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Fri, 12 Jul 2019 11:23:22 +0000 (UTC) Received: from dhcp-200-226.str.redhat.com (dhcp-200-226.str.redhat.com [10.33.200.226]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5CC6860BFB; Fri, 12 Jul 2019 11:23:19 +0000 (UTC) Date: Fri, 12 Jul 2019 13:23:18 +0200 From: Kevin Wolf To: Max Reitz Message-ID: <20190712112318.GG4514@dhcp-200-226.str.redhat.com> References: <20190711195804.30703-1-mreitz@redhat.com> <20190711195804.30703-2-mreitz@redhat.com> <20190712092419.GB4514@dhcp-200-226.str.redhat.com> <8ce2ce78-833e-c98f-ad3a-d44f6432ae4c@redhat.com> <20190712110110.GE4514@dhcp-200-226.str.redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="KDt/GgjP6HVcx58l" Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.11.3 (2019-02-01) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Fri, 12 Jul 2019 11:23:22 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [RFC 1/5] block/nbd: Fix hang in .bdrv_close() 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: Stefan Hajnoczi , qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --KDt/GgjP6HVcx58l Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Am 12.07.2019 um 13:09 hat Max Reitz geschrieben: > On 12.07.19 13:01, Kevin Wolf wrote: > > Am 12.07.2019 um 12:47 hat Max Reitz geschrieben: > >> On 12.07.19 11:24, Kevin Wolf wrote: > >>> Am 11.07.2019 um 21:58 hat Max Reitz geschrieben: > >>>> When nbd_close() is called from a coroutine, the connection_co never > >>>> gets to run, and thus nbd_teardown_connection() hangs. > >>>> > >>>> This is because aio_co_enter() only puts the connection_co into the = main > >>>> coroutine's wake-up queue, so this main coroutine needs to yield and > >>>> reschedule itself to let the connection_co run. > >>>> > >>>> Signed-off-by: Max Reitz > >>>> --- > >>>> block/nbd.c | 12 +++++++++++- > >>>> 1 file changed, 11 insertions(+), 1 deletion(-) > >>>> > >>>> diff --git a/block/nbd.c b/block/nbd.c > >>>> index 81edabbf35..b83b6cd43e 100644 > >>>> --- a/block/nbd.c > >>>> +++ b/block/nbd.c > >>>> @@ -135,7 +135,17 @@ static void nbd_teardown_connection(BlockDriver= State *bs) > >>>> qio_channel_shutdown(s->ioc, > >>>> QIO_CHANNEL_SHUTDOWN_BOTH, > >>>> NULL); > >>>> - BDRV_POLL_WHILE(bs, s->connection_co); > >>>> + > >>>> + if (qemu_in_coroutine()) { > >>>> + /* Let our caller poll and just yield until connection_co i= s done */ > >>>> + while (s->connection_co) { > >>>> + aio_co_schedule(qemu_get_current_aio_context(), > >>>> + qemu_coroutine_self()); > >>>> + qemu_coroutine_yield(); > >>>> + } > >>> > >>> Isn't this busy waiting? Why not let s->connection_co wake us up when > >>> it's about to terminate instead of immediately rescheduling ourselves? > >> > >> Yes, it is busy waiting, but I didn=E2=80=99t find that bad. The conn= ection_co > >> will be invoked in basically every iteration, and once there is no > >> pending data, it will quit. > >> > >> The answer to =E2=80=9Cwhy not...