From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
To: Emanuele Giuseppe Esposito <eesposit@redhat.com>, qemu-block@nongnu.org
Cc: John Snow <jsnow@redhat.com>, Kevin Wolf <kwolf@redhat.com>,
Max Reitz <mreitz@redhat.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Stefan Hajnoczi <stefanha@redhat.com>,
qemu-devel@nongnu.org
Subject: Re: [PATCH v3 4/5] block-copy: add a CoMutex
Date: Wed, 9 Jun 2021 15:25:39 +0300 [thread overview]
Message-ID: <94c227e0-e179-f675-cd77-d470e0aaec9b@virtuozzo.com> (raw)
In-Reply-To: <20210608073344.53637-5-eesposit@redhat.com>
08.06.2021 10:33, Emanuele Giuseppe Esposito wrote:
> Add a CoMutex to protect concurrent access of block-copy
> data structures.
>
> This mutex also protects .copy_bitmap, because its thread-safe
> API does not prevent it from assigning two tasks to the same
> bitmap region.
>
> .finished, .cancelled and reads to .ret and .error_is_read will be
> protected in the following patch, because are used also outside
> coroutines.
>
> Also set block_copy_task_create as coroutine_fn because:
> 1) it is static and only invoked by coroutine functions
> 2) this patch introduces and uses a CoMutex lock there
>
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
I missed, did you (where?) add a comment like "all APIs are thread-safe", or what is thread-safe?
> ---
> block/block-copy.c | 82 ++++++++++++++++++++++++++++++----------------
> 1 file changed, 54 insertions(+), 28 deletions(-)
>
> diff --git a/block/block-copy.c b/block/block-copy.c
> index e2adb5b2ea..56f62913e4 100644
> --- a/block/block-copy.c
> +++ b/block/block-copy.c
> @@ -61,6 +61,7 @@ typedef struct BlockCopyCallState {
>
> /* OUT parameters */
> bool cancelled;
> + /* Fields protected by lock in BlockCopyState */
> bool error_is_read;
> int ret;
> } BlockCopyCallState;
> @@ -78,7 +79,7 @@ typedef struct BlockCopyTask {
> int64_t bytes; /* only re-set in task_shrink, before running the task */
> BlockCopyMethod method; /* initialized in block_copy_dirty_clusters() */
>
> - /* State */
> + /* State. Protected by lock in BlockCopyState */
> CoQueue wait_queue; /* coroutines blocked on this task */
>
> /* To reference all call states from BlockCopyState */
> @@ -99,7 +100,8 @@ typedef struct BlockCopyState {
> BdrvChild *source;
> BdrvChild *target;
>
> - /* State */
> + /* State. Protected by lock */
> + CoMutex lock;
> int64_t in_flight_bytes;
> BlockCopyMethod method;
> QLIST_HEAD(, BlockCopyTask) tasks; /* All tasks from all block-copy calls */
> @@ -139,8 +141,10 @@ typedef struct BlockCopyState {
> bool skip_unallocated;
> } BlockCopyState;
>
May be nitpicking, but if we want block_copy_set_progress_meter to be threadsafe it should set s->progress under mutex. Or we should document that it's not threadsafe and called once.
> -static BlockCopyTask *find_conflicting_task(BlockCopyState *s,
> - int64_t offset, int64_t bytes)
> +/* Called with lock held */
> +static BlockCopyTask *find_conflicting_task_locked(BlockCopyState *s,
> + int64_t offset,
> + int64_t bytes)
> {
> BlockCopyTask *t;
>
> @@ -160,18 +164,22 @@ static BlockCopyTask *find_conflicting_task(BlockCopyState *s,
> static bool coroutine_fn block_copy_wait_one(BlockCopyState *s, int64_t offset,
> int64_t bytes)
> {
> - BlockCopyTask *task = find_conflicting_task(s, offset, bytes);
> + BlockCopyTask *task;
> +
> + QEMU_LOCK_GUARD(&s->lock);
> + task = find_conflicting_task_locked(s, offset, bytes);
>
> if (!task) {
> return false;
> }
>
> - qemu_co_queue_wait(&task->wait_queue, NULL);
> + qemu_co_queue_wait(&task->wait_queue, &s->lock);
>
> return true;
> }
>
> -static int64_t block_copy_chunk_size(BlockCopyState *s)
> +/* Called with lock held */
> +static int64_t block_copy_chunk_size_locked(BlockCopyState *s)
> {
> switch (s->method) {
> case COPY_READ_WRITE_CLUSTER:
> @@ -193,14 +201,16 @@ static int64_t block_copy_chunk_size(BlockCopyState *s)
> * Search for the first dirty area in offset/bytes range and create task at
> * the beginning of it.
