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Tue, 9 Jun 2026 15:49:04 +0000 (UTC) Received: from localhost (unknown [10.2.16.141]) by mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id E5B5736535; Tue, 9 Jun 2026 15:49:03 +0000 (UTC) Date: Tue, 9 Jun 2026 11:49:00 -0400 From: Stefan Hajnoczi To: Kevin Wolf Cc: Boudewijn van der Heide , qemu-block@nongnu.org, qemu-devel@nongnu.org, hreitz@redhat.com, pbonzini@redhat.com Subject: Re: [PATCH] block: switch to co_wrapper_mixed for blk_lock_medium() and blk_eject() Message-ID: <20260609154900.GC57581@fedora> References: <20260530175822.10321-1-boudewijn@delta-utec.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="vFodwuVgBO9vbVih" Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.6 on 10.30.177.95 Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.445, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --vFodwuVgBO9vbVih Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Mon, Jun 08, 2026 at 02:17:46PM +0200, Kevin Wolf wrote: > [Cc: Paolo, Stefan] >=20 > Am 30.05.2026 um 19:58 hat Boudewijn van der Heide geschrieben: > > scsi_disk_emulate_command() can be called from a coroutine; > > when req->cmd.buf[0] is ALLOW_MEDIUM_REMOVAL, the synchronous > > blk_lock_medium() is called, which hits assert(!qemu_in_coroutine()), > > and crashes: > >=20 > > qemu-system-hppa: block/block-gen.c:1692: blk_lock_medium: > > Assertion `!qemu_in_coroutine()' failed. > >=20 > > blk_eject() has the same problem, it can be called from coroutine, > > because the same vtable entry (SCSIReqOps.send_command, > > scsi_disk_emulate_command() here) calls blk_eject when req->cmd.buf[0] > > is START_STOP. > >=20 > > Fix by switching to co_wrapper_mixed for blk_lock_medium() and > > blk_eject() instead of just co_wrapper. > >=20 > > Signed-off-by: Boudewijn van der Heide >=20 > Yes, that will fix your immediate symptoms. I'm not completely sure if > that's the level where we want to fix things, though, which is why I > added Paolo and Stefan to Cc. >=20 > > --- > > Observed crash on fedora qemu-10.1.5-1.fc43 on x86_64 host using qemu-s= ystem-hppa. > >=20 > > trace: > >=20 > > Thread 1 (...): > > # 5 __assert_fail (...) at assert.c > > # 6 blk_lock_medium (...) at block/block-gen.c > > # 7 scsi_disk_emulate_command (...) at ../hw/scsi/scsi-disk.c > > # 8 scsi_req_enqueue (...) at ../hw/scsi/scsi-bus.c > > # 9 lsi_do_command (...) at ../hw/scsi/lsi53c895a.c > > # 10 lsi_execute_script (...) at ../hw/scsi/lsi53c895a.c > > # 11 scsi_read_complete_noio (...) at ../hw/scsi/scsi-disk.c > > # 12 blk_aio_complete (...) at ../block/block-backend.c > > # 14 blk_aio_read_entry (...) at ../block/block-backend.c > > # 15 in coroutine_trampoline (...) at ../util/coroutine-ucontex= t.c > >=20 > > blk_eject() has the same path, but req->cmd.buf[0] is START_STOP instead > > of ALLOW_MEDIUM_REMOVAL inside scsi_disk_emulate_command(), > > so fix that aswell. >=20 > This would be useful information to have in the commit message proper. >=20 > The step from #10 to #11 seems to be a rather big one that left out > intermediate function calls, so I'm not sure if I fully understand it. >=20 > Either way, I don't think lsi_execute_script() and the functions called > by it were ever written with the intention to run in a coroutine. I'm > almost sure that problems can be in more than just the two functions > you're changing here. So my question is mostly, should the path involve > a BH somewhere to break out of coroutine context, instead of making one > or two specific cases work? The code path in this bug is from the block layer (blk_aio_read_entry -> blk_aio_complete) to scsi-disk (scsi_read_complete_noio) into the LSI device (probably lsi_command_complete -> lsi_resume_script -> lsi_execute_script). I'm not sure if the LSI device, which is unaware of coroutines, should have to work around this. It seems cleaner for blk_aio_preadv() to invoke cb() from outside coroutine context since that API does not say anything about coroutine contexts. On the other hand, there will be a performance overhead for leaving coroutine context even when it would be fine to run in a coroutine... > If the general feeling is that we do want to make essentially all of the > SCSI code coroutine_mixed_fn and are prepared to fix any issues arising > from it, then this patch is fine as far as I am concerned. >=20 > Kevin >=20 > > --- > > include/system/block-backend-io.h | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > >=20 > > diff --git a/include/system/block-backend-io.h b/include/system/block-b= ackend-io.h > > index fd84723d9d..7368ad5c09 100644 > > --- a/include/system/block-backend-io.h > > +++ b/include/system/block-backend-io.h > > @@ -81,10 +81,10 @@ bool coroutine_fn GRAPH_RDLOCK blk_co_is_available(= BlockBackend *blk); > > bool co_wrapper_mixed_bdrv_rdlock blk_is_available(BlockBackend *blk); > > =20 > > void coroutine_fn blk_co_lock_medium(BlockBackend *blk, bool locked); > > -void co_wrapper blk_lock_medium(BlockBackend *blk, bool locked); > > +void co_wrapper_mixed blk_lock_medium(BlockBackend *blk, bool locked); > > =20 > > void coroutine_fn blk_co_eject(BlockBackend *blk, bool eject_flag); > > -void co_wrapper blk_eject(BlockBackend *blk, bool eject_flag); > > +void co_wrapper_mixed blk_eject(BlockBackend *blk, bool eject_flag); > > =20 > > int64_t coroutine_fn blk_co_getlength(BlockBackend *blk); > > int64_t co_wrapper_mixed blk_getlength(BlockBackend *blk); > > --=20 > > 2.54.0 > >=20 >=20 --vFodwuVgBO9vbVih Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQEzBAEBCgAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmooNewACgkQnKSrs4Gr c8gPFQf9FGq+hLl1Z4Mcaned4d8YsSmEHmdQD7yrddO+74dSS7+MZkdwaQ56aKP6 tFiki/u97wxlx/mURGP/WjVS/+6rgaQUOoL7xLnJFg9D9YGcJ2yC3UgrCgIg12Ny 7EZNFMrPh0fZmTiF/NKXqsBlSStXFY1kz+Qzdsks0U0FlvCu2yXcn4XyY8+HekDZ p/re5KBcIF9cm3DxQXBugB4FBNGrR8bMHJgQ3CgRIS5IrZX9stRGbKlryfAOB9JN z0aO2tslpkJ5CLVk1DoLN3LfO2qbJIjThb9sCKT8n81CAPGIP3jIlUz6cpim3eqX 3z8Oq7/7Y+cMlzO7veNXfI+7iAPV7A== =dqJE -----END PGP SIGNATURE----- --vFodwuVgBO9vbVih--