[PATCH 1/2] smp_call_function_many SMP race
From: Milton Miller
Date: Tue Jan 18 2011 - 16:07:30 EST
From: Anton Blanchard <anton@xxxxxxxxx>
I noticed a failure where we hit the following WARN_ON in
generic_smp_call_function_interrupt:
if (!cpumask_test_and_clear_cpu(cpu, data->cpumask))
continue;
data->csd.func(data->csd.info);
refs = atomic_dec_return(&data->refs);
WARN_ON(refs < 0); <-------------------------
We atomically tested and cleared our bit in the cpumask, and yet the
number of cpus left (ie refs) was 0. How can this be?
It turns out commit 54fdade1c3332391948ec43530c02c4794a38172
(generic-ipi: make struct call_function_data lockless)
is at fault. It removes locking from smp_call_function_many and in
doing so creates a rather complicated race.
The problem comes about because:
- The smp_call_function_many interrupt handler walks call_function.queue
without any locking.
- We reuse a percpu data structure in smp_call_function_many.
- We do not wait for any RCU grace period before starting the next
smp_call_function_many.
Imagine a scenario where CPU A does two smp_call_functions back to
back, and CPU B does an smp_call_function in between. We concentrate on
how CPU C handles the calls:
CPU A CPU B CPU C CPU D
smp_call_function
smp_call_function_interrupt
walks
call_function.queue sees
data from CPU A on list
smp_call_function
smp_call_function_interrupt
walks
call_function.queue sees
(stale) CPU A on list
smp_call_function int
clears last ref on A
list_del_rcu, unlock
smp_call_function reuses
percpu *data A
data->cpumask sees and
clears bit in cpumask
might be using old or new fn!
decrements refs below 0
set data->refs (too late!)
The important thing to note is since the interrupt handler walks a
potentially stale call_function.queue without any locking, then another
cpu can view the percpu *data structure at any time, even when the
owner is in the process of initialising it.
The following test case hits the WARN_ON 100% of the time on my PowerPC
box (having 128 threads does help :)
#include <linux/module.h>
#include <linux/init.h>
#define ITERATIONS 100
static void do_nothing_ipi(void *dummy)
{
}
static void do_ipis(struct work_struct *dummy)
{
int i;
for (i = 0; i < ITERATIONS; i++)
smp_call_function(do_nothing_ipi, NULL, 1);
printk(KERN_DEBUG "cpu %d finished\n", smp_processor_id());
}
static struct work_struct work[NR_CPUS];
static int __init testcase_init(void)
{
int cpu;
for_each_online_cpu(cpu) {
INIT_WORK(&work[cpu], do_ipis);
schedule_work_on(cpu, &work[cpu]);
}
return 0;
}
static void __exit testcase_exit(void)
{
}
module_init(testcase_init)
module_exit(testcase_exit)
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Anton Blanchard");
I tried to fix it by ordering the read and the write of ->cpumask and
->refs. In doing so I missed a critical case but Paul McKenney was able
to spot my bug thankfully :) To ensure we arent viewing previous
iterations the interrupt handler needs to read ->refs then ->cpumask
then ->refs _again_.
Thanks to Milton Miller and Paul McKenney for helping to debug this
issue.
[Milton Miller: add WARN_ON and BUG_ON, remove extra read of refs before
initial read of mask that doesn't help (also noted by Peter Zijlstra),
adjust comments, hopefully clarify senerio ]
Signed-off-by: Anton Blanchard <anton@xxxxxxxxx>
Revised-by: Milton Miller <miltonm@xxxxxxx> [ removed excess tests ]
Signed-off-by: Milton Miller <miltonm@xxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxx # 2.6.32 and later
Index: common/kernel/smp.c
===================================================================
--- common.orig/kernel/smp.c 2011-01-17 20:15:54.000000000 -0600
+++ common/kernel/smp.c 2011-01-17 20:16:18.000000000 -0600
@@ -194,6 +194,24 @@ void generic_smp_call_function_interrupt
list_for_each_entry_rcu(data, &call_function.queue, csd.list) {
int refs;
+ /*
+ * Since we walk the list without any locks, we might
+ * see an entry that was completed, removed from the
+ * list and is in the process of being reused.
+ *
+ * We must check that the cpu is in the cpumask before
+ * checking the refs, and both must be set before
+ * executing the callback on this cpu.
+ */
+
+ if (!cpumask_test_cpu(cpu, data->cpumask))
+ continue;
+
+ smp_rmb();
+
+ if (atomic_read(&data->refs) == 0)
+ continue;
+
if (!cpumask_test_and_clear_cpu(cpu, data->cpumask))
continue;
@@ -202,6 +220,8 @@ void generic_smp_call_function_interrupt
refs = atomic_dec_return(&data->refs);
WARN_ON(refs < 0);
if (!refs) {
+ WARN_ON(!cpumask_empty(data->cpumask));
+
raw_spin_lock(&call_function.lock);
list_del_rcu(&data->csd.list);
raw_spin_unlock(&call_function.lock);
@@ -453,11 +473,21 @@ void smp_call_function_many(const struct
data = &__get_cpu_var(cfd_data);
csd_lock(&data->csd);
+ BUG_ON(atomic_read(&data->refs) || !cpumask_empty(data->cpumask));
data->csd.func = func;
data->csd.info = info;
cpumask_and(data->cpumask, mask, cpu_online_mask);
cpumask_clear_cpu(this_cpu, data->cpumask);
+
+ /*
+ * To ensure the interrupt handler gets an complete view
+ * we order the cpumask and refs writes and order the read
+ * of them in the interrupt handler. In addition we may
+ * only clear our own cpu bit from the mask.
+ */
+ smp_wmb();
+
atomic_set(&data->refs, cpumask_weight(data->cpumask));
raw_spin_lock_irqsave(&call_function.lock, flags);
--
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