From 4cd4262034849da01eb88659af677b69f8169f06 Mon Sep 17 00:00:00 2001 From: Steven Rostedt Date: Wed, 26 Nov 2008 21:04:24 -0500 Subject: sched: prevent divide by zero error in cpu_avg_load_per_task Impact: fix divide by zero crash in scheduler rebalance irq While testing the branch profiler, I hit this crash: divide error: 0000 [#1] PREEMPT SMP [...] RIP: 0010:[] [] cpu_avg_load_per_task+0x50/0x7f [...] Call Trace: <0> [] find_busiest_group+0x3e5/0xcaa [] rebalance_domains+0x2da/0xa21 [] ? find_next_bit+0x1b2/0x1e6 [] run_rebalance_domains+0x112/0x19f [] __do_softirq+0xa8/0x232 [] call_softirq+0x1c/0x3e [] do_softirq+0x94/0x1cd [] irq_exit+0x6b/0x10e [] smp_apic_timer_interrupt+0xd3/0xff [] apic_timer_interrupt+0x13/0x20 The code for cpu_avg_load_per_task has: if (rq->nr_running) rq->avg_load_per_task = rq->load.weight / rq->nr_running; The runqueue lock is not held here, and there is nothing that prevents the rq->nr_running from going to zero after it passes the if condition. The branch profiler simply made the race window bigger. This patch saves off the rq->nr_running to a local variable and uses that for both the condition and the division. Signed-off-by: Steven Rostedt Peter Zijlstra Signed-off-by: Ingo Molnar --- kernel/sched.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) (limited to 'kernel') diff --git a/kernel/sched.c b/kernel/sched.c index 9b1e79371c20..700aa9a1413f 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -1453,9 +1453,10 @@ 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; - 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; -- cgit v1.2.2 From 1583715ddb61f822041807a0f18b3b4845e88c76 Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Tue, 25 Nov 2008 10:27:49 +0100 Subject: sched, cpusets: fix warning in kernel/cpuset.c MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit this warning: kernel/cpuset.c: In function ‘generate_sched_domains’: kernel/cpuset.c:588: warning: ‘ndoms’ may be used uninitialized in this function triggers because GCC does not recognize that ndoms stays uninitialized only if doms is NULL - but that flow is covered at the end of generate_sched_domains(). Help out GCC by initializing this variable to 0. (that's prudent anyway) Also, this function needs a splitup and code flow simplification: with 160 lines length it's clearly too long. Signed-off-by: Ingo Molnar --- kernel/cpuset.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'kernel') diff --git a/kernel/cpuset.c b/kernel/cpuset.c index da7ff6137f37..96c0ba13b8cd 100644 --- a/kernel/cpuset.c +++ b/kernel/cpuset.c @@ -585,7 +585,7 @@ static int generate_sched_domains(cpumask_t **domains, int i, j, k; /* indices for partition finding loops */ cpumask_t *doms; /* resulting partition; i.e. sched domains */ struct sched_domain_attr *dattr; /* attributes for custom domains */ - int ndoms; /* number of sched domains in result */ + int ndoms = 0; /* number of sched domains in result */ int nslot; /* next empty doms[] cpumask_t slot */ doms = NULL; -- cgit v1.2.2 From af6d596fd603219b054c1c90fb16672a9fd441bd Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Sat, 29 Nov 2008 20:45:15 +0100 Subject: 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 --- kernel/sched.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'kernel') diff --git a/kernel/sched.c b/kernel/sched.c index 700aa9a1413f..b7480fb5c3dc 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; -- cgit v1.2.2