> */
> -static BlockCopyTask *block_copy_task_create(BlockCopyState *s,
> - BlockCopyCallState *call_state,
> - int64_t offset, int64_t bytes)
> +static coroutine_fn BlockCopyTask *block_copy_task_create(BlockCopyState *s,
> + BlockCopyCallState *call_state,
> + int64_t offset, int64_t bytes)
> {
> BlockCopyTask *task;
> - int64_t max_chunk = block_copy_chunk_size(s);
> + int64_t max_chunk;
>
> - max_chunk = MIN_NON_ZERO(max_chunk, call_state->max_chunk);
> + QEMU_LOCK_GUARD(&s->lock);
> + max_chunk = MIN_NON_ZERO(block_copy_chunk_size_locked(s),
> + call_state->max_chunk);
> if (!bdrv_dirty_bitmap_next_dirty_area(s->copy_bitmap,
> offset, offset + bytes,
> max_chunk, &offset, &bytes))
> @@ -212,7 +222,7 @@ static BlockCopyTask *block_copy_task_create(BlockCopyState *s,
> bytes = QEMU_ALIGN_UP(bytes, s->cluster_size);
>
> /* region is dirty, so no existent tasks possible in it */
> - assert(!find_conflicting_task(s, offset, bytes));
> + assert(!find_conflicting_task_locked(s, offset, bytes));
>
> bdrv_reset_dirty_bitmap(s->copy_bitmap, offset, bytes);
> s->in_flight_bytes += bytes;
> @@ -248,16 +258,19 @@ static void coroutine_fn block_copy_task_shrink(BlockCopyTask *task,
>
The function reads task->bytes not under mutex.. It's safe, as only that function is modifying the field, and it's called once. Still, let's make critical section a little bit wider, just for simplicity. I mean, simple QEMU_LOCK_GUARD() at start of function.
> assert(new_bytes > 0 && new_bytes < task->bytes);
>
> - task->s->in_flight_bytes -= task->bytes - new_bytes;
> - bdrv_set_dirty_bitmap(task->s->copy_bitmap,
> - task->offset + new_bytes, task->bytes - new_bytes);
> -
> - task->bytes = new_bytes;
> - qemu_co_queue_restart_all(&task->wait_queue);
> + WITH_QEMU_LOCK_GUARD(&task->s->lock) {
> + task->s->in_flight_bytes -= task->bytes - new_bytes;
> + bdrv_set_dirty_bitmap(task->s->copy_bitmap,
> + task->offset + new_bytes,
> + task->bytes - new_bytes);
> + task->bytes = new_bytes;
> + qemu_co_queue_restart_all(&task->wait_queue);
> + }
> }
>
> static void coroutine_fn block_copy_task_end(BlockCopyTask *task, int ret)
> {
> + QEMU_LOCK_GUARD(&task->s->lock);
> task->s->in_flight_bytes -= task->bytes;
> if (ret < 0) {
> bdrv_set_dirty_bitmap(task->s->copy_bitmap, task->offset, task->bytes);
> @@ -335,6 +348,7 @@ BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
> }
>
> ratelimit_init(&s->rate_limit);
> + qemu_co_mutex_init(&s->lock);
> QLIST_INIT(&s->tasks);
> QLIST_INIT(&s->calls);
>
> @@ -390,6 +404,8 @@ static coroutine_fn int block_copy_task_run(AioTaskPool *pool,
Oops. seems block_copy_task_run misses block_copy_task_end() call befokre freeing the task. preexisting bug..
> * a full-size buffer or disabled if the copy_range attempt fails. The output
> * value of @method should be used for subsequent tasks.
> * Returns 0 on success.
> + *
> + * Called with lock held.
