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Wed, 03 Feb 2021 12:02:36 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 113EDAD78; Wed, 3 Feb 2021 12:02:35 +0000 (UTC) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: 9ad6f6a4-78ec-4a2f-85f4-a115add573fb X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1612353755; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=2kTGFphc2LZJ2ATGCnVNW5wh5u84pzQBs/EFF3UgR/k=; b=m5xiyPN/zmrdlZOrczt+uqGnGR6zP43X4ZuOTXyaLakz1+NGQUrcunbdcm+j5FiR32O8gK PTTHDzAyYBKw5psqHVqS6Gv1Vcr1myMwkj6D29TwiJDFpTky2XjYu0VdI8oiT/D1iMRDBF dSCKv+bK6GOG6pGfqBKREC5f62QF11U= To: Julien Grall , Dario Faggioli , =?UTF-8?Q?Anders_T=c3=b6rnqvist?= , xen-devel@lists.xenproject.org, Stefano Stabellini Cc: Jan Beulich , Andrew Cooper References: <207305e4e2998614767fdcc5ad83ced6de982820.camel@suse.com> <744ddde6-a228-82fc-76b9-401926d7963b@xen.org> <18ef4619-19ae-90d2-459c-9b5282b49176@codiax.se> <4760cbac-b006-78bc-b064-3265384f6707@xen.org> <311bb201bcacfd356f0c0b67856754eceae39e37.camel@suse.com> <7f2ec84a-9814-ffb1-0940-e936a80afbbb@xen.org> <501664dbdb736eaa4d9c05255dedfd7ad3e694fa.camel@suse.com> <1ab7ad80-c027-ffdd-8188-e1ab1fd53335@xen.org> <5ad8befd-75a1-4995-e0bb-e1a438f7556d@suse.com> From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= Subject: Re: Null scheduler and vwfi native problem Message-ID: <9a144000-e429-b9be-6562-ef8ba11b54ba@suse.com> Date: Wed, 3 Feb 2021 13:02:33 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="7wxiMZchvXXR5GW2xwgbBGrPq9ZxYCspR" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --7wxiMZchvXXR5GW2xwgbBGrPq9ZxYCspR Content-Type: multipart/mixed; boundary="3nxcv8NOxFK7IFjUud9Ja1aV6lqovGwjW"; protected-headers="v1" From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= To: Julien Grall , Dario Faggioli , =?UTF-8?Q?Anders_T=c3=b6rnqvist?= , xen-devel@lists.xenproject.org, Stefano Stabellini Cc: Jan Beulich , Andrew Cooper Message-ID: <9a144000-e429-b9be-6562-ef8ba11b54ba@suse.com> Subject: Re: Null scheduler and vwfi native problem References: <207305e4e2998614767fdcc5ad83ced6de982820.camel@suse.com> <744ddde6-a228-82fc-76b9-401926d7963b@xen.org> <18ef4619-19ae-90d2-459c-9b5282b49176@codiax.se> <4760cbac-b006-78bc-b064-3265384f6707@xen.org> <311bb201bcacfd356f0c0b67856754eceae39e37.camel@suse.com> <7f2ec84a-9814-ffb1-0940-e936a80afbbb@xen.org> <501664dbdb736eaa4d9c05255dedfd7ad3e694fa.camel@suse.com> <1ab7ad80-c027-ffdd-8188-e1ab1fd53335@xen.org> <5ad8befd-75a1-4995-e0bb-e1a438f7556d@suse.com> In-Reply-To: --3nxcv8NOxFK7IFjUud9Ja1aV6lqovGwjW Content-Type: multipart/mixed; boundary="------------35435530B3D7394E53ECF621" Content-Language: en-US This is a multi-part message in MIME format. --------------35435530B3D7394E53ECF621 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable On 03.02.21 12:20, Julien Grall wrote: > Hi Juergen, >=20 > On 03/02/2021 11:00, J=C3=BCrgen Gro=C3=9F wrote: >> On 03.02.21 10:19, Julien Grall wrote: >>> Hi, >>> >>> On 03/02/2021 07:31, Dario Faggioli wrote: >>>> On Tue, 2021-02-02 at 15:23 +0000, Julien Grall wrote: >>>>> In reality, it is probably still too early as a pCPU can be >>>>> considered >>>>> quiesced until a call to rcu_lock*() (such rcu_lock_domain()). >>>>> >>>> Well, yes, in theory, we could track down which is the first RCU rea= d >>>> side crit. section on this path, and put the call right before that = (if >>>> I understood what you mean). >>> >>> Oh, that's not what I meant. This will indeed be far more complex=20 >>> than I originally had in mind. >>> >>> AFAIU, the RCU uses critical section to protect data. So the=20 >>> "entering" could be used as "the pCPU is not quiesced" and "exiting" = >>> could be used as "the pCPU is quiesced". >>> >>> The concern with my approach is we would need to make sure that Xen=20 >>> correctly uses the rcu helpers. I know Juergen worked on that=20 >>> recently, but I don't know whether this is fully complete. >> >> I think it is complete, but I can't be sure, of course. >> >> One bit missing (for catching some wrong uses of the helpers) is this >> patch: >> >> https://lists.xen.org/archives/html/xen-devel/2020-03/msg01759.html >> >> I don't remember why it hasn't been taken, but I think there was a >> specific reason for that. >=20 > Looking at v8 and the patch is suitably reviewed by Jan. So I am a bit = > puzzled to why this wasn't committed... I had to go to v6 and notice th= e=20 > following message: >=20 > "albeit to be honest I'm not fully convinced we need to go this far." >=20 > Was the implication that his reviewed-by was conditional to someone els= e=20 > answering to the e-mail? I have no record for that being the case. Patches 1-3 of that series were needed for getting rid of stop_machine_run() in rcu handling and to fix other problems. Patch 4 was adding some additional ASSERT()s for making sure no potential deadlocks due to wrong rcu usage could creep in again. Patch 5 was more a "nice to have" addition in order to avoid any wrong usage of rcu which should have no real negative impact on the system stability. So I believe Jan as the committer didn't want to commit it himself, but was fine with the overall idea and implementation. I still think for code sanity it would be nice, but I was rather busy with Xenstore and event channel security work at that time, so I didn't urge anyone to take this patch. 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