From: Christoph Lameter <cl@linux.com>
Date: Wed, 12 Jul 2017 14:33:11 -0700
Subject: [PATCH] kernel/fork.c: virtually mapped stacks: do not disable
 interrupts
References: bnc#1060662 bnc#978907
Patch-mainline: v4.12.4
Git-commit: 112166f88cf83dd11486cf1818672d42b540865b

commit 112166f88cf83dd11486cf1818672d42b540865b upstream.

The reason to disable interrupts seems to be to avoid switching to a
different processor while handling per cpu data using individual loads and
stores.  If we use per cpu RMV primitives we will not have to disable
interrupts.

Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1705171055130.5898@east.gentwo.org
Signed-off-by: Christoph Lameter <cl@linux.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 kernel/fork.c | 16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index d927ec11aa7a..6440e0b70cad 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -205,19 +205,17 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
 	void *stack;
 	int i;
 
-	local_irq_disable();
 	for (i = 0; i < NR_CACHED_STACKS; i++) {
-		struct vm_struct *s = this_cpu_read(cached_stacks[i]);
+		struct vm_struct *s;
+
+		s = this_cpu_xchg(cached_stacks[i], NULL);
 
 		if (!s)
 			continue;
-		this_cpu_write(cached_stacks[i], NULL);
 
 		tsk->stack_vm_area = s;
-		local_irq_enable();
 		return s->addr;
 	}
-	local_irq_enable();
 
 	stack = __vmalloc_node_range(THREAD_SIZE, THREAD_SIZE,
 				     VMALLOC_START, VMALLOC_END,
@@ -245,19 +243,15 @@ static inline void free_thread_stack(struct task_struct *tsk)
 {
 #ifdef CONFIG_VMAP_STACK
 	if (task_stack_vm_area(tsk)) {
-		unsigned long flags;
 		int i;
 
-		local_irq_save(flags);
 		for (i = 0; i < NR_CACHED_STACKS; i++) {
-			if (this_cpu_read(cached_stacks[i]))
+			if (this_cpu_cmpxchg(cached_stacks[i],
+					NULL, tsk->stack_vm_area) != NULL)
 				continue;
 
-			this_cpu_write(cached_stacks[i], tsk->stack_vm_area);
-			local_irq_restore(flags);
 			return;
 		}
-		local_irq_restore(flags);
 
 		vfree_atomic(tsk->stack);
 		return;
-- 
2.14.2