=E2=80=9D of course is because it=E2= =80=99d be more complicated. > >> > >> But anyway. > >> > >> Adding a new function qemu_coroutine_run_after(target) that adds > >> qemu_coroutine_self() to the given @target coroutine=E2=80=99s wake-up= queue and > >> then using that instead of scheduling works, too, yes. > >> > >> I don=E2=80=99t really like being responsible for coroutine code, thou= gh... > >> > >> (And maybe it=E2=80=99d be better to make it qemu_coroutine_yield_for(= target), > >> which does the above and then yields?) > >=20 > > Or just do something like this, which is arguably not only a fix for the > > busy wait, but also a code simplification: >=20 > 1. Is that guaranteed to work? What if data sneaks in, the > connection_co handles that, and doesn=E2=80=99t wake up the teardown_co? = Will > it be re-scheduled? Then connection_co is buggy because we clearly requested that it terminate. It is possible that it does so only after handling another request, but this wouldn't be a problem. teardown_co would then just sleep for a few cycles more until connection_co is done and reaches the aio_co_wake() call. > 2. I precisely didn=E2=80=99t want to do this because we have this functi= onality > already in the form of Coroutine.co_queue_wakeup. Why duplicate it here? co_queue_wakeup contains coroutines to be run at the next yield point (or termination), which may be when connection_co is actually done, but it might also be earlier. My explicit aio_co_wake() at the end of connection_co is guaranteed to run only when connection_co is done. Kevin > > diff --git a/block/nbd.c b/block/nbd.c > > index b83b6cd43e..c061bd1bfc 100644 > > --- a/block/nbd.c > > +++ b/block/nbd.c > > @@ -61,6 +61,7 @@ typedef struct BDRVNBDState { > > CoMutex send_mutex; > > CoQueue free_sema; > > Coroutine *connection_co; > > + Coroutine *teardown_co; > > int in_flight; > >=20 > > NBDClientRequest requests[MAX_NBD_REQUESTS]; > > @@ -137,12 +138,9 @@ static void nbd_teardown_connection(BlockDriverSta= te *bs) > > NULL); > >=20 > > if (qemu_in_coroutine()) { > > - /* Let our caller poll and just yield until connection_co is d= one */ > > - while (s->connection_co) { > > - aio_co_schedule(qemu_get_current_aio_context(), > > - qemu_coroutine_self()); > > - qemu_coroutine_yield(); > > - } > > + /* just yield until connection_co is done */ > > + s->teardown_co =3D qemu_coroutine_self(); > > + qemu_coroutine_yield(); > > } else { > > BDRV_POLL_WHILE(bs, s->connection_co); > > } > > @@ -217,6 +215,9 @@ static coroutine_fn void nbd_connection_entry(void = *opaque) > > bdrv_dec_in_flight(s->bs); > >=20 > > s->connection_co =3D NULL; > > + if (s->teardown_co) { > > + aio_co_wake(s->teardown_co); > > + } > > aio_wait_kick(); > > } > >=20 >=20 >=20 --KDt/GgjP6HVcx58l Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIcBAEBAgAGBQJdKG2mAAoJEH8JsnLIjy/WsGYP/j1wPsqLxGgdVsuAjWvmloF8 39ydqT+CS/WVE3iNkBx3EpGRjMVeXA5/lZKXgR2bVKJY5GM/fy6L5/k9bPFa4gvL x1hamE2GNN+TyD1ILJvHN769HG9j8C1WLNPLarh3erjY9Bu5qkGPEBmy+wI5DdmW 8I+ccB4+DEJqwwdQB3Tl3WTOOKHP/ppTvkwzyoKmfDBfPD7HGYaUj/SDgQgCc/f1 MrVlqwey00xjToSZ8q3J5iUw+U9Z9nZdWEPm/7ZkE8ufzo3yciMHvunicuOWyINw Z7H1kngsNGAwESZDznMl0oPt3w+TFOaUHFy7dglHFNI8Ikbyjn5uVM637tdrSKP9 bbX5CRbHgIBj/e59/AXNFeFQXSaOsTObLNdC6rhU8VtCthm3pOYRtIAij8vD3x2O ZFEuu+7kJt4MjLFbscITl4vrn8fLDOulCeRpC3tyibDF5Ic1Q8v9nXpteLVCJSBN smKT3X+vP6N6TADNl1OG0UObr83bgn6J1pJ37/0PXMc4Y7MR3GpO4tDPINtDfqKW YxqPr6WiNTGoUMyJNf0pRpA/DT1GH0zAVQjujHuf6mRckaZp64fENTf0PRSM6nN3 nCnaZq10PF9UfD0mWnxj45fNik/z30ShNJdxGlX4zljuGwH3pUWyGYhDqE9rflUA gjaCvaC2LfLhUTB3JIt1 =npG/ -----END PGP SIGNATURE----- --KDt/GgjP6HVcx58l--