Patch "kernel/fork.c: virtually mapped stacks: do not disable interrupts" has been added to the 4.12-stable tree

This is a note to let you know that I've just added the patch titled

kernel/fork.c: virtually mapped stacks: do not disable interrupts

to the 4.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kernel-fork.c-virtually-mapped-stacks-do-not-disable-interrupts.patch
and it can be found in the queue-4.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Christoph Lameter <[email protected]>
Date: Wed, 12 Jul 2017 14:33:11 -0700
Subject: kernel/fork.c: virtually mapped stacks: do not disable interrupts

From: Christoph Lameter <[email protected]>

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.

---
 kernel/fork.c |   16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

@@ -205,19 +205,17 @@ static unsigned long *alloc_thread_stack
 	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(str
 {
 #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;

Patches currently in stable-queue which might be from [email protected] are

queue-4.12/kernel-fork.c-virtually-mapped-stacks-do-not-disable-interrupts.patch