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Subject[PATCH tip/core/rcu 10/10] cpu: Avoid puts_pending overflow
Date
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

A long string of get_online_cpus() with each followed by a
put_online_cpu() that fails to acquire cpu_hotplug.lock can result in
overflow of the cpu_hotplug.puts_pending counter. Although this is
perhaps improbably, a system with absolutely no CPU-hotplug operations
will have an arbitrarily long time in which this overflow could occur.
This commit therefore adds overflow checks to get_online_cpus() and
try_get_online_cpus().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/cpu.c | 19 +++++++++++++------
1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 90a3d017b90c..5d220234b3ca 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -86,6 +86,16 @@ static struct {
#define cpuhp_lock_acquire() lock_map_acquire(&cpu_hotplug.dep_map)
#define cpuhp_lock_release() lock_map_release(&cpu_hotplug.dep_map)

+static void apply_puts_pending(int max)
+{
+ int delta;
+
+ if (atomic_read(&cpu_hotplug.puts_pending) >= max) {
+ delta = atomic_xchg(&cpu_hotplug.puts_pending, 0);
+ cpu_hotplug.refcount -= delta;
+ }
+}
+
void get_online_cpus(void)
{
might_sleep();
@@ -93,6 +103,7 @@ void get_online_cpus(void)
return;
cpuhp_lock_acquire_read();
mutex_lock(&cpu_hotplug.lock);
+ apply_puts_pending(65536);
cpu_hotplug.refcount++;
mutex_unlock(&cpu_hotplug.lock);
}
@@ -105,6 +116,7 @@ bool try_get_online_cpus(void)
if (!mutex_trylock(&cpu_hotplug.lock))
return false;
cpuhp_lock_acquire_tryread();
+ apply_puts_pending(65536);
cpu_hotplug.refcount++;
mutex_unlock(&cpu_hotplug.lock);
return true;
@@ -161,12 +173,7 @@ void cpu_hotplug_begin(void)
cpuhp_lock_acquire();
for (;;) {
mutex_lock(&cpu_hotplug.lock);
- if (atomic_read(&cpu_hotplug.puts_pending)) {
- int delta;
-
- delta = atomic_xchg(&cpu_hotplug.puts_pending, 0);
- cpu_hotplug.refcount -= delta;
- }
+ apply_puts_pending(1);
if (likely(!cpu_hotplug.refcount))
break;
__set_current_state(TASK_UNINTERRUPTIBLE);
--
1.8.1.5


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