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794b70d024
If a device uses blk_inc/dec_in_flight() in order to build macro
operations that involve multiple requests for the block layer and that
need to be completed as a unit before the BlockBackend can be considered
drained, it sets the stage for a deadlock: When a drain is requested,
the inner request at the BlockBackend level will be queued in
blk_wait_while_drained() and wait until the drained section ends, but at
the same time, drain_begin can only return if the whole macro operation
at the device level has completed.
Introduce a new interface to allow implementing the logic correctly:
Instead of queueing individual requests, blk_co_start_request() calls
blk_wait_while_drained() once at the beginning. The individual requests
must then set BDRV_REQ_NO_QUEUE to avoid being queued and running into
the deadlock; being wrapped in blk_co_start/end_request() makes sure
that drain_begin waits for them and they don't sneak in when the
BlockBackend is supposed to already be quiescent.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20260421161132.99878-3-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 34a6763776)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>