From owner-freebsd-current@FreeBSD.ORG Fri May 13 15:51:11 2011 Return-Path: Delivered-To: freebsd-current@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 550FC1065673; Fri, 13 May 2011 15:51:11 +0000 (UTC) (envelope-from max@love2party.net) Received: from moutng.kundenserver.de (moutng.kundenserver.de [212.227.126.171]) by mx1.freebsd.org (Postfix) with ESMTP id 01D698FC16; Fri, 13 May 2011 15:51:10 +0000 (UTC) Received: from maxlap.localnet ([192.75.139.253]) by mrelayeu.kundenserver.de (node=mreu2) with ESMTP (Nemesis) id 0McAb5-1Q2Q9a0j6V-00JaQc; Fri, 13 May 2011 17:51:09 +0200 From: Max Laier To: freebsd-current@freebsd.org Date: Fri, 13 May 2011 11:50:57 -0400 User-Agent: KMail/1.13.6 (FreeBSD/9.0-CURRENT; KDE/4.6.1; amd64; ; ) References: <4DCD357D.6000109@FreeBSD.org> <201105131041.59981.max@love2party.net> <4DCD4E21.7020800@FreeBSD.org> In-Reply-To: <4DCD4E21.7020800@FreeBSD.org> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201105131150.57548.max@love2party.net> X-Provags-ID: V02:K0:uTzm/722e8pPk6/v0Lf3e+NUUyF+WAs5eO8J6wodgiJ Gl8nKW+/BeHFZVVY6ZdpeZhvac2039BV4SnkMjPardj0+RDoaF zDIZUdVe5OXnXV1u7XB1uAbFTtq8oqg7he4naVZPZEeUukazb2 AMGv3l3KAd/WxEJb77oDW1+32b9eihwr8ywYhiXU/4eGtzs7Pi lBJjY9zLU2eDlaJ2hQcuQ== Cc: Andriy Gapon Subject: Re: proposed smp_rendezvous change X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 13 May 2011 15:51:11 -0000 On Friday 13 May 2011 11:28:33 Andriy Gapon wrote: > on 13/05/2011 17:41 Max Laier said the following: > > this ncpus isn't the one you are looking for. > > Thank you! > > Here's an updated patch: Can you attach the patch, so I can apply it locally. This code is really hard to read without context. Some more comments inline ... > > Index: sys/kern/subr_smp.c > =================================================================== > --- sys/kern/subr_smp.c (revision 221835) > +++ sys/kern/subr_smp.c (working copy) > @@ -316,19 +316,14 @@ > void (*local_action_func)(void*) = smp_rv_action_func; > void (*local_teardown_func)(void*) = smp_rv_teardown_func; > > - /* Ensure we have up-to-date values. */ > - atomic_add_acq_int(&smp_rv_waiters[0], 1); > - while (smp_rv_waiters[0] < smp_rv_ncpus) > - cpu_spinwait(); > - You really need this for architectures that need the memory barrier to ensure consistency. We also need to move the reads of smp_rv_* below this point to provide a consistent view. > /* setup function */ > if (local_setup_func != smp_no_rendevous_barrier) { > if (smp_rv_setup_func != NULL) > smp_rv_setup_func(smp_rv_func_arg); > > /* spin on entry rendezvous */ > - atomic_add_int(&smp_rv_waiters[1], 1); > - while (smp_rv_waiters[1] < smp_rv_ncpus) > + atomic_add_int(&smp_rv_waiters[0], 1); > + while (smp_rv_waiters[0] < smp_rv_ncpus) > cpu_spinwait(); > } > > @@ -337,12 +332,16 @@ > local_action_func(local_func_arg); > > /* spin on exit rendezvous */ > - atomic_add_int(&smp_rv_waiters[2], 1); > - if (local_teardown_func == smp_no_rendevous_barrier) > + atomic_add_int(&smp_rv_waiters[1], 1); > + if (local_teardown_func == smp_no_rendevous_barrier) { > + atomic_add_int(&smp_rv_waiters[2], 1); > return; > - while (smp_rv_waiters[2] < smp_rv_ncpus) > + } > + while (smp_rv_waiters[1] < smp_rv_ncpus) > cpu_spinwait(); > > + atomic_add_int(&smp_rv_waiters[2], 1); > + > /* teardown function */ > if (local_teardown_func != NULL) > local_teardown_func(local_func_arg); > @@ -377,6 +376,10 @@ > /* obtain rendezvous lock */ > mtx_lock_spin(&smp_ipi_mtx); > > + /* Wait for any previous unwaited rendezvous to finish. */ > + while (atomic_load_acq_int(&smp_rv_waiters[2]) < smp_rv_ncpus) > + cpu_spinwait(); > + This does not help you at all. Imagine the following (unlikely, but not impossible) case: CPUA: start rendevouz including self, finish the action first (i.e. CPUA is the first one to see smp_rv_waiters[2] == smp_rv_ncpus, drop the lock and start a new rendevouz. smp_rv_waiters[2] == smp_rv_ncpus is still true on that CPU, but ... CPUB might have increased smp_rv_waiters[2] for the first rendevouz, but never saw smp_rv_waiters[2] == smp_rv_ncpus, still ... CPUA is allowed to start a new rendevouz which will leave CPUB stranded and can lead to a deadlock. I think this is also possible with another CPU starting the second rendevous. > /* set static function pointers */ > smp_rv_ncpus = ncpus; > smp_rv_setup_func = setup_func; > @@ -395,7 +398,7 @@ > smp_rendezvous_action(); > > if (teardown_func == smp_no_rendevous_barrier) > - while (atomic_load_acq_int(&smp_rv_waiters[2]) < ncpus) > + while (atomic_load_acq_int(&smp_rv_waiters[1]) < ncpus) > cpu_spinwait(); > > /* release lock */ > _______________________________________________ > freebsd-current@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-current > To unsubscribe, send any mail to "freebsd-current-unsubscribe@freebsd.org"