[PATCH 4.4 29/30] percpu: acquire pcpu_lock when updating pcpu_nr_empty_pop_pages

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Tahsin Erdogan <tah...@google.com>

commit 320661b08dd6f1746d5c7ab4eb435ec64b97cd45 upstream.

Update to pcpu_nr_empty_pop_pages in pcpu_alloc() is currently done
without holding pcpu_lock. This can lead to bad updates to the variable.
Add missing lock calls.

Fixes: b539b87fed37 ("percpu: implmeent pcpu_nr_empty_pop_pages and 
chunk->nr_populated")
Signed-off-by: Tahsin Erdogan <tah...@google.com>
Signed-off-by: Tejun Heo <t...@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 mm/percpu.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1012,8 +1012,11 @@ area_found:
                mutex_unlock(&pcpu_alloc_mutex);
        }
 
-       if (chunk != pcpu_reserved_chunk)
+       if (chunk != pcpu_reserved_chunk) {
+               spin_lock_irqsave(&pcpu_lock, flags);
                pcpu_nr_empty_pop_pages -= occ_pages;
+               spin_unlock_irqrestore(&pcpu_lock, flags);
+       }
 
        if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_LOW)
                pcpu_schedule_balance_work();