From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.efficios.com (mail.efficios.com [167.114.142.141]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3y7HYP1ZGtzDqs5 for ; Fri, 6 Oct 2017 03:00:41 +1100 (AEDT) Date: Thu, 5 Oct 2017 16:02:06 +0000 (UTC) From: Mathieu Desnoyers To: Peter Zijlstra Cc: "Paul E. McKenney" , linux-kernel , Ingo Molnar , Lai Jiangshan , dipankar , Andrew Morton , Josh Triplett , Thomas Gleixner , rostedt , David Howells , Eric Dumazet , fweisbec , Oleg Nesterov , Boqun Feng , Andrew Hunter , maged michael , gromer , Avi Kivity , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Dave Watson , Alan Stern , Will Deacon , Andy Lutomirski , Ingo Molnar , Alexander Viro , Nicholas Piggin , linuxppc-dev , linux-arch Message-ID: <312162.31738.1507219326334.JavaMail.zimbra@efficios.com> In-Reply-To: <20171005121250.prr5ff5kf3lxq6hx@hirez.programming.kicks-ass.net> References: <20171004213734.GA11463@linux.vnet.ibm.com> <1507153075-12345-1-git-send-email-paulmck@linux.vnet.ibm.com> <20171005121250.prr5ff5kf3lxq6hx@hirez.programming.kicks-ass.net> Subject: Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , ----- On Oct 5, 2017, at 8:12 AM, Peter Zijlstra peterz@infradead.org wrote: > On Wed, Oct 04, 2017 at 02:37:53PM -0700, Paul E. McKenney wrote: >> diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c >> new file mode 100644 >> index 000000000000..b0d79a5f5981 >> --- /dev/null >> +++ b/arch/powerpc/kernel/membarrier.c >> @@ -0,0 +1,45 @@ > >> +void membarrier_arch_register_private_expedited(struct task_struct *p) >> +{ >> + struct task_struct *t; >> + >> + if (get_nr_threads(p) == 1) { >> + set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED); >> + return; >> + } >> + /* >> + * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread >> + * fork is protected by siglock. >> + */ >> + spin_lock(&p->sighand->siglock); >> + for_each_thread(p, t) >> + set_ti_thread_flag(task_thread_info(t), >> + TIF_MEMBARRIER_PRIVATE_EXPEDITED); > > I'm not sure this works correctly vs CLONE_VM without CLONE_THREAD. The intent here is to hold the sighand siglock to provide mutual exclusion against invocation of membarrier_fork(p, clone_flags) by copy_process(). copy_process() grabs spin_lock(¤t->sighand->siglock) for both CLONE_THREAD and not CLONE_THREAD flags. What am I missing here ? > >> + spin_unlock(&p->sighand->siglock); >> + /* >> + * Ensure all future scheduler executions will observe the new >> + * thread flag state for this process. >> + */ >> + synchronize_sched(); > > This relies on the flag being read inside rq->lock, right? Yes. The flag is read by membarrier_arch_switch_mm(), invoked within switch_mm_irqs_off(), called by context_switch() before finish_task_switch() releases the rq lock. Is the comment clear enough, or do you have suggestions for improvements ? Thanks, Mathieu > > > +} -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com