From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: linux-next: build failure after merge of the rcu tree Date: Fri, 11 Aug 2017 13:12:47 -0700 Message-ID: <20170811201247.GH3730@linux.vnet.ibm.com> References: <20170811144352.585085e2@canb.auug.org.au> <20170811045453.GB3730@linux.vnet.ibm.com> <20170811091434.h6mkuuw3zcgkzg26@hirez.programming.kicks-ass.net> <20170811144150.26gowhxte7ri5fpk@hirez.programming.kicks-ass.net> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20170811144150.26gowhxte7ri5fpk@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org To: Peter Zijlstra Cc: Stephen Rothwell , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Linux-Next Mailing List , Linux Kernel Mailing List List-Id: linux-next.vger.kernel.org On Fri, Aug 11, 2017 at 04:41:50PM +0200, Peter Zijlstra wrote: > On Fri, Aug 11, 2017 at 11:14:34AM +0200, Peter Zijlstra wrote: > > On Thu, Aug 10, 2017 at 09:54:53PM -0700, Paul E. McKenney wrote: > > > On Fri, Aug 11, 2017 at 02:43:52PM +1000, Stephen Rothwell wrote: > > > > > > Looks like I need to rebase my patch on top of a9668cd6ee28, and > > > than put an smp_mb__after_spinlock() between the lock and the unlock. > > > > > > Peter, any objections to that approach? Other suggestions? > > > > Hurm.. I'll have to try and understand that comment there again it > > seems. > > OK, so per commit b5740f4b2cb3 ("sched: Fix ancient race in do_exit()") > the race is with try_to_wake_up(): > > down_read() > p->state = TASK_UNINTERRUPTIBLE; > > try_to_wake_up(p) > spin_lock(p->pi_lock); > /* sees TASK_UNINTERRUPTIBLE */ > ttwu_remote() > /* check stuff, no need to schedule() */ > p->state = TASK_RUNNING > > > p->state = TASK_DEAD > > p->state = TASK_RUNNING /* whoops! */ > spin_unlock(p->pi_lock); > > __schedule(false); > BUG(); > > > > > So given that, I think that: > > spin_lock(¤t->pi_lock); > spin_unlock(¤t->pi_lock); > > current->state = TASK_DEAD; > > is sufficient. I don't see a need for an additional smp_mb here. > > Either the concurrent ttwu is finished and we must observe its RUNNING > store, or it will observe our RUNNING store. Makes sense to me! Please see below for the updated commit. Thanx, Paul ------------------------------------------------------------------------ commit 23a9b748a3d27f67cdb078fcb891a920285e75d9 Author: Paul E. McKenney Date: Thu Jun 29 12:08:26 2017 -0700 sched: Replace spin_unlock_wait() with lock/unlock pair There is no agreed-upon definition of spin_unlock_wait()'s semantics, and it appears that all callers could do just as well with a lock/unlock pair. This commit therefore replaces the spin_unlock_wait() call in do_task_dead() with spin_lock() followed immediately by spin_unlock(). This should be safe from a performance perspective because the lock is this tasks ->pi_lock, and this is called only after the task exits. Signed-off-by: Paul E. McKenney Cc: Ingo Molnar Cc: Peter Zijlstra Cc: Will Deacon Cc: Alan Stern Cc: Andrea Parri Cc: Linus Torvalds [ paulmck: Drop smp_mb() based on Peter Zijlstra's analysis: http://lkml.kernel.org/r/20170811144150.26gowhxte7ri5fpk@hirez.programming.kicks-ass.net ] diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 17c667b427b4..5d22323ae099 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3352,8 +3352,8 @@ void __noreturn do_task_dead(void) * To avoid it, we have to wait for releasing tsk->pi_lock which * is held by try_to_wake_up() */ - smp_mb(); - raw_spin_unlock_wait(¤t->pi_lock); + raw_spin_lock_irq(¤t->pi_lock); + raw_spin_unlock_irq(¤t->pi_lock); /* Causes final put_task_struct in finish_task_switch(): */ __set_current_state(TASK_DEAD);