Re: [PULL 11/33] scsi: only access SCSIDevice->requests from one thread

[Stefan Hajnoczi]

On Wed, Jan 24, 2024 at 01:12:47PM +0100, Hanna Czenczek wrote:
> On 23.01.24 18:10, Kevin Wolf wrote:
> > Am 23.01.2024 um 17:40 hat Hanna Czenczek geschrieben:
> > > On 21.12.23 22:23, Kevin Wolf wrote:
> > > > From: Stefan Hajnoczi<stefanha@redhat.com>
> > > > 
> > > > Stop depending on the AioContext lock and instead access
> > > > SCSIDevice->requests from only one thread at a time:
> > > > - When the VM is running only the BlockBackend's AioContext may access
> > > >     the requests list.
> > > > - When the VM is stopped only the main loop may access the requests
> > > >     list.
> > > > 
> > > > These constraints protect the requests list without the need for locking
> > > > in the I/O code path.
> > > > 
> > > > Note that multiple IOThreads are not supported yet because the code
> > > > assumes all SCSIRequests are executed from a single AioContext. Leave
> > > > that as future work.
> > > > 
> > > > Signed-off-by: Stefan Hajnoczi<stefanha@redhat.com>
> > > > Reviewed-by: Eric Blake<eblake@redhat.com>
> > > > Message-ID:<20231204164259.1515217-2-stefanha@redhat.com>
> > > > Signed-off-by: Kevin Wolf<kwolf@redhat.com>
> > > > ---
> > > >    include/hw/scsi/scsi.h |   7 +-
> > > >    hw/scsi/scsi-bus.c     | 181 
> > > > ++++++++++++++++++++++++++++-------------
> > > >    2 files changed, 131 insertions(+), 57 deletions(-)
> > > My reproducer forhttps://issues.redhat.com/browse/RHEL-3934  now breaks 
> > > more
> > > often because of this commit than because of the original bug, i.e. when
> > > repeatedly hot-plugging and unplugging a virtio-scsi and a scsi-hd device,
> > > this tends to happen when unplugging the scsi-hd:
> > > 
> > > {"execute":"device_del","arguments":{"id":"stg0"}}
> > > {"return": {}}
> > > qemu-system-x86_64: ../block/block-backend.c:2429: blk_get_aio_context:
> > > Assertion `ctx == blk->ctx' failed.
> 
> [...]
> 
> > I don't know anything about the problem either, but since you already
> > speculated about the cause, let me speculate about the solution:
> > Can we hold the graph writer lock for the tran_commit() call in
> > bdrv_try_change_aio_context()? And of course take the reader lock for
> > blk_get_aio_context(), but that should be completely unproblematic.
> 
> I tried this, and it’s not easy taking the lock just for tran_commit(),
> because some callers of bdrv_try_change_aio_context() already hold the write
> lock (specifically bdrv_attach_child_common(),
> bdrv_attach_child_common_abort(), and bdrv_root_unref_child()[1]), and
> qmp_x_blockdev_set_iothread() holds the read lock.  Other callers don’t hold
> any lock[2].
> 
> So I’m not sure whether we should mark all of bdrv_try_change_aio_context()
> as GRAPH_WRLOCK and then make all callers take the lock, or really only take
> it for tran_commit(), and have callers release the lock around
> bdrv_try_change_aio_context(). Former sounds better to naïve me.
> 
> (In any case, FWIW, having blk_set_aio_context() take the write lock, and
> scsi_device_for_each_req_async_bh() take the read lock[3], does fix the
> assertion failure.)

I wonder if a simpler solution is blk_inc_in_flight() in
scsi_device_for_each_req_async() and blk_dec_in_flight() in
scsi_device_for_each_req_async_bh() so that drain
waits for the BH.

There is a drain around the AioContext change, so as long as
scsi_device_for_each_req_async() was called before blk_set_aio_context()
and not _during_ aio_poll(), we would prevent inconsistent BB vs BDS
aio_contexts.

Stefan

> 
> Hanna
> 
> [1] bdrv_root_unref_child() is not marked as GRAPH_WRLOCK, but it’s callers
> generally seem to ensure that the lock is taken when calling it.
> 
> [2] blk_set_aio_context() (evidently), blk_exp_add(),
> external_snapshot_abort(), {blockdev,drive}_backup_action(),
> qmp_{blockdev,drive}_mirror()
> 
> [3] I’ve made the _bh a coroutine (for bdrv_graph_co_rdlock()) and replaced
> the aio_bh_schedule_oneshot() by aio_co_enter() – hope that’s right.

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