From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52648) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bm3tT-0007fk-Ia for qemu-devel@nongnu.org; Mon, 19 Sep 2016 15:06:20 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bm3tQ-0004Ta-9G for qemu-devel@nongnu.org; Mon, 19 Sep 2016 15:06:19 -0400 Received: from out2-smtp.messagingengine.com ([66.111.4.26]:58111) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bm3tN-0004Q2-SW for qemu-devel@nongnu.org; Mon, 19 Sep 2016 15:06:16 -0400 Date: Mon, 19 Sep 2016 15:06:04 -0400 From: "Emilio G. Cota" Message-ID: <20160919190604.GA11819@flamenco> References: <20160919155139.28371-1-alex.bennee@linaro.org> <20160919155139.28371-8-alex.bennee@linaro.org> <20160919180614.GA8713@flamenco> <704acb80-5680-c375-7437-25b3230af599@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <704acb80-5680-c375-7437-25b3230af599@redhat.com> Subject: Re: [Qemu-devel] [RFC 7/8] util/qht: atomically set b->hashes List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: Alex =?iso-8859-1?Q?Benn=E9e?= , qemu-devel@nongnu.org, stefanha@redhat.com, kwolf@redhat.com, mttcg@greensocs.com, fred.konrad@greensocs.com, a.rigo@virtualopensystems.com, bobby.prani@gmail.com, nikunj@linux.vnet.ibm.com, mark.burton@greensocs.com, jan.kiszka@siemens.com, serge.fdrv@gmail.com, rth@twiddle.net, peter.maydell@linaro.org, claudio.fontana@huawei.com On Mon, Sep 19, 2016 at 20:37:06 +0200, Paolo Bonzini wrote: > On 19/09/2016 20:06, Emilio G. Cota wrote: > > On Mon, Sep 19, 2016 at 16:51:38 +0100, Alex Bennée wrote: > >> > ThreadSanitizer detects a possible race between reading/writing the > >> > hashes. As ordering semantics are already documented for qht we just > >> > need to ensure a race can't tear the hash value so we can use the > >> > relaxed atomic_set/read functions. > > This was discussed here: > > > > https://lists.gnu.org/archive/html/qemu-devel/2016-05/msg03658.html > > > > To reiterate: reading torn hash values is fine, since the retry will > > happen regardless (and all pointers[] remain valid through the RCU > > read-critical section). > > True, but C11 says data races are undefined, not merely unspecified. > seqlock-protected data requires a relaxed read and write, because they > are read concurrently in the read and write sides. Ah I see. Let me then just point out that this comes at a small perf loss. Running 'taskset -c 0 tests/qht-bench -n 1 -d 10' (i.e. all lookups) 10 times, we get: before the patch: $ ./mean.pl 34.04 34.24 34.38 34.25 34.18 34.51 34.46 34.44 34.29 34.08 34.287 +- 0.160072900059109 after: $ ./mean.pl 33.94 34.00 33.52 33.46 33.55 33.71 34.27 34.06 34.28 34.58 33.937 +- 0.374731014640279 But hey we can live with that. Cheers, E.