Peter Zijlstra 95913d9791 sched/core: Fix TASK_DEAD race in finish_task_switch() ... So the problem this patch is trying to address is as follows: CPU0 CPU1 context_switch(A, B) ttwu(A) LOCK A->pi_lock A->on_cpu == 0 finish_task_switch(A) prev_state = A->state <-. WMB | A->on_cpu = 0; | UNLOCK rq0->lock | | context_switch(C, A) `-- A->state = TASK_DEAD prev_state == TASK_DEAD put_task_struct(A) context_switch(A, C) finish_task_switch(A) A->state == TASK_DEAD put_task_struct(A) The argument being that the WMB will allow the load of A->state on CPU0 to cross over and observe CPU1's store of A->state, which will then result in a double-drop and use-after-free. Now the comment states (and this was true once upon a long time ago) that we need to observe A->state while holding rq->lock because that will order us against the wakeup; however the wakeup will not in fact acquire (that) rq->lock; it takes A->pi_lock these days. We can obviously fix this by upgrading the WMB to an MB, but that is expensive, so we'd rather avoid that. The alternative this patch takes is: smp_store_release(&A->on_cpu, 0), which avoids the MB on some archs, but not important ones like ARM. Reported-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Linus Torvalds <torvalds@linux-foundation.org> Cc: <stable@vger.kernel.org> # v3.1+ Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-kernel@vger.kernel.org Cc: manfred@colorfullife.com Cc: will.deacon@arm.com Fixes: e4a52bcb9a ("sched: Remove rq->lock from the first half of ttwu()") Link: http://lkml.kernel.org/r/20150929124509.GG3816@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar <mingo@kernel.org>		2015-10-06 17:05:17 +02:00
..
auto_group.c
auto_group.h
clock.c
completion.c
core.c	sched/core: Fix TASK_DEAD race in finish_task_switch()	2015-10-06 17:05:17 +02:00
cpuacct.c
cpuacct.h
cpudeadline.c
cpudeadline.h
cpupri.c
cpupri.h
cputime.c
deadline.c
debug.c
fair.c
features.h
idle.c
idle_task.c
loadavg.c
Makefile
rt.c
sched.h	sched/core: Fix TASK_DEAD race in finish_task_switch()	2015-10-06 17:05:17 +02:00
stats.c
stats.h
stop_task.c
wait.c