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Tue, 26 May 2020 16:42:17 +0000 (UTC) Date: Tue, 26 May 2020 18:42:15 +0200 From: Kevin Wolf To: Stefan Reiter Subject: Re: [RFC PATCH 2/3] block: Allow bdrv_run_co() from different AioContext Message-ID: <20200526164215.GA8886@linux.fritz.box> References: <20200512144318.181049-1-kwolf@redhat.com> <20200512144318.181049-3-kwolf@redhat.com> <2ab7bae7-03c3-f269-1db1-202aeb41bdf3@proxmox.com> <20200525164150.GD19863@linux.fritz.box> MIME-Version: 1.0 In-Reply-To: <20200525164150.GD19863@linux.fritz.box> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.120; envelope-from=kwolf@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/26 01:51:57 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=0.001, 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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-block@nongnu.org, armbru@redhat.com, qemu-devel@nongnu.org, mreitz@redhat.com, stefanha@redhat.com, t.lamprecht@proxmox.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 25.05.2020 um 18:41 hat Kevin Wolf geschrieben: > Am 25.05.2020 um 16:18 hat Stefan Reiter geschrieben: > > On 5/12/20 4:43 PM, Kevin Wolf wrote: > > > Coroutine functions that are entered through bdrv_run_co() are already > > > safe to call from synchronous code in a different AioContext because > > > bdrv_coroutine_enter() will schedule them in the context of the node. > > > > > > However, the coroutine fastpath still requires that we're already in the > > > right AioContext when called in coroutine context. > > > > > > In order to make the behaviour more consistent and to make life a bit > > > easier for callers, let's check the AioContext and automatically move > > > the current coroutine around if we're not in the right context yet. > > > > > > Signed-off-by: Kevin Wolf > > > --- > > > block/io.c | 15 ++++++++++++++- > > > 1 file changed, 14 insertions(+), 1 deletion(-) > > > > > > diff --git a/block/io.c b/block/io.c > > > index c1badaadc9..7808e8bdc0 100644 > > > --- a/block/io.c > > > +++ b/block/io.c > > > @@ -895,8 +895,21 @@ static int bdrv_run_co(BlockDriverState *bs, CoroutineEntry *entry, > > > void *opaque, int *ret) > > > { > > > if (qemu_in_coroutine()) { > > > - /* Fast-path if already in coroutine context */ > > > + Coroutine *self = qemu_coroutine_self(); > > > + AioContext *bs_ctx = bdrv_get_aio_context(bs); > > > + AioContext *co_ctx = qemu_coroutine_get_aio_context(self); > > > + > > > + if (bs_ctx != co_ctx) { > > > + /* Move to the iothread of the node */ > > > + aio_co_schedule(bs_ctx, self); > > > + qemu_coroutine_yield(); > > > > I'm pretty sure this can lead to a race: When the thread we're re-scheduling > > to is faster to schedule us than we can reach qemu_coroutine_yield, then > > we'll get an abort ("Co-routine re-entered recursively"), since co->caller > > is still set. > > > > I've seen this happen in our code when I try to do the scheduling fandangle > > there. > > Ah, crap. I guess letting a coroutine re-schedule itself is only safe > within the same thread then. > > > Is there a safer way to have a coroutine reschedule itself? Some lock > > missing? > > There is no problem that can't be solved by adding another level of > indirection... We would have to schedule a BH in the original thread > that will only schedule the coroutine in its new thread after it has > yielded. > > Maybe we should actually introduce a helper function that moves the > current coroutine to a different AioContext this way. Like this: https://repo.or.cz/qemu/kevin.git/commitdiff/ed0244ba4ac699f7e8eaf7512ff25645cf43bda2 The series for which I need this isn't quite ready yet, so I haven't sent it as a patch yet, but if it proves useful in other contexts, we can always commit it without the rest. Kevin