_resv_get_excl_fence(), we want > them to get either the old fence or the new fence but never NULL. > NULL would imply that the object is idle which it probably isn't in > any sort of pipelined world. The thing is, the kref_get_unless_zero _only_ fails when the object could have been idle meanwhile and it's exclusive fence slot NULL. Maybe no one wrote that NULL, but from thread A's pov there's no difference between those. Therefore returning NULL in that case is totally fine. It is _not_ possible for that kref_get_unless_zero to fail, while the fence isn't signalled yet. I think we might need to go through this on irc a bit ... -Daniel > > Ergo dma_fence_get_rcu is enough. If it's not, we've screwed up really > > big time. The only reason you need _unsafe is if you have > > typesafe_by_rcu, or maybe if you yolo your fence ordering a bit much > > and break the DAG property in a few cases. > > > > > I agree with Christian that that part of dma_fence_get_rcu_safe needs > > > to stay. I was missing that until I did my giant "let's walk through > > > the code" e-mail. > > > > Well if I'm wrong there's a _ton_ of broken code in upstream right > > now, even in dma-buf/dma-resv.c. We're using dma_fence_get_rcu a lot. > = > Yup. 19 times. What I'm trying to understand is how much of that > code depends on properly catching a pointer-switch race and how much > is ok with a NULL failure mode. This trybot seems to imply that most > things are ok with the NULL failure mode: > = > https://patchwork.freedesktop.org/series/91267/ > = > Of course, as we discussed on IRC, I'm not sure how much I trust > proof-by-trybot here. :-) > = > > Also the timing is all backwards: get_rcu_safe was added as a fix for > > when i915 made its dma_fence typesafe_by_rcu. We didn't have any need > > for this beforehand. So I'm really not quite buying this story here > > yet you're all trying to sell me on. > = > Yeah, that's really concerning. It's possible that many of the uses > of get_rcu_safe were added because someone had recycle bugs and others > were added because of pointer chase bugs and people weren't entirely > clear on which. > = > --Jason > = > > -Daniel > > > > > > > > --Jason > > > > > > > > > This is completely unrelated to SLAB_TYPESAFE_BY_RCU. See the > > > > > > dma_fence_chain usage for reference. > > > > > > > > > > > > What you can remove is the sequence number handling in dma-buf.= That > > > > > > should make adding fences quite a bit quicker. > > > > > > > > > > I'll look at that and try to understand what's going on there. > > > > > > > > Hm I thought the seqlock was to make sure we have a consistent set = of > > > > fences across exclusive and all shared slot. Not to protect against > > > > the fence disappearing due to typesafe_by_rcu. > > > > -Daniel > > > > > > > > > --Jason > > > > > > > > > > > Regards, > > > > > > Christian. > > > > > > > > > > > > > > > > > > > > Signed-off-by: Jason Ekstrand > > > > > > > Cc: Daniel Vetter > > > > > > > Cc: Christian K=F6nig > > > > > > > Cc: Matthew Auld > > > > > > > Cc: Maarten Lankhorst > > > > > > > --- > > > > > > > drivers/dma-buf/dma-fence-chain.c | 8 ++-- > > > > > > > drivers/dma-buf/dma-resv.c | 4 +- > > > > > > > drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c | 4 +- > > > > > > > drivers/gpu/drm/i915/i915_active.h | 4 +- > > > > > > > drivers/gpu/drm/i915/i915_vma.c | 4 +- > > > > > > > include/drm/drm_syncobj.h | 4 +- > > > > > > > include/linux/dma-fence.h | 50 ------------= ----------- > > > > > > > include/linux/dma-resv.h | 4 +- > > > > > > > 8 files changed, 23 insertions(+), 59 deletions(-) > > > > > > > > > > > > > > diff --git a/drivers/dma-buf/dma-fence-chain.c b/drivers/dma-= buf/dma-fence-chain.c > > > > > > > index 