Fix incomplete __mntput locking

Getting this wrong caused

	WARNING: at fs/namespace.c:636 mntput_no_expire+0xac/0xf2()

due to optimistically checking cpu_writer->mnt outside the spinlock.

Here's what we really want:
 * we know that nobody will set cpu_writer->mnt to mnt from now on
 * all changes to that sucker are done under cpu_writer->lock
 * we want the laziest equivalent of
	spin_lock(&cpu_writer->lock);
	if (likely(cpu_writer->mnt != mnt)) {
		spin_unlock(&cpu_writer->lock);
		continue;
	}
	/* do stuff */
  that would make sure we won't miss earlier setting of ->mnt done by
  another CPU.

Anyway, for now we just move the spin_lock() earlier and move the test
into the properly locked region.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-and-tested-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 228d8c4bfd18..06f8e63f6cb1 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -614,9 +614,11 @@ static inline void __mntput(struct vfsmount *mnt)
          */
         for_each_possible_cpu(cpu) {
                 struct mnt_writer *cpu_writer = &per_cpu(mnt_writers, cpu);
-                if (cpu_writer->mnt != mnt)
-                        continue;
                 spin_lock(&cpu_writer->lock);
+                if (cpu_writer->mnt != mnt) {
+                        spin_unlock(&cpu_writer->lock);
+                        continue;
+                }
                 atomic_add(cpu_writer->count, &mnt->__mnt_writers);
                 cpu_writer->count = 0;
                 /*