Messages in this thread | | | Date | Fri, 11 Aug 2017 16:41:50 +0200 | From | Peter Zijlstra <> | Subject | Re: linux-next: build failure after merge of the rcu tree |
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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.
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