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Tue, 23 Jul 2019 10:21:10 +0000 (UTC) Received: from dresden.str.redhat.com (unknown [10.40.205.118]) by smtp.corp.redhat.com (Postfix) with ESMTPS id A17D65C230; Tue, 23 Jul 2019 10:21:09 +0000 (UTC) To: Kevin Wolf References: <20190722133054.21781-1-mreitz@redhat.com> <20190722133054.21781-3-mreitz@redhat.com> <20190723085236.GA5296@localhost.localdomain> <91179292-29aa-4d00-78ac-a8861fd5f308@redhat.com> <20190723100243.GA11008@localhost.localdomain> From: Max Reitz Openpgp: preference=signencrypt Autocrypt: addr=mreitz@redhat.com; prefer-encrypt=mutual; keydata= mQENBFXOJlcBCADEyyhOTsoa/2ujoTRAJj4MKA21dkxxELVj3cuILpLTmtachWj7QW+TVG8U /PsMCFbpwsQR7oEy8eHHZwuGQsNpEtNC2G/L8Yka0BIBzv7dEgrPzIu+W3anZXQW4702+uES U29G8TP/NGfXRRHGlbBIH9KNUnOSUD2vRtpOLXkWsV5CN6vQFYgQfFvmp5ZpPeUe6xNplu8V mcTw8OSEDW/ZnxJc8TekCKZSpdzYoxfzjm7xGmZqB18VFwgJZlIibt1HE0EB4w5GsD7x5ekh awIe3RwoZgZDLQMdOitJ1tUc8aqaxvgA4tz6J6st8D8pS//m1gAoYJWGwwIVj1DjTYLtABEB AAG0HU1heCBSZWl0eiA8bXJlaXR6QHJlZGhhdC5jb20+iQFTBBMBCAA9AhsDBQkSzAMABQsJ CAcCBhUICQoLAgQWAgMBAh4BAheABQJVzie5FRhoa3A6Ly9rZXlzLmdudXBnLm5ldAAKCRD0 B9sAYdXPQDcIB/9uNkbYEex1rHKz3mr12uxYMwLOOFY9fstP5aoVJQ1nWQVB6m2cfKGdcRe1 2/nFaHSNAzT0NnKz2MjhZVmcrpyd2Gp2QyISCfb1FbT82GMtXFj1wiHmPb3CixYmWGQUUh+I AvUqsevLA+WihgBUyaJq/vuDVM1/K9Un+w+Tz5vpeMidlIsTYhcsMhn0L9wlCjoucljvbDy/ 8C9L2DUdgi3XTa0ORKeflUhdL4gucWoAMrKX2nmPjBMKLgU7WLBc8AtV+84b9OWFML6NEyo4 4cP7cM/07VlJK53pqNg5cHtnWwjHcbpGkQvx6RUx6F1My3y52vM24rNUA3+ligVEgPYBuQEN BFXOJlcBCADAmcVUNTWT6yLWQHvxZ0o47KCP8OcLqD+67T0RCe6d0LP8GsWtrJdeDIQk+T+F xO7DolQPS6iQ6Ak2/lJaPX8L0BkEAiMuLCKFU6Bn3lFOkrQeKp3u05wCSV1iKnhg0UPji9V2 W5eNfy8F4ZQHpeGUGy+liGXlxqkeRVhLyevUqfU0WgNqAJpfhHSGpBgihUupmyUg7lfUPeRM DzAN1pIqoFuxnN+BRHdAecpsLcbR8sQddXmDg9BpSKozO/JyBmaS1RlquI8HERQoe6EynJhd 64aICHDfj61rp+/0jTIcevxIIAzW70IadoS/y3DVIkuhncgDBvGbF3aBtjrJVP+5ABEBAAGJ ASUEGAEIAA8FAlXOJlcCGwwFCRLMAwAACgkQ9AfbAGHVz0CbFwf9F/PXxQR9i4N0iipISYjU sxVdjJOM2TMut+ZZcQ6NSMvhZ0ogQxJ+iEQ5OjnIputKvPVd5U7WRh+4lF1lB/NQGrGZQ1ic alkj6ocscQyFwfib+xIe9w8TG1CVGkII7+TbS5pXHRxZH1niaRpoi/hYtgzkuOPp35jJyqT/ /ELbqQTDAWcqtJhzxKLE/ugcOMK520dJDeb6x2xVES+S5LXby0D4juZlvUj+1fwZu+7Io5+B bkhSVPb/QdOVTpnz7zWNyNw+OONo1aBUKkhq2UIByYXgORPFnbfMY7QWHcjpBVw9MgC4tGeF R4bv+1nAMMxKmb5VvQCExr0eFhJUAHAhVg== Message-ID: <083850e7-81ab-a9df-9fbd-1bb49adf1712@redhat.com> Date: Tue, 23 Jul 2019 12:21:07 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20190723100243.GA11008@localhost.localdomain> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="UxbUjkn1G74qajStHXwRZ7EoFUlHPNs4Z" X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.48]); Tue, 23 Jul 2019 10:21:10 +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] [PATCH for-4.1 2/2] block: Only the main loop can change AioContexts 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-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --UxbUjkn1G74qajStHXwRZ7EoFUlHPNs4Z Content-Type: multipart/mixed; boundary="6kjBipt3HJh7fvPrKz6hqq9JUU7RPrALJ"; protected-headers="v1" From: Max Reitz To: Kevin Wolf Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org Message-ID: <083850e7-81ab-a9df-9fbd-1bb49adf1712@redhat.com> Subject: Re: [PATCH for-4.1 2/2] block: Only the main loop can change AioContexts References: <20190722133054.21781-1-mreitz@redhat.com> <20190722133054.21781-3-mreitz@redhat.com> <20190723085236.GA5296@localhost.localdomain> <91179292-29aa-4d00-78ac-a8861fd5f308@redhat.com> <20190723100243.GA11008@localhost.localdomain> In-Reply-To: <20190723100243.GA11008@localhost.localdomain> --6kjBipt3HJh7fvPrKz6hqq9JUU7RPrALJ Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 23.07.19 12:02, Kevin Wolf wrote: > Am 23.07.2019 um 11:41 hat Max Reitz geschrieben: >> On 23.07.19 10:52, Kevin Wolf wrote: >>> Am 22.07.2019 um 15:30 hat Max Reitz geschrieben: >>>> bdrv_set_aio_context_ignore() can only work in the main loop: >>>> bdrv_drained_begin() only works in the main loop and the node's (old= ) >>>> AioContext; and bdrv_drained_end() really only works in the main loo= p >>>> and the node's (new) AioContext (contrary to its current