Date: Wed, 25 Jul 2012 22:23:51 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Jim Harris <jim.harris@gmail.com> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Andriy Gapon <avg@freebsd.org>, Bruce Evans <brde@optusnet.com.au> Subject: Re: svn commit: r238755 - head/sys/x86/x86 Message-ID: <20120725192351.GR2676@deviant.kiev.zoral.com.ua> In-Reply-To: <CAJP=Hc9Ys6uou2wfcO=r9D%2BAaC34mVeHOK1bgFWtBz8kfvWo6A@mail.gmail.com> References: <201207242210.q6OMACqV079603@svn.freebsd.org> <500F9E22.4080608@FreeBSD.org> <20120725102130.GH2676@deviant.kiev.zoral.com.ua> <20120725233033.N5406@besplex.bde.org> <20120725173212.GN2676@deviant.kiev.zoral.com.ua> <CAJP=Hc9Ys6uou2wfcO=r9D%2BAaC34mVeHOK1bgFWtBz8kfvWo6A@mail.gmail.com>
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--DPQvABvJ16HvCXZu Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Jul 25, 2012 at 11:00:41AM -0700, Jim Harris wrote: > On Wed, Jul 25, 2012 at 10:32 AM, Konstantin Belousov > <kostikbel@gmail.com> wrote: > > I also asked Jim to test whether the cause the TSC sync test failure > > is the lack of synchronization between gathering data and tasting it, > > but ut appeared that the reason is genuine timecounter value going > > backward. >=20 > I wonder if instead of timecounter going backward, that TSC test > fails because CPU speculatively performs rdtsc instruction in relation > to waiter checks in smp_rendezvous_action. Or maybe we are saying > the same thing. Ok, the definition of the 'timecounter goes back', as I understand it: you have two events A and B in two threads, provable ordered, say, A is a lock release and B is the same lock acquisition. Assume that you take rdtsc values tA and tB under the scope of the lock right before A and right after B. Then it should be impossible to have tA > tB. I do not think that we can ever observe tA > tB if both threads are executing on the same CPU. --DPQvABvJ16HvCXZu Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAlAQR8cACgkQC3+MBN1Mb4j+dQCgp7HrGcliuvwV1trGrXhrkND8 u7wAnRrJEgINxLU1l1fGSA5HB5F2LXpr =jvTV -----END PGP SIGNATURE----- --DPQvABvJ16HvCXZu--
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