linux-kernel - Virtually Mapped Stacks: Do not disable interrupts

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Date:   Wed, 17 May 2017 10:58:26 -0500 (CDT)
From:   Christoph Lameter <cl@...ux.com>
To:     Andy Lutomirski <luto@...capital.net>
cc:     linux-kernel@...r.kernel.org
Subject: Virtually Mapped Stacks: Do not disable interrupts

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.

Signed-off-by: Christoph Lameter <cl@...ux.com>

Index: linux/kernel/fork.c
===================================================================
--- linux.orig/kernel/fork.c
+++ linux/kernel/fork.c
@@ -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;