7d129e68ac701..46dfc7d94d8ed 100644 > > > > > > > --- a/drivers/dma-buf/dma-fence-chain.c > > > > > > > +++ b/drivers/dma-buf/dma-fence-chain.c > > > > > > > @@ -15,15 +15,17 @@ static bool dma_fence_chain_enable_signal= ing(struct dma_fence *fence); > > > > > > > * dma_fence_chain_get_prev - use RCU to get a reference to= the previous fence > > > > > > > * @chain: chain node to get the previous node from > > > > > > > * > > > > > > > - * Use dma_fence_get_rcu_safe to get a reference to the prev= ious fence of the > > > > > > > - * chain node. > > > > > > > + * Use rcu_dereference and dma_fence_get_rcu to get a refere= nce to the > > > > > > > + * previous fence of the chain node. > > > > > > > */ > > > > > > > static struct dma_fence *dma_fence_chain_get_prev(struct dm= a_fence_chain *chain) > > > > > > > { > > > > > > > struct dma_fence *prev; > > > > > > > > > > > > > > rcu_read_lock(); > > > > > > > - prev =3D dma_fence_get_rcu_safe(&chain->prev); > > > > > > > + prev =3D rcu_dereference(chain->prev); > > > > > > > + if (prev) > > > > > > > + prev =3D dma_fence_get_rcu(prev); > > > > > > > rcu_read_unlock(); > > > > > > > return prev; > > > > > > > } > > > > > > > diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma= -resv.c > > > > > > > index f26c71747d43a..cfe0db3cca292 100644 > > > > > > > --- a/drivers/dma-buf/dma-resv.c > > > > > > > +++ b/drivers/dma-buf/dma-resv.c > > > > > > > @@ -376,7 +376,9 @@ int dma_resv_copy_fences(struct dma_resv = *dst, struct dma_resv *src) > > > > > > > dst_list =3D NULL; > > > > > > > } > > > > > > > > > > > > > > - new =3D dma_fence_get_rcu_safe(&src->fence_excl); > > > > > > > + new =3D rcu_dereference(src->fence_excl); > > > > > > > + if (new) > > > > > > > + new =3D dma_fence_get_rcu(new); > > > > > > > rcu_read_unlock(); > > > > > > > > > > > > > > src_list =3D dma_resv_shared_list(dst); > > > > > > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c b/driv= ers/gpu/drm/amd/amdgpu/amdgpu_fence.c > > > > > > > index 72d9b92b17547..0aeb6117f3893 100644 > > > > > > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c > > > > > > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c > > > > > > > @@ -161,7 +161,9 @@ int amdgpu_fence_emit(struct amdgpu_ring = *ring, struct dma_fence **f, > > > > > > > struct dma_fence *old; > > > > > > > > > > > > > > rcu_read_lock(); > > > > > > > - old =3D dma_fence_get_rcu_safe(ptr); > > > > > > > + old =3D rcu_dereference(*ptr); > > > > > > > + if (old) > > > > > > > + old =3D dma_fence_get_rcu(old); > > > > > > > rcu_read_unlock(); > > > > > > > > > > > > > > if (old) { > > > > > > > diff --git a/drivers/gpu/drm/i915/i915_active.h b/drivers/gpu= /drm/i915/i915_active.h > > > > > > > index d0feda68b874f..bd89cfc806ca5 100644 > > > > > > > --- a/drivers/gpu/drm/i915/i915_active.h > > > > > > > +++ b/drivers/gpu/drm/i915/i915_active.h > > > > > > > @@ -103,7 +103,9 @@ i915_active_fence_get(struct i915_active_= fence *active) > > > > > > > struct dma_fence *fence; > > > > > > > > > > > > > > rcu_read_lock(); > > > > > > > - fence =3D dma_fence_get_rcu_safe(&active->fence); > > > > > > > + fence =3D rcu_dereference(active->fence); > > > > > > > + if (fence) > > > > > > > + fence =3D dma_fence_get_rcu(fence); > > > > > > > rcu_read_unlock(); > > > > > > > > > > > > > > return fence; > > > > > > > diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/dr= m/i915/i915_vma.c > > > > > > > index 0f227f28b2802..ed0388d99197e 100644 > > > > > > > --- a/drivers/gpu/drm/i915/i915_vma.c > > > > > > > +++ b/drivers/gpu/drm/i915/i915_vma.c > > > > > > > @@ -351,7 +351,9 @@ int i915_vma_wait_for_bind(struct i915_vm= a *vma) > > > > > > > struct dma_fence *fence; > > > > > > > > > > > > > > rcu_read_lock(); > > > > > > > - fence =3D dma_fence_get_rcu_safe(&vma->active.e= xcl.fence); > > > > > > > + fence =3D rcu_dereference(vma->active.excl.fenc= e); > > > > > > > + if (fence) > > > > > > > + fence =3D dma_fence_get_rcu(fence); > > > > > > > rcu_read_unlock(); > > > > > > > if (fence) { > > > > > > > err =3D dma_fence_wait(fence, MAX_SCHED= ULE_TIMEOUT); > > > > > > > diff --git a/include/drm/drm_syncobj.h b/include/drm/drm_sync= obj.h > > > > > > > index 6cf7243a1dc5e..6c45d52988bcc 100644 > > > > > > > --- a/include/drm/drm_syncobj.h > > > > > > > +++ b/include/drm/drm_syncobj.h > > > > > > > @@ -105,7 +105,9 @@ drm_syncobj_fence_get(struct drm_syncobj = *syncobj) > > > > > > > struct dma_fence *fence; > > > > > > > > > > > > > > rcu_read_lock(); > > > > > > > - fence =3D dma_fence_get_rcu_safe(&syncobj->fence); > > > > > > > + fence =3D rcu_dereference(syncobj->fence); > > > > > > > + if (fence) > > > > > > > + fence =3D dma_fence_get_rcu(syncobj->fence); > > > > > > > rcu_read_unlock(); > > > > > > > > > > > > > > return fence; > > > > > > > diff --git a/include/linux/dma-fence.h b/include/linux/dma-fe= nce.h > > > > > > > index 6ffb4b2c63715..f4a2ab2b1ae46 100644 > > > > > > > --- a/include/linux/dma-fence.h > > > > > > > +++ b/include/linux/dma-fence.h > > > > > > > @@ -307,56 +307,6 @@ static inline struct dma_fence *dma_fenc= e_get_rcu(struct dma_fence *fence) > > > > > > > return NULL; > > > > > > > } > > > > > > > > > > > > > > -/** > > > > > > > - * dma_fence_get_rcu_safe - acquire a reference to an RCU t= racked fence > > > > > > > - * @fencep: pointer to fence to increase refcount of > > > > > > > - * > > > > > > > - * Function returns NULL if no refcount could be obtained, o= r the fence. > > > > > > > - * This function handles acquiring a reference to a fence th= at may be > > > > > > > - * reallocated within the RCU grace period (such as with SLA= B_TYPESAFE_BY_RCU), > > > > > > > - * so long as the caller is using RCU on the pointer to the = fence. > > > > > > > - * > > > > > > > - * An alternative mechanism is to employ a seqlock to protec= t a bunch of > > > > > > > - * fences, such as used by struct dma_resv. When using a seq= lock, > > > > > > > - * the seqlock must be taken before and checked after a refe= rence to the > > > > > > > - * fence is acquired (as shown here). > > > > > > > - * > > > > > > > - * The caller is required to hold the RCU read lock. > > > > > > > - */ > > > > > > > -static inline struct dma_fence * > > > > > > > -dma_fence_get_rcu_safe(struct dma_fence __rcu **fencep) > > > > > > > -{ > > > > > > > - do { > > > > > > > - struct dma_fence *fence; > > > > > > > - > > > > > > > - fence =3D rcu_dereference(*fencep); > > > > > > > - if (!fence) > > > > > > > - return NULL; > > > > > > > - > > > > > > > - if (!dma_fence_get_rcu(fence)) > > > > > > > - continue; > > > > > > > - > > > > > > > - /* The atomic_inc_not_zero() inside dma_fence_g= et_rcu() > > > > > > > - * provides a full memory barrier upon success = (such as now). > > > > > > > - * This is paired with the write barrier from a= ssigning > > > > > > > - * to the __rcu protected fence pointer so that= if that > > > > > > > - * pointer still matches the current fence, we = know we > > > > > > > - * have successfully acquire a reference to it.