Re: linux-next: build failure after merge of the rcu tree

Paul E. McKenney
2017-08-11T20:12:47Z

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(&current->pi_lock);
> spin_unlock(&current->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 <paulmck@linux.vnet.ibm.com>
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 <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Andrea Parri <parri.andrea@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
[ 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(&current->pi_lock);
+ raw_spin_lock_irq(&current->pi_lock);
+ raw_spin_unlock_irq(&current->pi_lock);

/* Causes final put_task_struct in finish_task_switch(): */
__set_current_state(TASK_DEAD);