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Fri, 27 Nov 2020 13:31:40 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 4a92843e-e17a-47c7-a52a-b7cb9a48a700; Fri, 27 Nov 2020 13:31:38 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 00AE4AC90; Fri, 27 Nov 2020 13:31:37 +0000 (UTC) Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kidqd-0000hD-Vn for xen-devel@lists.xenproject.org; Fri, 27 Nov 2020 13:31:40 +0000 X-Inumbo-ID: 4a92843e-e17a-47c7-a52a-b7cb9a48a700 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 4a92843e-e17a-47c7-a52a-b7cb9a48a700; Fri, 27 Nov 2020 13:31:38 +0000 (UTC) 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=1606483898; 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=LN3Ob1rDFoFX3TlASGbBlsqWYmGkMsYCozvzQ697bjU=; b=GFRhAQvKJt9IwsE9cBena4hdJa24dGO+iNcFFIBNiv3NpS5zqAOxToqua6EnxfCrFO0Z/M mrx0c4FenpV8vJ6TWI2Gfi0M9hnYe8gLZj+kfdhJ4mvhKO4su4zpqkjYpTNu/lwIjpdNoR hcOcBN0FYbKXhGwDAFze2eRvceRJX40= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 00AE4AC90; Fri, 27 Nov 2020 13:31:37 +0000 (UTC) To: Jan Beulich Cc: Andrew Cooper , George Dunlap , Ian Jackson , Julien Grall , Stefano Stabellini , Wei Liu , xen-devel@lists.xenproject.org References: <20201125105122.3650-1-jgross@suse.com> <20201125105122.3650-3-jgross@suse.com> <68bbc88a-075e-327b-47e1-1dca1f9a3772@suse.com> From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= Subject: Re: [PATCH v8 2/3] xen/events: rework fifo queue locking Message-ID: <65eaa8b6-601f-e522-9ad9-2d88e905d447@suse.com> Date: Fri, 27 Nov 2020 14:31:37 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.4.0 MIME-Version: 1.0 In-Reply-To: <68bbc88a-075e-327b-47e1-1dca1f9a3772@suse.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="2rHiV5LjUhONZVLnswYGHrQTfEeZp1OpH" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --2rHiV5LjUhONZVLnswYGHrQTfEeZp1OpH Content-Type: multipart/mixed; boundary="SWmKRM9IfMPz2noWTThsUTRu6sebOSi82"; protected-headers="v1" From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= To: Jan Beulich Cc: Andrew Cooper , George Dunlap , Ian Jackson , Julien Grall , Stefano Stabellini , Wei Liu , xen-devel@lists.xenproject.org Message-ID: <65eaa8b6-601f-e522-9ad9-2d88e905d447@suse.com> Subject: Re: [PATCH v8 2/3] xen/events: rework fifo queue locking References: <20201125105122.3650-1-jgross@suse.com> <20201125105122.3650-3-jgross@suse.com> <68bbc88a-075e-327b-47e1-1dca1f9a3772@suse.com> In-Reply-To: <68bbc88a-075e-327b-47e1-1dca1f9a3772@suse.com> --SWmKRM9IfMPz2noWTThsUTRu6sebOSi82 Content-Type: multipart/mixed; boundary="------------CFC6A8A3632F14E22D795896" Content-Language: en-US This is a multi-part message in MIME format. --------------CFC6A8A3632F14E22D795896 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable On 27.11.20 14:11, Jan Beulich wrote: > On 25.11.2020 11:51, Juergen Gross wrote: >> Two cpus entering evtchn_fifo_set_pending() for the same event channel= >> can race in case the first one gets interrupted after setting >> EVTCHN_FIFO_PENDING and when the other one manages to set >> EVTCHN_FIFO_LINKED before the first one is testing that bit. This can >> lead to evtchn_check_pollers() being called before the event is put >> properly into the queue, resulting eventually in the guest not seeing >> the event pending and thus blocking forever afterwards. >> >> Note that commit 5f2df45ead7c1195 ("xen/evtchn: rework per event chann= el >> lock") made the race just more obvious, while the fifo event channel >> implementation had this race from the beginning when an unmask operati= on >> was running in parallel with an event channel send operation. >=20 > I notice you've altered the Fixes: tag, but you still say "from > the beginning" here. Oh, indeed. Thanks for spotting. >=20 >> Using a spinlock for the per event channel lock is problematic due to >> some paths needing to take the lock are called with interrupts off, so= >> the lock would need to disable interrupts, which in turn breaks some >> use cases related to vm events. >=20 > This reads as if it got put here by mistake. May I suggest to start > with "Using ... had turned out problematic ..." and then add > something like "Therefore that lock was switched to an rw one"? Fine with me. >=20 >> For avoiding this race the queue locking in evtchn_fifo_set_pending() >> needs to be reworked to cover the test of EVTCHN_FIFO_PENDING, >> EVTCHN_FIFO_MASKED and EVTCHN_FIFO_LINKED, too. Additionally when an >> event channel needs to change queues both queues need to be locked >> initially. >=20 > "... in order to avoid having a window with no lock held at all"? Yes, this is better. >=20 >> @@ -204,17 +175,67 @@ static void evtchn_fifo_set_pending(struct vcpu = *v, struct evtchn *evtchn) >> return; >> } >> =20 >> + /* >> + * Lock all queues related to the event channel (in case of a que= ue change >> + * this might be two). >> + * It is mandatory to do that before setting and testing the PEND= ING bit >> + * and to hold the current queue lock until the event has put int= o the >=20 > "has been" or "was" I think? "has been". >=20 > With adjustments along these lines (which I guess could again be > done while committing) or reasons against supplied > Reviewed-by: Jan Beulich Thanks. >=20 > One aspect which I wonder whether it wants adding to the description > is that this change makes a bad situation worse: Back at the time > per-channel locks were added to avoid the bottleneck of the per- > domain event lock. While a per-queue lock's scope at least isn't the > entire domain, their use on the send path still has been preventing > full parallelism here. And this patch widens the lock holding region. As the description already says that additional operations are to be guarded by the lock I think it is rather clear that the lock holding region is being widened. OTOH I wouldn't reject such an addition to the commit message if you think it is required. 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