= If it no > > > > > > > - * longer matches, we are holding a reference t= o some other > > > > > > > - * reallocated pointer. This is possible if the= allocator > > > > > > > - * is using a freelist like SLAB_TYPESAFE_BY_RC= U where the > > > > > > > - * fence remains valid for the RCU grace period= , but it > > > > > > > - * may be reallocated. When using such allocato= rs, we are > > > > > > > - * responsible for ensuring the reference we ge= t is to > > > > > > > - * the right fence, as below. > > > > > > > - */ > > > > > > > - if (fence =3D=3D rcu_access_pointer(*fencep)) > > > > > > > - return rcu_pointer_handoff(fence); > > > > > > > - > > > > > > > - dma_fence_put(fence); > > > > > > > - } while (1); > > > > > > > -} > > > > > > > - > > > > > > > #ifdef CONFIG_LOCKDEP > > > > > > > bool dma_fence_begin_signalling(void); > > > > > > > void dma_fence_end_signalling(bool cookie); > > > > > > > diff --git a/include/linux/dma-resv.h b/include/linux/dma-res= v.h > > > > > > > index 562b885cf9c3d..a38c021f379af 100644 > > > > > > > --- a/include/linux/dma-resv.h > > > > > > > +++ b/include/linux/dma-resv.h > > > > > > > @@ -248,7 +248,9 @@ dma_resv_get_excl_unlocked(struct dma_res= v *obj) > > > > > > > return NULL; > > > > > > > > > > > > > > rcu_read_lock(); > > > > > > > - fence =3D dma_fence_get_rcu_safe(&obj->fence_excl); > > > > > > > + fence =3D rcu_dereference(obj->fence_excl); > > > > > > > + if (fence) > > > > > > > + fence =3D dma_fence_get_rcu(fence); > > > > > > > rcu_read_unlock(); > > > > > > > > > > > > > > return fence; > > > > > > > > > > > > > > > > > > > > > > -- > > > > Daniel Vetter > > > > Software Engineer, Intel Corporation > > > > http://blog.ffwll.ch > > > > > > > > -- > > Daniel Vetter > > Software Engineer, Intel Corporation > > http://blog.ffwll.ch -- = Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ Intel-gfx mailing list Intel-gfx@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/intel-gfx From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 62734C48BDF for ; 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Thu, 10 Jun 2021 10:06:53 -0700 (PDT) Received: from phenom.ffwll.local ([2a02:168:57f4:0:efd0:b9e5:5ae6:c2fa]) by smtp.gmail.com with ESMTPSA id q5sm3494635wmc.0.2021.06.10.10.06.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Jun 2021 10:06:52 -0700 (PDT) Date: Thu, 10 Jun 2021 19:06:51 +0200 From: Daniel Vetter To: Jason Ekstrand Subject: Re: [PATCH 5/5] DONOTMERGE: dma-buf: Get rid of dma_fence_get_rcu_safe Message-ID: References: <20210609212959.471209-1-jason@jlekstrand.net> <20210609212959.471209-6-jason@jlekstrand.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Operating-System: Linux phenom 5.10.32scarlett+ X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Daniel Vetter , Intel GFX , Maling list - DRI developers , Matthew Auld , Christian =?iso-8859-1?Q?K=F6nig?= Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Thu, Jun 10, 2021 at 11:52:23AM -0500, Jason Ekstrand wrote: > On Thu, Jun 10, 2021 at 11:38 AM Daniel Vetter wrote: > > > > On Thu, Jun 10, 2021 at 6:24 PM Jason Ekstrand wrote: > > > > > > On Thu, Jun 10, 2021 at 10:13 AM Daniel Vetter wrote: > > > > > > > > On Thu, Jun 10, 2021 at 3:59 PM Jason Ekstrand wrote: > > > > > > > > > > On Thu, Jun 10, 2021 at 1:51 AM Christian König > > > > > wrote: > > > > > > > > > > > > Am 09.06.21 um 23:29 schrieb Jason Ekstrand: > > > > > > > This helper existed to handle the weird corner-cases caused by using > > > > > > > SLAB_TYPESAFE_BY_RCU for backing dma_fence. Now that no one is using > > > > > > > that anymore (i915 was the only real user), dma_fence_get_rcu is > > > > > > > sufficient. The one slightly annoying thing we have to deal with here > > > > > > > is that dma_fence_get_rcu_safe did an rcu_dereference as well as a > > > > > > > SLAB_TYPESAFE_BY_RCU-safe dma_fence_get_rcu. This means each call site > > > > > > > ends up being 3 lines instead of 1. > > > > > > > > > > > > That's an outright NAK. > > > > > > > > > > > > The loop in dma_fence_get_rcu_safe is necessary because the underlying > > > > > > fence object can be replaced while taking the reference. > > > > > > > > > > Right. I had missed a bit of