[34-longterm 064/247] kernel/smp.c: fix smp_call_function_many() SMP race

Paul Gortmaker
2011-06-23T17:36:36Z

From: Anton Blanchard <anton@samba.org>

-------------------
This is a commit scheduled for the next v2.6.34 longterm release.
If you see a problem with using this for longterm, please comment.
-------------------

commit 6dc19899958e420a931274b94019e267e2396d3e upstream.

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.

[miltonm@bga.com: 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 scenario ]
[miltonm@bga.com: remove excess tests]
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Milton Miller <miltonm@bga.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---
kernel/smp.c | 30 ++++++++++++++++++++++++++++++
1 files changed, 30 insertions(+), 0 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index 3fc6973..0a9e6bd 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -194,6 +194,24 @@ void generic_smp_call_function_interrupt(void)
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(void)
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);
@@ -442,11 +462,21 @@ void smp_call_function_many(const struct cpumask *mask,

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);
--
1.7.4.4