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Subject[RFC PATCH for 5.4 7/7] sched/membarrier: Return -ENOMEM to userspace on memory allocation failure
Date
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@efficios.com>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
---
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
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