From: Luis Chamberlain <mcgrof@kernel.org>
To: David Laight <David.Laight@aculab.com>
Cc: "tj@kernel.org" <tj@kernel.org>,
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Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Subject: Re: [PATCH v7 09/12] sysfs: fix deadlock race with module removal
Date: Tue, 21 Sep 2021 08:48:49 -0700 [thread overview]
Message-ID: <CAB=NE6WjupsJFwsj94sC_j3gcYn2Qo0sx1=tMv=WUZ83jq_DFw@mail.gmail.com> (raw)
In-Reply-To: <c70dcb03e27e43c5b5311e184357df39@AcuMS.aculab.com>
On Tue, Sep 21, 2021 at 1:24 AM David Laight <David.Laight@aculab.com> wrote:
>
> From: Luis Chamberlain
> > Sent: 17 September 2021 20:47
> >
> > When sysfs attributes use a lock also used on module removal we can
> > race to deadlock. This happens when for instance a sysfs file on
> > a driver is used, then at the same time we have module removal call
> > trigger. The module removal call code holds a lock, and then the sysfs
> > file entry waits for the same lock. While holding the lock the module
> > removal tries to remove the sysfs entries, but these cannot be removed
> > yet as one is waiting for a lock. This won't complete as the lock is
> > already held. Likewise module removal cannot complete, and so we deadlock.
>
> Isn't the real problem the race between a sysfs file action and the
> removal of the sysfs node?
Nope, that is taken care of by kernfs.
> This isn't really related to module unload - except that may
> well remove some sysfs nodes.
Nope, the issue is a deadlock that can happen due to a shared lock on
module removal and a driver sysfs operation.
> This is the same problem as removing any other kind of driver callback.
> There are three basic solutions:
> 1) Use a global lock - not usually useful.
> 2) Have the remove call sleep until any callbacks are complete.
> 3) Have the remove just request removal and have a final
> callback (from a different context).
Kernfs already does a sort of combination of 1) and 2) but 1) is using
atomic reference counts.
> If the remove can sleep (as in 2) then there is a requirement
> on the driver code to not hold any locks across the 'remove'
> that can be acquired during the callbacks.
And this is the part that kernfs has no control over since the removal
and sysfs operation are implementation specific.
> Now, for sysfs, you probably only want to sleep the remove code
> while a read/write is in progress - not just because the node
> is open.
> That probably requires marking an open node 'invalid' and
> deferring delete to close.
This is already done by kernfs.
> None of this requires a reference count on the module.
You are missing the point to the other aspect of the try_module_get(),
it lets you also check if module exit has been entered. By using
try_module_get() you let the module exit trump proceeding with an
operation, therefore also preventing any potential use of a shared
lock on module exit and the driver specific sysfs operation.
Luis
next parent reply other threads:[~2021-09-21 15:49 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20210917194709.3562413-1-mcgrof@kernel.org>
[not found] ` <20210917194709.3562413-10-mcgrof@kernel.org>
[not found] ` <c70dcb03e27e43c5b5311e184357df39@AcuMS.aculab.com>
2021-09-21 15:48 ` Luis Chamberlain [this message]
[not found] <20210918050430.3671227-1-mcgrof@kernel.org>
2021-09-18 5:04 ` [PATCH v7 09/12] sysfs: fix deadlock race with module removal Luis Chamberlain
2021-09-20 17:53 ` Tejun Heo
2021-09-20 19:15 ` Luis Chamberlain
2021-09-20 19:22 ` Tejun Heo
2021-09-20 19:38 ` Luis Chamberlain
2021-09-20 20:52 ` Dan Williams
2021-09-20 21:17 ` Luis Chamberlain
2021-09-20 21:55 ` Dan Williams
2021-09-21 0:03 ` Luis Chamberlain
2021-09-20 21:36 ` Bart Van Assche
2021-09-20 21:43 ` Luis Chamberlain
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