Virtually Mapped Stacks: Do not disable interrupts

From: Christoph Lameter
Date: Wed May 17 2017 - 11:58:34 EST


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@xxxxxxxxx>

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;