[RFC PATCH for 5.4 7/7] sched/membarrier: Return -ENOMEM to userspace on memory allocation failure

[Mathieu Desnoyers]

Remove the IPI fallback code from membarrier to deal with very
infrequent cpumask memory allocation failure. Use GFP_KERNEL rather
than GFP_NOWAIT, and relax the blocking guarantees for the expedited
membarrier system call commands, allowing it to block if waiting for
memory to be made available.

In addition, now -ENOMEM can be returned to user-space if the cpumask
memory allocation fails.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Russell King - ARM Linux admin <linux@xxxxxxxxxxxxxxx>
Cc: Chris Metcalf <cmetcalf@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Kirill Tkhai <tkhai@xxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/membarrier.c | 61 ++++++++++++---------------------------
 1 file changed, 19 insertions(+), 42 deletions(-)

diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index 8afbdf92be0a..1420c656e8f0 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -66,7 +66,6 @@ void membarrier_exec_mmap(struct mm_struct *mm)
 static int membarrier_global_expedited(void)
 {
 	int cpu;
-	bool fallback = false;
 	cpumask_var_t tmpmask;
 
 	if (num_online_cpus() == 1)
@@ -78,15 +77,8 @@ static int membarrier_global_expedited(void)
 	 */
 	smp_mb();	/* system call entry is not a mb. */
 
-	/*
-	 * Expedited membarrier commands guarantee that they won't
-	 * block, hence the GFP_NOWAIT allocation flag and fallback
-	 * implementation.
-	 */
-	if (!zalloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
-		/* Fallback for OOM. */
-		fallback = true;
-	}
+	if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
+		return -ENOMEM;
 
 	cpus_read_lock();
 	rcu_read_lock();
@@ -117,18 +109,15 @@ static int membarrier_global_expedited(void)
 		if (p->flags & PF_KTHREAD)
 			continue;
 
-		if (!fallback)
-			__cpumask_set_cpu(cpu, tmpmask);
-		else
-			smp_call_function_single(cpu, ipi_mb, NULL, 1);
+		__cpumask_set_cpu(cpu, tmpmask);
 	}
 	rcu_read_unlock();
-	if (!fallback) {
-		preempt_disable();
-		smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
-		preempt_enable();
-		free_cpumask_var(tmpmask);
-	}
+
+	preempt_disable();
+	smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
+	preempt_enable();
+
+	free_cpumask_var(tmpmask);
 	cpus_read_unlock();
 
 	/*
@@ -143,7 +132,6 @@ static int membarrier_global_expedited(void)
 static int membarrier_private_expedited(int flags)
 {
 	int cpu;
-	bool fallback = false;
 	cpumask_var_t tmpmask;
 	struct mm_struct *mm = current->mm;
 
@@ -168,15 +156,8 @@ static int membarrier_private_expedited(int flags)
 	 */
 	smp_mb();	/* system call entry is not a mb. */
 
-	/*
-	 * Expedited membarrier commands guarantee that they won't
-	 * block, hence the GFP_NOWAIT allocation flag and fallback
-	 * implementation.
-	 */
-	if (!zalloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
-		/* Fallback for OOM. */
-		fallback = true;
-	}
+	if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
+		return -ENOMEM;
 
 	cpus_read_lock();
 	rcu_read_lock();
@@ -194,20 +175,16 @@ static int membarrier_private_expedited(int flags)
 		if (cpu == raw_smp_processor_id())
 			continue;
 		p = task_rcu_dereference(&cpu_rq(cpu)->curr);
-		if (p && p->mm == mm) {
-			if (!fallback)
-				__cpumask_set_cpu(cpu, tmpmask);
-			else
-				smp_call_function_single(cpu, ipi_mb, NULL, 1);
-		}
+		if (p && p->mm == mm)
+			__cpumask_set_cpu(cpu, tmpmask);
 	}
 	rcu_read_unlock();
-	if (!fallback) {
-		preempt_disable();
-		smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
-		preempt_enable();
-		free_cpumask_var(tmpmask);
-	}
+
+	preempt_disable();
+	smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
+	preempt_enable();
+
+	free_cpumask_var(tmpmask);
 	cpus_read_unlock();
 
 	/*
-- 
2.17.1