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authorIngo Molnar <mingo@elte.hu>2008-11-29 20:45:15 +0100
committerIngo Molnar <mingo@elte.hu>2008-11-29 20:45:15 +0100
commitaf6d596fd603219b054c1c90fb16672a9fd441bd (patch)
treee3e5a85532079aedc9b0b194a319ec084b8a4449
parent1583715ddb61f822041807a0f18b3b4845e88c76 (diff)
sched: prevent divide by zero error in cpu_avg_load_per_task, update
Regarding the bug addressed in: 4cd4262: sched: prevent divide by zero error in cpu_avg_load_per_task Linus points out that the fix is not complete: > There's nothing that keeps gcc from deciding not to reload > rq->nr_running. > > Of course, in _practice_, I don't think gcc ever will (if it decides > that it will spill, gcc is likely going to decide that it will > literally spill the local variable to the stack rather than decide to > reload off the pointer), but it's a valid compiler optimization, and > it even has a name (rematerialization). > > So I suspect that your patch does fix the bug, but it still leaves the > fairly unlikely _potential_ for it to re-appear at some point. > > We have ACCESS_ONCE() as a macro to guarantee that the compiler > doesn't rematerialize a pointer access. That also would clarify > the fact that we access something unsafe outside a lock. So make sure our nr_running value is immutable and cannot change after we check it for nonzero. Signed-off-by: Ingo Molnar <mingo@elte.hu>
-rw-r--r--kernel/sched.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/kernel/sched.c b/kernel/sched.c
index 700aa9a1413..b7480fb5c3d 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1453,7 +1453,7 @@ static int task_hot(struct task_struct *p, u64 now, struct sched_domain *sd);
static unsigned long cpu_avg_load_per_task(int cpu)
{
struct rq *rq = cpu_rq(cpu);
- unsigned long nr_running = rq->nr_running;
+ unsigned long nr_running = ACCESS_ONCE(rq->nr_running);
if (nr_running)
rq->avg_load_per_task = rq->load.weight / nr_running;