Am 26.05.2025 um 15:21 hat Fiona Ebner geschrieben: > This is in preparation to mark bdrv_drained_begin() as GRAPH_UNLOCKED. > > Note that even if bdrv_drained_begin() were already marked as > GRAPH_UNLOCKED, TSA would not complain about the instance in > bdrv_change_aio_context() before this change, because it is preceded > by a bdrv_graph_rdunlock_main_loop() call. It is not correct to > release the lock here, and in case the caller holds a write lock, it > wouldn't actually release the lock. > > In combination with block-stream, there is a deadlock that can happen > because of this [0]. In particular, it can happen that > main thread IO thread > 1. acquires write lock > in blk_co_do_preadv_part(): > 2. have non-zero blk->in_flight > 3. try to acquire read lock > 4. begin drain > > Steps 3 and 4 might be switched. Draining will poll and get stuck, > because it will see the non-zero in_flight counter. But the IO thread > will not make any progress either, because it cannot acquire the read > lock. > > After this change, all paths to bdrv_change_aio_context() drain: > bdrv_change_aio_context() is called by: > 1. bdrv_child_cb_change_aio_ctx() which is only called via the > change_aio_ctx() callback, see below. > 2. bdrv_child_change_aio_context(), see below. > 3. bdrv_try_change_aio_context(), where a drained section is > introduced. > > The change_aio_ctx() callback is called by: > 1. bdrv_attach_child_common_abort(), where a drained section is > introduced. > 2. bdrv_attach_child_common(), where a drained section is introduced. > 3. bdrv_parent_change_aio_context(), see below. > > bdrv_child_change_aio_context() is called by: > 1. bdrv_change_aio_context(), i.e. recursive, so being in a drained > section is invariant. > 2. child_job_change_aio_ctx(), which is only called via the > change_aio_ctx() callback, see above. > > bdrv_parent_change_aio_context() is called by: > 1. bdrv_change_aio_context(), i.e. recursive, so being in a drained > section is invariant. > > This resolves all code paths. Note that bdrv_attach_child_common() > and bdrv_attach_child_common_abort() hold the graph write lock and > callers of bdrv_try_change_aio_context() might too, so they are not > actually allowed to drain either. This will be addressed in the > following commits. > > More granular draining is not trivially possible, because > bdrv_change_aio_context() can recursively call itself e.g. via > bdrv_child_change_aio_context(). > > [0]: > https://lore.kernel.org/qemu-devel/73839c04-7616-407e-b057-80ca69e63...@virtuozzo.com/ > > Reported-by: Andrey Drobyshev <andrey.drobys...@virtuozzo.com> > Signed-off-by: Fiona Ebner <f.eb...@proxmox.com>
Reviewed-by: Kevin Wolf <kw...@redhat.com>