that when I first read through it. I > > > > > see the need for the loop now. But there are some other tricky bits > > > > > in there besides just the loop. > > > > > > > > I thought that's what the kref_get_unless_zero was for in > > > > dma_fence_get_rcu? Otherwise I guess I'm not seeing why still have > > > > dma_fence_get_rcu around, since that should either be a kref_get or > > > > it's just unsafe to call it ... > > > > > > AFAICT, dma_fence_get_rcu is unsafe unless you somehow know that it's > > > your fence and it's never recycled. > > > > > > Where the loop comes in is if you have someone come along, under the > > > RCU write lock or not, and swap out the pointer and unref it while > > > you're trying to fetch it. In this case, if you just write the three > > > lines I duplicated throughout this patch, you'll end up with NULL if > > > you (partially) lose the race. The loop exists to ensure that you get > > > either the old pointer or the new pointer and you only ever get NULL > > > if somewhere during the mess, the pointer actually gets set to NULL. > > > > It's not that easy. At least not for dma_resv. > > > > The thing is, you can't just go in and replace the write fence with > > something else. There's supposed to be some ordering here (how much we > > actually still follow that or not is a bit another question, that I'm > > trying to answer with an audit of lots of drivers), which means if you > > replace e.g. the exclusive fence, the previous fence will _not_ just > > get freed. Because the next exclusive fence needs to wait for that to > > finish first. > > > > Conceptually the refcount will _only_ go to 0 once all later > > dependencies have seen it get signalled, and once the fence itself has > > been signalled. A signalled fence might as well not exist, so if > > that's what happened in that tiny window, then yes a legal scenario > > is the following: > > > > thread A: > > - rcu_dereference(resv->exclusive_fence); > > > > thread B: > > - dma_fence signals, retires, drops refcount to 0 > > - sets the exclusive fence to NULL > > - creates a new dma_fence > > - sets the exclusive fence to that new fence > > > > thread A: > > - kref_get_unless_zero fails, we report that the exclusive fence slot is NULL > > > > Ofc normally we're fully pipeline, and we lazily clear slots, so no > > one ever writes the fence ptr to NULL. But conceptually it's totally > > fine, and an indistinguishable sequence of events from the point of > > view of thread A. > > How is reporting that the exclusive fence is NULL ok in that scenario? > If someone comes along and calls dma_resv_get_excl_fence(), we want > them to get either the old fence or the new fence but never NULL. > NULL would imply that the object is idle which it probably isn't in > any sort of pipelined world. The thing is, the kref_get_unless_zero _only_ fails when the object could have been idle meanwhile and it's exclusive fence slot NULL. Maybe no one wrote that NULL, but from thread A's pov there's no difference between those. Therefore returning NULL in that case is totally fine. It is _not_ possible for that kref_get_unless_zero to fail, while the fence isn't signalled yet. I think we might need to go through this on irc a bit ... -Daniel > > Ergo dma_fence_get_rcu is enough. If it's not, we've screwed up really > > big time. The only reason you need _unsafe is if you have > > typesafe_by_rcu, or maybe if you yolo your fence ordering a bit much > > and break the DAG property in a few cases. > > > > > I agree with Christian that that part of dma_fence_get_rcu_safe needs > > > to stay. I was missing that until I did my giant "let's walk through > > > the code" e-mail. > > > > Well if I'm wrong there's a _ton_ of broken code in