> */
> static int coroutine_fn block_copy_do_copy(BlockCopyState *s,
> int64_t offset, int64_t bytes,
> @@ -476,16 +492,20 @@ static coroutine_fn int block_copy_task_entry(AioTask *task)
> int ret;
>
> ret = block_copy_do_copy(s, t->offset, t->bytes, &method, &error_is_read);
> - if (s->method == t->method) {
> - s->method = method;
> - }
> - if (ret < 0) {
> - if (!t->call_state->ret) {
> - t->call_state->ret = ret;
> - t->call_state->error_is_read = error_is_read;
> +
> + WITH_QEMU_LOCK_GUARD(&t->s->lock) {
> + if (s->method == t->method) {
> + s->method = method;
> + }
> +
> + if (ret < 0) {
> + if (!t->call_state->ret) {
> + t->call_state->ret = ret;
> + t->call_state->error_is_read = error_is_read;
> + }
> + } else {
> + progress_work_done(t->s->progress, t->bytes);
> }
> - } else {
> - progress_work_done(t->s->progress, t->bytes);
> }
> co_put_to_shres(t->s->mem, t->bytes);
> block_copy_task_end(t, ret);
> @@ -587,10 +607,12 @@ int64_t block_copy_reset_unallocated(BlockCopyState *s,
> bytes = clusters * s->cluster_size;
>
> if (!ret) {
> + qemu_co_mutex_lock(&s->lock);
> bdrv_reset_dirty_bitmap(s->copy_bitmap, offset, bytes);
> progress_set_remaining(s->progress,
> bdrv_get_dirty_count(s->copy_bitmap) +
> s->in_flight_bytes);
> + qemu_co_mutex_unlock(&s->lock);
> }
>
> *count = bytes;
> @@ -729,7 +751,9 @@ static int coroutine_fn block_copy_common(BlockCopyCallState *call_state)
> {
> int ret;
>
> + qemu_co_mutex_lock(&call_state->s->lock);
> QLIST_INSERT_HEAD(&call_state->s->calls, call_state, list);
> + qemu_co_mutex_unlock(&call_state->s->lock);
>
> do {
> ret = block_copy_dirty_clusters(call_state);
> @@ -756,7 +780,9 @@ static int coroutine_fn block_copy_common(BlockCopyCallState *call_state)
> call_state->cb(call_state->cb_opaque);
> }
>
> + qemu_co_mutex_lock(&call_state->s->lock);
> QLIST_REMOVE(call_state, list);
> + qemu_co_mutex_unlock(&call_state->s->lock);
>
> return ret;
> }
>
I looked through the whole file on top of the series, and it seems overall OK to me. I still don't really like additional atomics, but they probably should be refactored together with refactoring all status-getters into one block_copy_call_status().. So it's a work for some future day, I will not do it in parallel :)
I don't insist, but for me patches 2,4,5 only make sense as a whole, so, I'd merge them into one patch called "make block-copy APIs thread-safe". Otherwise, thread-safety comes only in last patch, and patches 2 and 4 are a kind of preparations that hard to review in separate.
Anyway, reviewing of such change is a walk through the whole file trying to understand, how much is it thread-safe now.
--
Best regards,
Vladimir
next prev parent reply other threads:[~2021-06-09 12:27 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-08 7:33 [PATCH v3 0/5] block-copy: protect block-copy internal structures Emanuele Giuseppe Esposito
2021-06-08 7:33 ` [PATCH v3 1/5] block-copy: streamline choice of copy_range vs. read/write Emanuele Giuseppe Esposito
2021-06-09 8:51 ` Vladimir Sementsov-Ogievskiy
2021-06-09 9:33 ` Paolo Bonzini
2021-06-09 10:09 ` Vladimir Sementsov-Ogievskiy
2021-06-09 10:54 ` Vladimir Sementsov-Ogievskiy
2021-06-08 7:33 ` [PATCH v3 2/5] block-copy: improve comments of BlockCopyTask and BlockCopyState types and functions Emanuele Giuseppe Esposito
2021-06-09 9:12 ` Vladimir Sementsov-Ogievskiy
2021-06-10 10:14 ` Emanuele Giuseppe Esposito
2021-06-10 10:27 ` Vladimir Sementsov-Ogievskiy
2021-06-10 10:46 ` Emanuele Giuseppe Esposito
2021-06-10 11:12 ` Vladimir Sementsov-Ogievskiy
2021-06-10 14:21 ` Emanuele Giuseppe Esposito
2021-06-10 15:05 ` Vladimir Sementsov-Ogievskiy
2021-06-08 7:33 ` [PATCH v3 3/5] block-copy: move progress_set_remaining in block_copy_task_end Emanuele Giuseppe Esposito
2021-06-08 7:33 ` [PATCH v3 4/5] block-copy: add a CoMutex Emanuele Giuseppe Esposito
2021-06-09 12:25 ` Vladimir Sementsov-Ogievskiy [this message]
2021-06-10 14:49 ` Emanuele Giuseppe Esposito
2021-06-08 7:33 ` [PATCH v3 5/5] block-copy: atomic .cancelled and .finished fields in BlockCopyCallState Emanuele Giuseppe Esposito
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