linux-kernel - [PATCH] mm/vmalloc: Fix unlock order in s

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Message-Id: <20201213180843.16938-1-longman@redhat.com>
Date:   Sun, 13 Dec 2020 13:08:43 -0500
From:   Waiman Long <longman@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Waiman Long <longman@...hat.com>
Subject: [PATCH] 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.

Fixes: e36176be1c39 ("mm/vmalloc: rework vmap_area_lock")
Signed-off-by: Waiman Long <longman@...hat.com>
---
 mm/vmalloc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 6ae491a8b210..75913f685c71 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3448,11 +3448,11 @@ static void *s_next(struct seq_file *m, void *p, loff_t *pos)
 }
 
 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)
-- 
2.18.1