upstream right > > now, even in dma-buf/dma-resv.c. We're using dma_fence_get_rcu a lot. > > Yup. 19 times. What I'm trying to understand is how much of that > code depends on properly catching a pointer-switch race and how much > is ok with a NULL failure mode. This trybot seems to imply that most > things are ok with the NULL failure mode: > > https://patchwork.freedesktop.org/series/91267/ > > Of course, as we discussed on IRC, I'm not sure how much I trust > proof-by-trybot here. :-) > > > Also the timing is all backwards: get_rcu_safe was added as a fix for > > when i915 made its dma_fence typesafe_by_rcu. We didn't have any need > > for this beforehand. So I'm really not quite buying this story here > > yet you're all trying to sell me on. > > Yeah, that's really concerning. It's possible that many of the uses > of get_rcu_safe were added because someone had recycle bugs and others > were added because of pointer chase bugs and people weren't entirely > clear on which. > > --Jason > > > -Daniel > > > > > > > > --Jason > > > > > > > > > This is completely unrelated to SLAB_TYPESAFE_BY_RCU. See the > > > > > > dma_fence_chain usage for reference. > > > > > > > > > > > > What you can remove is the sequence number handling in dma-buf. That > > > > > > should make adding fences quite a bit quicker. > > > > > > > > > > I'll look at that and try to understand what's going on there. > > > > > > > > Hm I thought the seqlock was to make sure we have a consistent set of > > > > fences across exclusive and all shared slot. Not to protect against > > > > the fence disappearing due to typesafe_by_rcu. > > > > -Daniel > > > > > > > > > --Jason > > > > > > > > > > > Regards, > > > > > > Christian. > > > > > > > > > > > > > > > > > > > > Signed-off-by: Jason Ekstrand > > > > > > > Cc: Daniel Vetter > > > > > > > Cc: Christian König > > > > > > > Cc: Matthew Auld > > > > > > > Cc: Maarten Lankhorst > > > > > > > --- > > > > > > > drivers/dma-buf/dma-fence-chain.c | 8 ++-- > > > > > > > drivers/dma-buf/dma-resv.c | 4 +- > > > > > > > drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c | 4 +- > > > > > > > drivers/gpu/drm/i915/i915_active.h | 4 +- > > > > > > > drivers/gpu/drm/i915/i915_vma.c | 4 +- > > > > > > > include/drm/drm_syncobj.h | 4 +- > > > > > > > include/linux/dma-fence.h | 50 ----------------------- > > > > > > > include/linux/dma-resv.h | 4 +- > > > > > > > 8 files changed, 23 insertions(+), 59 deletions(-) > > > > > > > > > > > > > > diff --git a/drivers/dma-buf/dma-fence-chain.c b/drivers/dma-buf/dma-fence-chain.c > > > > > > > index 7d129e68ac701..46dfc7d94d8ed 100644 > > > > > > > --- a/drivers/dma-buf/dma-fence-chain.c > > > > > > > +++ b/drivers/dma-buf/dma-fence-chain.c > > > > > > > @@ -15,15 +15,17 @@ static bool dma_fence_chain_enable_signaling(struct dma_fence *fence); > > > > > > > * dma_fence_chain_get_prev - use RCU to get a reference to the previous fence > > > > > > > * @chain: chain node to get the previous node from > > > > > > > * > > > > > > > - * Use dma_fence_get_rcu_safe to get a reference to the previous fence of the > > > > > > > - * chain node. > > > > > > > + * Use rcu_dereference and dma_fence_get_rcu to get a reference to the > > > > > > > + * previous fence of the chain node. > > > > > > > */ > > > > > > > static struct dma_fence *dma_fence_chain_get_prev(struct dma_fence_chain *chain) > > > > > > > { > > > > > > > struct dma_fence *prev; > > > > > > > > > > > > > > rcu_read_lock(); > > > > > > > - prev = dma_fence_get_rcu_safe(&chain->prev); > > > > > > > + prev = rcu_dereference(chain->prev); > > > > > > > + if (prev) > > > > > > > + prev = dma_fence_get_rcu(prev); > > > > > > > rcu_read_unlock(); > > > > > > > return prev; > > > > > > > } > > > > > > > diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c > > > > > > > index f26c71747d43a..cfe0db3cca292 100644 > > > > > > > --- a/drivers/dma-buf/dma-resv.c > > > > > > > +++ b/drivers/dma-buf/dma-resv.c > > > > > > > @@ -376,7 +376,9 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src) > > > > > > > dst_list = NULL; > > > > > > > } > > > > > > > > > > > > > > - new = dma_fence_get_rcu_safe(&src->fence_excl); > > > > > > > + new = rcu_dereference(src->fence_excl); > > > > > > > + if (new) > > > > > > > + new = dma_fence_get_rcu(new); > > > > > > > rcu_read_unlock(); > > > > > > > > > > > > > > src_list = dma_resv_shared_list(dst); > > > > > > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c > > > > > > > index 72d9b92b17547..0aeb6117f3893 100644 > > > > > > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c > > > > > > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c > > > > > > > @@ -161,7 +161,9 @@ int amdgpu_fence_emit(struct amdgpu_ring *ring, struct dma_fence **f, > > > > > > > struct dma_fence *old; > > > > > > > > > > > > > > rcu_read_lock(); > > > > > > > - old = dma_fence_get_rcu_safe(ptr); > > > > > > > + old = rcu_dereference(*ptr); > > > > > > > + if (old) > > > > > > > + old = dma_fence_get_rcu(old); > > > > > > > rcu_read_unlock(); > > > > > > > > > > > > > > if (old) { > > > > > > > diff --git a/drivers/gpu/drm/i915/i915_active.h b/drivers/gpu/drm/i915/i915_active.h > > > > > > > index d0feda68b874f..bd89cfc806ca5 100644 > > > > > > > --- a/drivers/gpu/drm/i915/i915_active.h > > > > > > > +++ b/drivers/gpu/drm/i915/i915_active.h > > > > > > > @@ -103,7 +103,9 @@ i915_active_fence_get(struct i915_active_fence *active) > > > > > > > struct dma_fence *fence; > > > > > > > > > > > > > > rcu_read_lock(); > > > > > > > - fence = dma_fence_get_rcu_safe(&active->fence); > > > > > > > + fence = rcu_dereference(active->fence); > > > > > > > + if (fence) > > > > > > > + fence = dma_fence_get_rcu(fence); > > > > > > > rcu_read_unlock(); > > > > > > > > > > > > > > return fence; > > > > > > > diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c > > > > > > > index 0f227f28b2802..ed0388d99197e 100644 > > > > > > > --- a/drivers/gpu/drm/i915/i915_vma.c > > > > > > > +++ b/drivers/gpu/drm/i915/i915_vma.c > > > > > > > @@ -351,7 +351,9 @@ int i915_vma_wait_for_bind(struct i915_vma *vma) > > > > > > > struct dma_fence *fence; > > > > > > > > > > > > > > rcu_read_lock(); > > > > > > > - fence = dma_fence_get_rcu_safe(&vma->active.excl.fence); > > > > > > > + fence = rcu_dereference(vma->active.excl.fence); > > > > > > > + if (fence) > > > > > > > + fence = dma_fence_get_rcu(fence); > > > > > > > rcu_read_unlock(); > > > > > > > if (fence) { > > > > > > > err = dma_fence_wait(fence, MAX_SCHEDULE_TIMEOUT); > > > > > > > diff --git a/include/drm/drm_syncobj.h b/include/drm/drm_syncobj.h > > > > > > > index 6cf7243a1dc5e..6c45d52988bcc 100644 > > > > > > > --- a/include/drm/drm_syncobj.h > > > > > > > +++ b/include/drm/drm_syncobj.h > > > > > > > @@ -105,7 +105,9 @@ drm_syncobj_fence_get(struct drm_syncobj *syncobj) > > > > > > > struct dma_fence *fence; > > > > > > > > > > > > > > rcu_read_lock(); > > > > > > > - fence = dma_fence_get_rcu_safe(&syncobj->fence); > > > > > > > + fence = rcu_dereference(syncobj->fence); > > > > > > > + if (fence) > > > > > > > + fence = dma_fence_get_rcu(syncobj->fence); > > > > > > > rcu_read_unlock(); > > > > > > > > > > > > > > return fence; > > > > > > > diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h > > > > > > > index 6ffb4b2c63715..f4a2ab2b1ae46 100644 > > > > > > > --- a/include/linux/dma-fence.h > > > > > > > +++ b/include/linux/dma-fence.h > > > > > > > @@ -307,56 +307,6 @@ static inline struct dma_fence *dma_fence_get_rcu(struct dma_fence *fence) > > > > > > > return NULL; > > > > > > > } > > > > > > > > > > > > > > -/** > > > > > > > - * dma_fence_get_rcu_safe - acquire a reference to an RCU tracked fence > > > > > > > - * @fencep: pointer to fence to increase refcount of > > > > > > > - * > > > > > > > - * Function returns NULL if no refcount could be obtained, or the fence. > > > > > > > - * This function handles acquiring a reference to a fence that may be > > > > > > > - * reallocated within the RCU grace period (such as with SLAB_TYPESAFE_BY_RCU), > > > > > > > - * so long as the caller is using RCU on the pointer to the fence. > > > > > > > - * > > > > > > > - * An alternative mechanism is to employ a seqlock to protect a bunch of > > > > > > > - * fences, such as used by struct dma_resv. When using a seqlock, > > > > > > > - * the seqlock must be taken before and checked after a reference to the > > > > > > > - * fence is acquired (as shown here). > > > > > > > - * > > > > > > > - * The caller is required to hold the RCU read lock. > > > > > > > - */ > > > > > > > -static inline struct dma_fence * > > > > > > > -dma_fence_get_rcu_safe(struct dma_fence __rcu **fencep) > > > > > > > -{ > > > > > > > - do { > > > > > > > - struct dma_fence *fence; > > > > > > > - > > > > > > > - fence = rcu_dereference(*fencep); > > > > > > > - if (!fence) > > > > > > > - return NULL; > > > > > > > - > > > > > > > - if (!dma_fence_get_rcu(fence)) > > > > > > > - continue; > > > > > > > - > > > > > > > - /* The atomic_inc_not_zero() inside dma_fence_get_rcu() > > > > > > > - * provides a full memory barrier upon success (such as now). > > > > > > > - * This is paired with the write barrier from assigning > > > > > > > - * to the __rcu protected fence pointer so that if that > > > > > > > - * pointer still matches the current fence, we know we > > > > > > > - * have successfully acquire a reference to it. If it no > > > > > > > - * longer matches, we are holding a reference to some other > > > > > > > - * reallocated pointer. This is possible if the allocator > > > > > > > - * is using a freelist like SLAB_TYPESAFE_BY_RCU where the > > > > > > > - * fence remains valid for the RCU grace period, but it > > > > > > > - * may be reallocated. When using such allocators, we are > > > > > > > - * responsible for ensuring the reference we get is to > > > > > > > - * the right fence, as below. > > > > > > > - */ > > > > > > > - if (fence == rcu_access_pointer(*fencep)) > > > > > > > - return rcu_pointer_handoff(fence); > > > > > > > - > > > > > > > - dma_fence_put(fence); > > > > > > > - } while (1); > > > > > > > -} > > > > > > > - > > > > > > > #ifdef CONFIG_LOCKDEP > > > > > > > bool dma_fence_begin_signalling(void); > > > > > > > void dma_fence_end_signalling(bool cookie); > > > > > > > diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h > > > > > > > index 562b885cf9c3d..a38c021f379af 100644 > > > > > > > --- a/include/linux/dma-resv.h > > > > > > > +++ b/include/linux/dma-resv.h > > > > > > > @@ -248,7 +248,9 @@ dma_resv_get_excl_unlocked(struct dma_resv *obj) > > > > > > > return NULL; > > > > > > > > > > > > > > rcu_read_lock(); > > > > > > > - fence = dma_fence_get_rcu_safe(&obj->fence_excl); > > > > > > > + fence = rcu_dereference(obj->fence_excl); > > > > > > > + if (fence) > > > > > > > + fence = dma_fence_get_rcu(fence); > > > > > > > rcu_read_unlock(); > > > > > > > > > > > > > > return fence; > > > > > > > > > > > > > > > > > > > > > > -- > > > > Daniel Vetter > > > > Software Engineer, Intel Corporation > > > > http://blog.ffwll.ch > > > > > > > > -- > > Daniel Vetter > > Software Engineer, Intel Corporation > > http://blog.ffwll.ch -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch