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Fri, 12 Jul 2019 11:01:14 +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 551E660BFB; Fri, 12 Jul 2019 11:01:11 +0000 (UTC) Date: Fri, 12 Jul 2019 13:01:10 +0200 From: Kevin Wolf To: Max Reitz Message-ID: <20190712110110.GE4514@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> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="9jxsPFA5p3P2qPhR" Content-Disposition: inline In-Reply-To: <8ce2ce78-833e-c98f-ad3a-d44f6432ae4c@redhat.com> 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 [10.5.110.27]); Fri, 12 Jul 2019 11:01:14 +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" --9jxsPFA5p3P2qPhR Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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 ma= in > >> 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(BlockDriverSt= ate *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 is = done */ > >> + while (s->connection_co) { > >> + aio_co_schedule(qemu_get_current_aio_context(), > >> + qemu_coroutine_self()); > >> + qemu_coroutine_yield(); > >> + } > >=20 > > 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? >=20 > Yes, it is busy waiting, but I didn=E2=80=99t find that bad. The connect= ion_co > will be invoked in basically every iteration, and once there is no > pending data, it will quit. >=20 > The answer to =E2=80=9Cwhy not...=E2=80=9D of course is because it=E2=80= =99d be more complicated. >=20 > But anyway. >=20 > Adding a new function qemu_coroutine_run_after(target) that adds > qemu_coroutine_self() to the given @target coroutine=E2=80=99s wake-up qu= eue and > then using that instead of scheduling works, too, yes. >=20 > I don=E2=80=99t really like being responsible for coroutine code, though.= =2E. >=20 > (And maybe it=E2=80=99d be better to make it qemu_coroutine_yield_for(tar= get), > which does the above and then yields?) Or just do something like this, which is arguably not only a fix for the busy wait, but also a code simplification: 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; NBDClientRequest requests[MAX_NBD_REQUESTS]; @@ -137,12 +138,9 @@ static void nbd_teardown_connection(BlockDriverState *= bs) NULL); if (qemu_in_coroutine()) { - /* Let our caller poll and just yield until connection_co is done = */ - 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 *opa= que) bdrv_dec_in_flight(s->bs); s->connection_co =3D NULL; + if (s->teardown_co) { + aio_co_wake(s->teardown_co); + } aio_wait_kick(); } --9jxsPFA5p3P2qPhR Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIcBAEBAgAGBQJdKGh1AAoJEH8JsnLIjy/Wo4EQAJL94aC7j74DtzNHlH5yFIr4 0p5z/Cni/LIiZPsId7rw4XdkAAkc1bIHm0v3aydQkCK671EhWSKRK01mkw17rytk hYxPWaWeA3taPkHJ1yoMz4Vao0iTCs9TiM2d+PVppvHn2am2n3bwUkq+IbYiTbFf LNBeV2jaU4LabrAh55immVToLrrGwR0TigLoYYlv+msCLXVvtG0N1m+LWJW0NYSl we+ztPLenWjhuQdyG+L9xqykR9yeXYVAx4WIBtFQNOGK0SdnS5os+uZTnjmSQYJe Z5Bsg5sDcoshy4kqSpEhqR0CmdzDuweXxu2e2hsmuvJX4SLAjqT7spgVnKBvspgu RTJAgj1JkKYqOOrH+OivMbRjwzz9K592I/xQ+8vVekIi3bmXe/PiX7Yjw1Ez/iBZ vflHfelMTV26Ab5mbX167oQWcZ9oF18J7nd+6QfaU8B0snmuWDZ5oM+7rn6CDBX9 9v9A2WkDiB7p60bsqeI48DyvluO6CxOdJsbrp/bRZfJXYmFPf+xc7ck0pi+cbFW4 53pUlaWT/jcexpUuNrxy0JjhtaW3QB82OHtVkEl8W+VRHS1DeshLnPTzIdbjj53C 8Fp34bAveOY5Iquuvm3Fi06T2eZYnSB+ZZNkl2XnvM81/dT0ntrCr0Ozw+If+39m doxxoSX03Qyab6EnieiD =hkvt -----END PGP SIGNATURE----- --9jxsPFA5p3P2qPhR--