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Wed, 31 Jan 2024 20:35:40 +0000 (UTC) Received: from localhost (unknown [10.39.192.56]) by smtp.corp.redhat.com (Postfix) with ESMTP id 73B382166B31; Wed, 31 Jan 2024 20:35:39 +0000 (UTC) Date: Wed, 31 Jan 2024 15:35:37 -0500 From: Stefan Hajnoczi To: Hanna Czenczek Cc: Kevin Wolf , qemu-block@nongnu.org, qemu-devel@nongnu.org Subject: Re: [PULL 11/33] scsi: only access SCSIDevice->requests from one thread Message-ID: <20240131203537.GC396296@fedora> References: <20231221212339.164439-1-kwolf@redhat.com> <20231221212339.164439-12-kwolf@redhat.com> <73e752b2-a037-4b10-a903-56fa6ad75c6e@redhat.com> <08a66849-f190-4756-9b01-666f0d66afb6@redhat.com> <4c4173f2-b8fc-4c6f-88e1-8c31c4411837@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="tuppfXptz9SgFWCy" Content-Disposition: inline In-Reply-To: <4c4173f2-b8fc-4c6f-88e1-8c31c4411837@redhat.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.6 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: -33 X-Spam_score: -3.4 X-Spam_bar: --- X-Spam_report: (-3.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.292, 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_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: 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 --tuppfXptz9SgFWCy Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Jan 26, 2024 at 04:24:49PM +0100, Hanna Czenczek wrote: > On 26.01.24 14:18, Kevin Wolf wrote: > > Am 25.01.2024 um 18:32 hat Hanna Czenczek geschrieben: > > > 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 > > > > > >=20 > > > > > > 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 requ= ests > > > > > > list. > > > > > >=20 > > > > > > These constraints protect the requests list without the need fo= r locking > > > > > > in the I/O code path. > > > > > >=20 > > > > > > 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. > > > > > >=20 > > > > > > Signed-off-by: Stefan Hajnoczi > > > > > > Reviewed-by: Eric Blake > > > > > > Message-ID:<20231204164259.1515217-2-stefanha@redhat.com> > > > > > > Signed-off-by: Kevin Wolf > > > > > > --- > > > > > > 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-h= d device, > > > > > this tends to happen when unplugging the scsi-hd: >=20 > Note: We (on issues.redhat.com) have a separate report that seems to be > concerning this very problem: https://issues.redhat.com/browse/RHEL-19381 >=20 > > > > > {"execute":"device_del","arguments":{"id":"stg0"}} > > > > > {"return": {}} > > > > > qemu-system-x86_64: ../block/block-backend.c:2429: blk_get_aio_co= ntext: > > > > > Assertion `ctx =3D=3D blk->ctx' failed. > > > [...] > > >=20 > > > > > I don=E2=80=99t know anything about the problem yet, but as usual= , I like > > > > > speculation and discovering how wrong I was later on, so one thin= g I came > > > > > across that=E2=80=99s funny about virtio-scsi is that requests ca= n happen even while > > > > > a disk is being attached or detached.=C2=A0 That is, Linux seems = to probe all > > > > > LUNs when a new virtio-scsi device is being attached, and it won= =E2=80=99t stop just > > > > > because a disk is being attached or removed.=C2=A0 So maybe that= =E2=80=99s part of the > > > > > problem, that we get a request while the BB is being detached, and > > > > > temporarily in an inconsistent state (BDS context differs from BB= context). > > > > I don't know anything about the problem either, but since you alrea= dy > > > > 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 f= or > > > > blk_get_aio_context(), but that should be completely unproblematic. > > > Actually, now that completely unproblematic part is giving me trouble= =2E=C2=A0 I > > > wanted to just put a graph lock into blk_get_aio_context() (making it= a > > > coroutine with a wrapper) > > Which is the first thing I neglected and already not great. We have > > calls of blk_get_aio_context() in the SCSI I/O path, and creating a > > coroutine and doing at least two context switches simply for this call > > is a lot of overhead... > >=20 > > > but callers of blk_get_aio_context() generally assume the context is > > > going to stay the BB=E2=80=99s context for as long as their AioContex= t * > > > variable is in scope. > > I'm not so sure about that. And taking another step back, I'm actually > > also not sure how much it still matters now that they can submit I/O > > from any thread. >=20 > That=E2=80=99s my impression, too, but =E2=80=9Cnot sure=E2=80=9D doesn= =E2=80=99t feel great. :) > scsi_device_for_each_req_async_bh() specifically double-checks whether it= =E2=80=99s > still in the right context before invoking the specified function, so it > seems there was some intention to continue to run in the context associat= ed > with the BB. >=20 > (Not judging whether that intent makes sense or not, yet.) >=20 > > Maybe the correct solution is to remove the assertion from > > blk_get_aio_context() and just always return blk->ctx. If it's in the > > middle of a change, you'll either get the old one or the new one. Either > > one is fine to submit I/O from, and if you care about changes for other > > reasons (like SCSI does), then you need explicit code to protect it > > anyway (which SCSI apparently has, but it doesn't work). >=20 > I think most callers do just assume the BB stays in the context they got > (without any proof, admittedly), but I agree that under re-evaluation, it > probably doesn=E2=80=99t actually matter to them, really. And yes, basica= lly, if the > caller doesn=E2=80=99t need to take a lock because it doesn=E2=80=99t rea= lly matter whether > blk->ctx changes while its still using the old value, blk_get_aio_context= () > in turn doesn=E2=80=99t need to double-check blk->ctx against the root no= de=E2=80=99s > context either, and nobody needs a lock. >=20 > So I agree, it=E2=80=99s on the caller to protect against a potentially c= hanging > context, blk_get_aio_context() should just return blk->ctx and not check > against the root node. >=20 > (On a tangent: blk_drain() is a caller of blk_get_aio_context(), and it > polls that context.=C2=A0 Why does it need to poll that context specifica= lly when > requests may be in any context?=C2=A0 Is it because if there are requests= in the > main thread, we must poll that, but otherwise it=E2=80=99s fine to poll a= ny thread, > and we can only have requests in the main thread if that=E2=80=99s the BB= =E2=80=99s > context?) >=20 > > > I was tempted to think callers know what happens to the BB they pass > > > to blk_get_aio_context(), and it won=E2=80=99t change contexts so eas= ily, but > > > then I remembered this is exactly what happens in this case; we run > > > scsi_device_for_each_req_async_bh() in one thread (which calls > > > blk_get_aio_context()), and in the other, we change the BB=E2=80=99s = context. > > Let's think a bit more about scsi_device_for_each_req_async() > > specifically. This is a function that runs in the main thread. Nothing > > will change any AioContext assignment if it doesn't call it. It wants to > > make sure that scsi_device_for_each_req_async_bh() is called in the > > same AioContext where the virtqueue is processed, so it schedules a BH > > and waits for it. >=20 > I don=E2=80=99t quite follow, it doesn=E2=80=99t wait for the BH.=C2=A0 I= t uses > aio_bh_schedule_oneshot(), not aio_wait_bh_oneshot().=C2=A0 While you=E2= =80=99re right > that if it did wait, the BB context might still change, in practice we > wouldn=E2=80=99t have the problem at hand because the caller is actually = the one to > change the context, concurrently while the BH is running. >=20 > > Waiting for it means running a nested event loop that could do anything, > > including changing AioContexts. So this is what needs the locking, not > > the blk_get_aio_context() call in scsi_device_for_each_req_async_bh(). > > If we lock before the nested event loop and unlock in the BH, the check > > in the BH can become an assertion. (It is important that we unlock in > > the BH rather than after waiting because if something takes the writer > > lock, we need to unlock during the nested event loop of bdrv_wrlock() to > > avoid a deadlock.) > >=20 > > And spawning a coroutine for this looks a lot more acceptable because > > it's on a slow path anyway. > >=20 > > In fact, we probably don't technically need a coroutine to take the > > reader lock here. We can have a new graph lock function that asserts > > that there is no writer (we know because we're running in the main loop) > > and then atomically increments the reader count. But maybe that already > > complicates things again... >=20 > So as far as I understand we can=E2=80=99t just use aio_wait_bh_oneshot()= and wrap > it in bdrv_graph_rd{,un}lock_main_loop(), because that doesn=E2=80=99t ac= tually lock > the graph.=C2=A0 I feel like adding a new graph lock function for this qu= ite > highly specific case could be dangerous, because it seems easy to use the > wrong way. >=20 > Just having a trampoline coroutine to call bdrv_graph_co_rd{,un}lock() se= ems > simple enough and reasonable here (not a hot path).=C2=A0 Can we have that > coroutine then use aio_wait_bh_oneshot() with the existing _bh function, = or > should that be made a coroutine, too? There is a reason for running in the context associated with the BB: the virtio-scsi code assumes all request processing happens in the BB's AioContext. The SCSI request list and other SCSI emulation code is not thread-safe! The invariant is that SCSI request processing must only happen in one AioContext. Other parts of QEMU may perform block I/O from other AioContexts because they don't run SCSI emulation for this device. Stefan --tuppfXptz9SgFWCy Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmW6rxkACgkQnKSrs4Gr c8hlPQf9GLROlU7zNY9At2mNXLeIqf773C+PCptI8B+HofgpXK/ejvXz4+3F4fHp yFpNaYOdKYIE9JUeORvR4rvGENiZareHB5a2Vk32cJAgB8SNSsqIcbDvf9VxqoQT YGoCMth0iEB8bcIAYGXgA2Fi+5byEpbTonebu7v4u7aIWacj4qGKgja7wZ4svmqC QZKW+yyodCMvBD2bww7jT5RQL81HkgjNKLfRe5oaA97G7Cy+JrXkUQctxHXwnOWF PBYPmEpazSjI53uU1REj2r0UrbHKuow30nIvvMJSyjpejsDE59FtQusivO+lszne 0rKGJ+k8tAJyNZ0XDhSHpdSaqa6/3Q== =E/HN -----END PGP SIGNATURE----- --tuppfXptz9SgFWCy--