[patch 098/200] mm/vmalloc: Fix unlock order in s_stop()

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From: Waiman Long <longman@xxxxxxxxxx>
Subject: mm/vmalloc: Fix unlock order in s_stop()

When multiple locks are acquired, they should be released in reverse
order. For s_start() and s_stop() in mm/vmalloc.c, that is not the
case.

  s_start: mutex_lock(&vmap_purge_lock); spin_lock(&vmap_area_lock);
  s_stop : mutex_unlock(&vmap_purge_lock); spin_unlock(&vmap_area_lock);

This unlock sequence, though allowed, is not optimal. If a waiter is
present, mutex_unlock() will need to go through the slowpath of waking
up the waiter with preemption disabled. Fix that by releasing the
spinlock first before the mutex.

Link: https://lkml.kernel.org/r/20201213180843.16938-1-longman@xxxxxxxxxx
Fixes: e36176be1c39 ("mm/vmalloc: rework vmap_area_lock")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/vmalloc.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/vmalloc.c~mm-vmalloc-fix-unlock-order-in-s_stop
+++ a/mm/vmalloc.c
@@ -3465,11 +3465,11 @@ static void *s_next(struct seq_file *m,
 }
 
 static void s_stop(struct seq_file *m, void *p)
-	__releases(&vmap_purge_lock)
 	__releases(&vmap_area_lock)
+	__releases(&vmap_purge_lock)
 {
-	mutex_unlock(&vmap_purge_lock);
 	spin_unlock(&vmap_area_lock);
+	mutex_unlock(&vmap_purge_lock);
 }
 
 static void show_numa_info(struct seq_file *m, struct vm_struct *v)
_




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