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91ba0e1c38
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-b057-80ca69e63f51@virtuozzo.com/
Reported-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20250530151125.955508-9-f.ebner@proxmox.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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| disas | ||
| exec | ||
| fpu | ||
| gdbstub | ||
| hw | ||
| io | ||
| libdecnumber | ||
| migration | ||
| monitor | ||
| net | ||
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| standard-headers | ||
| system | ||
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| elf.h | ||
| glib-compat.h | ||
| qemu-io.h | ||
| qemu-main.h | ||