Re: [Qemu-devel] [PATCH v2 00/20] dataplane: remove RFifoLock

[Fam Zheng <famz@redhat.com>]

On Mon, 10/17 15:54, Paolo Bonzini wrote:
> This patch reorganizes aio_poll callers to establish new rules for
> dataplane locking.  The idea is that I/O operations on a dataplane
> BDS (i.e. one where the AioContext is not the main one) do not call
> aio_poll anymore.  Instead, they wait for the operation to end in the
> other I/O thread, at which point the other I/O thread calls bdrv_wakeup
> to wake up the main thread.
> 
> With this change, only one thread runs aio_poll for an AioContext.
> While aio_context_acquire/release is still needed to protect the BDSes,
> it need not interrupt the other thread's event loop anymore, and therefore
> it does not need contention callbacks anymore.  Thus the patch can remove
> RFifoLock.  This fixes possible hangs in bdrv_drain_all, reproducible (for
> example) by unplugging a virtio-scsi-dataplane device while there is I/O
> going on for a virtio-blk-dataplane on the same I/O thread.
> 
> Patch 1 is a bugfix that I already posted.
> 
> Patch 2 makes blockjobs independent of aio_poll, the reason for which
> should be apparent from the explanation above.
> 
> Patch 3 is an independent mirror bugfix, that I wanted to submit separately
> but happens to fix a hang in COLO replication.  Like patch 1 I believe
> it's pre-existing and merely exposed by these patches.
> 
> Patches 4 to 10 introduce the infrastructure to wake up the main thread
> while bdrv_drain or other synchronous operations are running.  Patches 11
> to 16 do other changes to prepare for this.  Notably bdrv_drain_all
> needs to be called without holding any AioContext lock, so bdrv_reopen
> releases the lock temporarily (and callers of bdrv_reopen needs fixing).
> 
> Patch 17 then does the big change, after which there are just some
> cleanups left to do.
> 
> Paolo
> 
> Fam Zheng (1):
>   qed: Implement .bdrv_drain
> 
> Paolo Bonzini (19):
>   replication: interrupt failover if the main device is closed
>   blockjob: introduce .drain callback for jobs
>   mirror: use bdrv_drained_begin/bdrv_drained_end
>   block: add BDS field to count in-flight requests
>   block: change drain to look only at one child at a time
>   block: introduce BDRV_POLL_WHILE
>   nfs: move nfs_set_events out of the while loops
>   nfs: use BDRV_POLL_WHILE
>   sheepdog: use BDRV_POLL_WHILE
>   aio: introduce qemu_get_current_aio_context
>   iothread: detach all block devices before stopping them
>   replication: pass BlockDriverState to reopen_backing_file
>   block: prepare bdrv_reopen_multiple to release AioContext
>   qemu-io: acquire AioContext
>   qemu-img: call aio_context_acquire/release around block job
>   block: only call aio_poll on the current thread's AioContext
>   iothread: release AioContext around aio_poll
>   qemu-thread: introduce QemuRecMutex
>   aio: convert from RFifoLock to QemuRecMutex

Modulo the one harmful question, series:

Reviewed-by: Fam Zheng <famz@redhat.com>