comment, wh= ich >>>> is just wrong). >>>> >>>> Consequentially, bdrv_set_aio_context_ignore() must be called from t= he >>>> main loop. Luckily, assuming that we can make block graph changes o= nly >>>> from the main loop as well, all its callers do that already. >>>> >>>> Note that changing a node's context in a sense is an operation that >>>> changes the block graph, so it actually makes sense to require this >>>> function to be called from the main loop. >>>> >>>> Also, fix bdrv_drained_end()'s description. You can only use it fro= m >>>> the main loop or the node's AioContext, and in the latter case, the >>>> whole subtree must be in the same context. >>>> >>>> Fixes: e037c09c78520cbdb6da7cfc6ad0256d5870b814 >>>> Signed-off-by: Max Reitz >>>> --- >>>> include/block/block.h | 8 +++----- >>>> block.c | 13 ++++++++----- >>>> 2 files changed, 11 insertions(+), 10 deletions(-) >>>> >>>> diff --git a/include/block/block.h b/include/block/block.h >>>> index 60f00479e0..50a07c1c33 100644 >>>> --- a/include/block/block.h >>>> +++ b/include/block/block.h >>>> @@ -667,11 +667,9 @@ void bdrv_subtree_drained_begin(BlockDriverStat= e *bs); >>>> * >>>> * This polls @bs's AioContext until all scheduled sub-drained_ends= >>>> * have settled. On one hand, that may result in graph changes. O= n >>>> - * the other, this requires that all involved nodes (@bs and all of= >>>> - * its parents) are in the same AioContext, and that the caller has= >>>> - * acquired it. >>>> - * If there are any nodes that are in different contexts from @bs, >>>> - * these contexts must not be acquired. >>>> + * the other, this requires that the caller either runs in the main= >>>> + * loop; or that all involved nodes (@bs and all of its parents) ar= e >>>> + * in the caller's AioContext. >>>> */ >>>> void bdrv_drained_end(BlockDriverState *bs); >>> >>> I think you are right about the requirement that bdrv_drained_end() i= s >>> only called from the main or the BDS AioContext, which is a requireme= nt >>> that directly comes from AIO_WAIT_WHILE(). >>> >>> However, I don't see why we have requirements on the AioContext of th= e >>> parent nodes (or any other nodes), except possibly not holding their >>> lock. We don't poll their context, so it shouldn't matter in which >>> context they are? >> >> Hm. I don=E2=80=99t know how I got confused there, you=E2=80=99re rig= ht. >> >> I don=E2=80=99t think the (falsely given) restriction hurts, though, b= ecause a >> subtree should be within a single context anyway (unless you=E2=80=99r= e in >> bdrv_set_aio_context_ignore(), which needs to be in the main context).= >> >> So, hm, yes, I messed up this comment a bit now. But now it=E2=80=99s= just more >> restrictive than it needs to be and I don=E2=80=99t think callers are = going to >> care, so... >=20 > Nothing that should hold up your pull request, but I'd prefer to fix th= e > comment in a follow-up. On second thought, does aio_wait_kick() wake up any context but the main context? I was under the impression that it doesn=E2=80=99t. If it does= n=E2=80=99t, I don=E2=80=99t know how bdrv_drained_end()=E2=80=99s AIO_WAIT_WHILE() will= be woken up if it doesn=E2=80=99t run in the main context and it has to wait for yet ano= ther thread. Max > One thing where I could imagine it becoming relevant in the future is > cross-context block jobs. At the moment, we automatically pull the > target node into the AioContext of the source and fail if this isn't > possible, but that's really overly restrictive. >=20 > Kevin --6kjBipt3HJh7fvPrKz6hqq9JUU7RPrALJ-- --UxbUjkn1G74qajStHXwRZ7EoFUlHPNs4Z Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl0235MACgkQ9AfbAGHV z0CEEQgAj4+TCnmc/YPvlkTXoIaqDkqHW0OQS/qOGIg5xJ0vYfzA+TlvVSvoXZe1 NvwVT+mT/yayAI8ZArMLqSDLKBTjvg3wtHYpqI+vu9WXIMtZATEsvDBjqLOmsTjL 9FbbJPk8psbf7S6khtDI93yy3nFpjiVJTc3VOnHs5r0Ax/OwE1ybtDclU/dtDEZ6 KECltbEeuSTOcWuA1WuFExTnK4prSYhWeQsNXzbKkYX0belS2dei6Fept+xUBrU6 NBeOvANDb8yvZEDgwwgTTL2y3VjoZCgjtzzDjv1QZbyx1Mr6BS0wxeZz74CJfsky iA7/UbuFXvOOtrZg8/mpRGJFHHEtJg== =03WF -----END PGP SIGNATURE----- --UxbUjkn1G74qajStHXwRZ7EoFUlHPNs4Z--