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Fri, 26 May 2023 08:40:09 +0000 (UTC) Received: from redhat.com (unknown [10.39.193.231]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C782020296C6; Fri, 26 May 2023 08:40:08 +0000 (UTC) Date: Fri, 26 May 2023 10:40:07 +0200 From: Kevin Wolf To: Stefan Hajnoczi Cc: qemu-block@nongnu.org, hreitz@redhat.com, eblake@redhat.com, qemu-devel@nongnu.org Subject: Re: [PATCH 06/12] qcow2: Fix open with 'file' in iothread Message-ID: References: <20230525124713.401149-1-kwolf@redhat.com> <20230525124713.401149-7-kwolf@redhat.com> <20230525185125.GH132697@fedora> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="a3oIJt+SE2znpY7y" Content-Disposition: inline In-Reply-To: <20230525185125.GH132697@fedora> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.4 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com 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, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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: 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 --a3oIJt+SE2znpY7y Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Am 25.05.2023 um 20:51 hat Stefan Hajnoczi geschrieben: > On Thu, May 25, 2023 at 02:47:07PM +0200, Kevin Wolf wrote: > > qcow2_open() doesn't work correctly when opening the 'file' child moves > > bs to an iothread, for several reasons: > >=20 > > - It uses BDRV_POLL_WHILE() to wait for the qcow2_open_entry() > > coroutine, which involves dropping the AioContext lock for bs when it > > is not in the main context - but we don't hold it, so this crashes. > >=20 > > - It runs the qcow2_open_entry() coroutine in the current thread instead > > of the new AioContext of bs. > >=20 > > - qcow2_open_entry() doesn't notify the main loop when it's done. > >=20 > > This patches fixes these issues around delegating work to a coroutine. > > Temporarily dropping the main AioContext lock is not necessary because > > we know we run in the main thread. > >=20 > > Signed-off-by: Kevin Wolf > > --- > > block/qcow2.c | 8 ++++++-- > > 1 file changed, 6 insertions(+), 2 deletions(-) > >=20 > > diff --git a/block/qcow2.c b/block/qcow2.c > > index b00b4e7575..7f3948360d 100644 > > --- a/block/qcow2.c > > +++ b/block/qcow2.c > > @@ -1904,6 +1904,8 @@ static void coroutine_fn qcow2_open_entry(void *o= paque) > > qoc->ret =3D qcow2_do_open(qoc->bs, qoc->options, qoc->flags, true, > > qoc->errp); > > qemu_co_mutex_unlock(&s->lock); > > + > > + aio_wait_kick(); > > } > > =20 > > static int qcow2_open(BlockDriverState *bs, QDict *options, int flags, > > @@ -1929,8 +1931,10 @@ static int qcow2_open(BlockDriverState *bs, QDic= t *options, int flags, > > =20 > > assert(!qemu_in_coroutine()); > > assert(qemu_get_current_aio_context() =3D=3D qemu_get_aio_context(= )); > > - qemu_coroutine_enter(qemu_coroutine_create(qcow2_open_entry, &qoc)= ); > > - BDRV_POLL_WHILE(bs, qoc.ret =3D=3D -EINPROGRESS); > > + > > + aio_co_enter(bdrv_get_aio_context(bs), > > + qemu_coroutine_create(qcow2_open_entry, &qoc)); > > + AIO_WAIT_WHILE_UNLOCKED(NULL, qoc.ret =3D=3D -EINPROGRESS); >=20 > Want to update the doc comment for bdrv_open_file_child() with a warning > that @parent's AioContext can change? Ok, I'll squash in the following. I seem to remember that bdrv_open_child() is actually wrong, too, regarding AioContext locking, but I didn't need to fix it for this test case. We need more test cases to break everything. :-) And the earlier we can get rid of the AioContext lock the better, because it seems really hard to get right across the board. Kevin diff --git a/block.c b/block.c index a2f8d5a0c0..263e1e22f3 100644 --- a/block.c +++ b/block.c @@ -3644,6 +3644,9 @@ done: * BlockdevRef. * * The BlockdevRef will be removed from the options QDict. + * + * @parent can move to a different AioContext in this functions. Callers m= ust + * make sure that their AioContext locking is still correct after this. */ BdrvChild *bdrv_open_child(const char *filename, QDict *options, const char *bdref_key, @@ -3668,6 +3671,9 @@ BdrvChild *bdrv_open_child(const char *filename, /* * Wrapper on bdrv_open_child() for most popular case: open primary child = of bs. + * + * @parent can move to a different AioContext in this functions. Callers m= ust + * make sure that their AioContext locking is still correct after this. */ int bdrv_open_file_child(const char *filename, QDict *options, const char *bdref_key, --a3oIJt+SE2znpY7y Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEE3D3rFZqa+V09dFb+fwmycsiPL9YFAmRwcGcACgkQfwmycsiP L9aS/w//f1wVg+IFrsHxHlLuMCVkmAq5EHGJ8qzZDQlYWYuFcZ+tm4mNF4acTCds g2Qlz6ef+SkOdbOjR1WMfaXsMkQqBsCsd0G5f6yrH65/MxQwUACk6uP3RYqbdJEY vpUhuae/IkzFRit+TmG2UreyWmZzK9zurVsoZzllWcyKo1bph2ILGkEXDewe1BTC MiFmY05i7vL7Pp15dgJcve9FBcdaHdn1c8AqTNUuKi4+jRGcH5P/q4VStvrfa4mm HBkA2pkMe8RoijW05Up8X7bsbQ6enKnQ6hNBbNWTfD0TROGKabDVouCvx+9WeFqa wHWw3jJ3FfwKsweygno622Bd5iUfrNhq8JDemJHAt0MHOThMp4fm4FIFYVVitc91 wXYgkBhJ6vPi3M+DcPGyahAEPEJxMlwE/JkXGgKDiWNBpvKlxSmzpbYUUi+TtWBX XId0teYi5pcbSo6Rmi4dLPw6YVPzWroO2T5BdayJI65S8ziQmORnwgUKf4kXMkNQ 5dOoTpxCCQm1bgLfU16rpoyWkiy8TpyZSobZ7TZO90Lny3QmSQUsXlGAHIi3OSOy 9ToVrhBc5xxow05JCe4z02YWS0Oyr9ppUyYEvMItVSHcYeqbQbC+nEQEfXZOpvoY HBnGsVi/4gNLZKE4ABsAd5/tp6ylf2e7slUlLvejCv8R0g7eWzg= =4W+t -----END PGP SIGNATURE----- --a3oIJt+SE2znpY7y--