Messages in this thread Patch in this message | | | Date | Sat, 29 Nov 2008 20:50:59 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 1/1] sched: prevent divide by zero error in cpu_avg_load_per_task |
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Wed, 26 Nov 2008, Steven Rostedt wrote: > > { > > struct rq *rq = cpu_rq(cpu); > > + unsigned long nr_running = rq->nr_running; > > > > - if (rq->nr_running) > > - rq->avg_load_per_task = rq->load.weight / rq->nr_running; > > + if (nr_running) > > + rq->avg_load_per_task = rq->load.weight / nr_running; > > else > > rq->avg_load_per_task = 0; > > I don't think this necessarily fixes it. > > 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.
Okay - i've queued up the fix below, to be on the safe side.
Ingo
----------------> From af6d596fd603219b054c1c90fb16672a9fd441bd Mon Sep 17 00:00:00 2001 From: Ingo Molnar <mingo@elte.hu> Date: Sat, 29 Nov 2008 20:45:15 +0100 Subject: [PATCH] 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> --- kernel/sched.c | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c index 700aa9a..b7480fb 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;
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