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authorSteven Rostedt <rostedt@goodmis.org>2011-08-05 08:27:49 -0400
committerIngo Molnar <mingo@elte.hu>2011-08-14 12:01:07 +0200
commitd473750b4073f16f23f46f30dc1bd3de45c35754 (patch)
treecefe6613df0f129836be5e218a768d02c1c7b814 /kernel/sched_cpupri.c
parentc92211d9b772792a9dea530c042efb4ab5562f50 (diff)
downloadlwn-d473750b4073f16f23f46f30dc1bd3de45c35754.tar.gz
lwn-d473750b4073f16f23f46f30dc1bd3de45c35754.zip
sched/cpupri: Fix memory barriers for vec updates to always be in order
[ This patch actually compiles. Thanks to Mike Galbraith for pointing that out. I compiled and booted this patch with no issues. ] Re-examining the cpupri patch, I see there's a possible race because the update of the two priorities vec->counts are not protected by a memory barrier. When a RT runqueue is overloaded and wants to push an RT task to another runqueue, it scans the RT priority vectors in a loop from lowest priority to highest. When we queue or dequeue an RT task that changes a runqueue's highest priority task, we update the vectors to show that a runqueue is rated at a different priority. To do this, we first set the new priority mask, and increment the vec->count, and then set the old priority mask by decrementing the vec->count. If we are lowering the runqueue's RT priority rating, it will trigger a RT pull, and we do not care if we miss pushing to this runqueue or not. But if we raise the priority, but the priority is still lower than an RT task that is looking to be pushed, we must make sure that this runqueue is still seen by the push algorithm (the loop). Because the loop reads from lowest to highest, and the new priority is set before the old one is cleared, we will either see the new or old priority set and the vector will be checked. But! Since there's no memory barrier between the updates of the two, the old count may be decremented first before the new count is incremented. This means the loop may see the old count of zero and skip it, and also the new count of zero before it was updated. A possible runqueue that the RT task could move to could be missed. A conditional memory barrier is placed between the vec->count updates and is only called when both updates are done. The smp_wmb() has also been changed to smp_mb__before_atomic_inc/dec(), as they are not needed by archs that already synchronize atomic_inc/dec(). The smp_rmb() has been moved to be called at every iteration of the loop so that the race between seeing the two updates is visible by each iteration of the loop, as an arch is free to optimize the reading of memory of the counters in the loop. Signed-off-by: Steven Rostedt <rostedt@goodmis.org> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Nick Piggin <npiggin@kernel.dk> Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/1312547269.18583.194.camel@gandalf.stny.rr.com Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'kernel/sched_cpupri.c')
-rw-r--r--kernel/sched_cpupri.c28
1 files changed, 25 insertions, 3 deletions
diff --git a/kernel/sched_cpupri.c b/kernel/sched_cpupri.c
index 7761a2669fff..90faffdbdf98 100644
--- a/kernel/sched_cpupri.c
+++ b/kernel/sched_cpupri.c
@@ -73,9 +73,10 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
for (idx = 0; idx < task_pri; idx++) {
struct cpupri_vec *vec = &cp->pri_to_cpu[idx];
+ int skip = 0;
if (!atomic_read(&(vec)->count))
- continue;
+ skip = 1;
/*
* When looking at the vector, we need to read the counter,
* do a memory barrier, then read the mask.
@@ -96,6 +97,10 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
*/
smp_rmb();
+ /* Need to do the rmb for every iteration */
+ if (skip)
+ continue;
+
if (cpumask_any_and(&p->cpus_allowed, vec->mask) >= nr_cpu_ids)
continue;
@@ -134,6 +139,7 @@ void cpupri_set(struct cpupri *cp, int cpu, int newpri)
{
int *currpri = &cp->cpu_to_pri[cpu];
int oldpri = *currpri;
+ int do_mb = 0;
newpri = convert_prio(newpri);
@@ -158,18 +164,34 @@ void cpupri_set(struct cpupri *cp, int cpu, int newpri)
* do a write memory barrier, and then update the count, to
* make sure the vector is visible when count is set.
*/
- smp_wmb();
+ smp_mb__before_atomic_inc();
atomic_inc(&(vec)->count);
+ do_mb = 1;
}
if (likely(oldpri != CPUPRI_INVALID)) {
struct cpupri_vec *vec = &cp->pri_to_cpu[oldpri];
/*
+ * Because the order of modification of the vec->count
+ * is important, we must make sure that the update
+ * of the new prio is seen before we decrement the
+ * old prio. This makes sure that the loop sees
+ * one or the other when we raise the priority of
+ * the run queue. We don't care about when we lower the
+ * priority, as that will trigger an rt pull anyway.
+ *
+ * We only need to do a memory barrier if we updated
+ * the new priority vec.
+ */
+ if (do_mb)
+ smp_mb__after_atomic_inc();
+
+ /*
* When removing from the vector, we decrement the counter first
* do a memory barrier and then clear the mask.
*/
atomic_dec(&(vec)->count);
- smp_wmb();
+ smp_mb__after_atomic_inc();
cpumask_clear_cpu(cpu, vec->mask);
}