Re: [PATCH 05/12] block: remove AioContext locking

[Eric Blake <eblake@redhat.com>]

On Wed, Nov 29, 2023 at 02:55:46PM -0500, Stefan Hajnoczi wrote:
> This is the big patch that removes
> aio_context_acquire()/aio_context_release() from the block layer and
> affected block layer users.
> 
> There isn't a clean way to split this patch and the reviewers are likely
> the same group of people, so I decided to do it in one patch.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---

> +++ b/block.c
> @@ -7585,29 +7433,12 @@ void coroutine_fn bdrv_co_leave(BlockDriverState *bs, AioContext *old_ctx)
>  
>  void coroutine_fn bdrv_co_lock(BlockDriverState *bs)
>  {
> -    AioContext *ctx = bdrv_get_aio_context(bs);
> -
> -    /* In the main thread, bs->aio_context won't change concurrently */
> -    assert(qemu_get_current_aio_context() == qemu_get_aio_context());
> -
> -    /*
> -     * We're in coroutine context, so we already hold the lock of the main
> -     * loop AioContext. Don't lock it twice to avoid deadlocks.
> -     */
> -    assert(qemu_in_coroutine());

Is this assertion worth keeping in the short term?...

> -    if (ctx != qemu_get_aio_context()) {
> -        aio_context_acquire(ctx);
> -    }
> +    /* TODO removed in next patch */
>  }

...I guess I'll see in the next patch.

>  
>  void coroutine_fn bdrv_co_unlock(BlockDriverState *bs)
>  {
> -    AioContext *ctx = bdrv_get_aio_context(bs);
> -
> -    assert(qemu_in_coroutine());
> -    if (ctx != qemu_get_aio_context()) {
> -        aio_context_release(ctx);
> -    }
> +    /* TODO removed in next patch */
>  }

Same comment.

> +++ b/blockdev.c
> @@ -1395,7 +1352,6 @@ static void external_snapshot_action(TransactionAction *action,
>      /* File name of the new image (for 'blockdev-snapshot-sync') */
>      const char *new_image_file;
>      ExternalSnapshotState *state = g_new0(ExternalSnapshotState, 1);
> -    AioContext *aio_context;
>      uint64_t perm, shared;
>  
>      /* TODO We'll eventually have to take a writer lock in this function */

I'm guessing removal of the locking gets us one step closer to
implementing this TODO at a later time?  Or is it now a stale comment?
Either way, it doesn't affect this patch.

> +++ b/migration/block.c
> @@ -270,7 +270,6 @@ static int mig_save_device_bulk(QEMUFile *f, BlkMigDevState *bmds)
>  
>      if (bmds->shared_base) {
>          qemu_mutex_lock_iothread();
> -        aio_context_acquire(blk_get_aio_context(bb));
...
> @@ -313,22 +311,10 @@ static int mig_save_device_bulk(QEMUFile *f, BlkMigDevState *bmds)
>      block_mig_state.submitted++;
>      blk_mig_unlock();
>  
> -    /* We do not know if bs is under the main thread (and thus does
> -     * not acquire the AioContext when doing AIO) or rather under
> -     * dataplane.  Thus acquire both the iothread mutex and the
> -     * AioContext.
> -     *
> -     * This is ugly and will disappear when we make bdrv_* thread-safe,
> -     * without the need to acquire the AioContext.
> -     */
> -    qemu_mutex_lock_iothread();
> -    aio_context_acquire(blk_get_aio_context(bmds->blk));

Will conflict, but with trivial resolution, with your other thread
renaming things to qemu_bql_lock().


> +++ b/tests/unit/test-blockjob.c

> -static void test_complete_in_standby(void)
> -{

> @@ -531,13 +402,5 @@ int main(int argc, char **argv)
>      g_test_add_func("/blockjob/cancel/standby", test_cancel_standby);
>      g_test_add_func("/blockjob/cancel/pending", test_cancel_pending);
>      g_test_add_func("/blockjob/cancel/concluded", test_cancel_concluded);
> -
> -    /*
> -     * This test is flaky and sometimes fails in CI and otherwise:
> -     * don't run unless user opts in via environment variable.
> -     */
> -    if (getenv("QEMU_TEST_FLAKY_TESTS")) {
> -        g_test_add_func("/blockjob/complete_in_standby", test_complete_in_standby);
> -    }

Looks like you ripped out this entire test, because it is no longer
viable.  I might have mentioned it in the commit message, or squashed
the removal of this test into the earlier 02/12 patch.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org