[PATCH 5.10 370/379] sched/membarrier: reduce the ability to hammer on sys_membarrier

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5.10-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxx>

commit 944d5fe50f3f03daacfea16300e656a1691c4a23 upstream.

On some systems, sys_membarrier can be very expensive, causing overall
slowdowns for everything.  So put a lock on the path in order to
serialize the accesses to prevent the ability for this to be called at
too high of a frequency and saturate the machine.

Reviewed-and-tested-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Acked-by: Borislav Petkov <bp@xxxxxxxxx>
Fixes: 22e4ebb97582 ("membarrier: Provide expedited private command")
Fixes: c5f58bd58f43 ("membarrier: Provide GLOBAL_EXPEDITED command")
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[ converted to explicit mutex_*() calls - cleanup.h is not in this stable
  branch - gregkh ]
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/sched/membarrier.c |    9 +++++++++
 1 file changed, 9 insertions(+)

--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -34,6 +34,8 @@
 	| MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK		\
 	| MEMBARRIER_PRIVATE_EXPEDITED_RSEQ_BITMASK)
 
+static DEFINE_MUTEX(membarrier_ipi_mutex);
+
 static void ipi_mb(void *info)
 {
 	smp_mb();	/* IPIs should be serializing but paranoid. */
@@ -119,6 +121,7 @@ static int membarrier_global_expedited(v
 	if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
 		return -ENOMEM;
 
+	mutex_lock(&membarrier_ipi_mutex);
 	cpus_read_lock();
 	rcu_read_lock();
 	for_each_online_cpu(cpu) {
@@ -165,6 +168,8 @@ static int membarrier_global_expedited(v
 	 * rq->curr modification in scheduler.
 	 */
 	smp_mb();	/* exit from system call is not a mb */
+	mutex_unlock(&membarrier_ipi_mutex);
+
 	return 0;
 }
 
@@ -208,6 +213,7 @@ static int membarrier_private_expedited(
 	if (cpu_id < 0 && !zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
 		return -ENOMEM;
 
+	mutex_lock(&membarrier_ipi_mutex);
 	cpus_read_lock();
 
 	if (cpu_id >= 0) {
@@ -280,6 +286,7 @@ out:
 	 * rq->curr modification in scheduler.
 	 */
 	smp_mb();	/* exit from system call is not a mb */
+	mutex_unlock(&membarrier_ipi_mutex);
 
 	return 0;
 }
@@ -321,6 +328,7 @@ static int sync_runqueues_membarrier_sta
 	 * between threads which are users of @mm has its membarrier state
 	 * updated.
 	 */
+	mutex_lock(&membarrier_ipi_mutex);
 	cpus_read_lock();
 	rcu_read_lock();
 	for_each_online_cpu(cpu) {
@@ -337,6 +345,7 @@ static int sync_runqueues_membarrier_sta
 
 	free_cpumask_var(tmpmask);
 	cpus_read_unlock();
+	mutex_unlock(&membarrier_ipi_mutex);
 
 	return 0;
 }






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