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Fri, 26 May 2023 09:04:20 +0000 (UTC) Received: from redhat.com (unknown [10.39.193.231]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 6EFA4140E95D; Fri, 26 May 2023 09:04:19 +0000 (UTC) Date: Fri, 26 May 2023 11:04:18 +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="y/t9MsWFPAhfVgfa" Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.7 Received-SPF: pass client-ip=170.10.129.124; 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 --y/t9MsWFPAhfVgfa Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Am 26.05.2023 um 10:40 hat Kevin Wolf geschrieben: > 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 mov= es > > > 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 inst= ead > > > 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 = *opaque) > > > qoc->ret =3D qcow2_do_open(qoc->bs, qoc->options, qoc->flags, tr= ue, > > > qoc->errp); > > > qemu_co_mutex_unlock(&s->lock); > > > + > > > + aio_wait_kick(); > > > } > > > =20 > > > static int qcow2_open(BlockDriverState *bs, QDict *options, int flag= s, > > > @@ -1929,8 +1931,10 @@ static int qcow2_open(BlockDriverState *bs, QD= ict *options, int flags, > > > =20 > > > assert(!qemu_in_coroutine()); > > > assert(qemu_get_current_aio_context() =3D=3D qemu_get_aio_contex= t()); > > > - qemu_coroutine_enter(qemu_coroutine_create(qcow2_open_entry, &qo= c)); > > > - 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? >=20 > Ok, I'll squash in the following. >=20 > 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. :-) >=20 > And the earlier we can get rid of the AioContext lock the better, > because it seems really hard to get right across the board. >=20 > Kevin >=20 > 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= must s/functions/function/, in fact. :-) > + * 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, >=20 > /* > * Wrapper on bdrv_open_child() for most popular case: open primary chil= d of bs. > + * > + * @parent can move to a different AioContext in this functions. Callers= must And here, too. > + * 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, Kevin --y/t9MsWFPAhfVgfa Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEE3D3rFZqa+V09dFb+fwmycsiPL9YFAmRwdhIACgkQfwmycsiP L9ZKgw//dAohnRSyzUKiyiHhd+zigIRb23mbmzUtPn9oC+dLE/VfNoPDk/Vylswr uEHvNoGMT6O9+L1osHGm/8vnaxUjROfX2r2jT73ucQm6IWsw7m4+QyPjgDrSRb1S qmK+fL/S1PGaiD4lJZlZwwbQbKk5K9XTQNYzN0BdV3C2k16hzfsSCk+exruwJf5j jEe8ftcRrZoMIqwPy1uzlL9z3I9nbWXsJ/2ymZDXJ+Te2FTRM8hBF6Lpfo3/c0dY 37+dg5bHTyLYRqKOb02MluoQWIW9A1/iSQYqdusRhfJO7a0vGBjQMmV2VNf/ZSlg SUHypn/sMRnHRWbm3+StTO+hT2Qbf3CVhAULdX2+aO9S8+oQ+2vEciGZaKehenMX FaddckARvK05TGFUpJK7ekKfVskji4AlIrOtN5LRaB1Nhfl4zajZbQ3FGuvBH4lV aSZYW2mc7gFRvYJfIeTvH1CruIASW/FZFcizvWkup2tyTobTge9PavIheAErVKH0 uMJO6YNhr+mkzHkeCjTh+H1zkSbM6A+MW8bdFV/cNBgPQoCvE9HxvvjZndTzs3hP nnQLRd5tQZoZ1Z5KYfQWQ4Q2EosffulrvAYhAViLAJMLYRro1GJRY+viOcL3t0IZ x7gq6CmvxIqUE5GxNgG80lwSb5wso23HjzI6F6kkvWQWgUJiBi8= =l1FF -----END PGP SIGNATURE----- --y/t9MsWFPAhfVgfa--