proc: loadavg reading race

The avenrun[] values are supposed to be protected by xtime_lock.
loadavg_read_proc does not use it.  Theoretically this may result in an
occasional glitch when the value read from /proc/loadavg would be as much
as 1<<11 times higher than it should be.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/proc/proc_misc.c b/fs/proc/proc_misc.c
index 383ff06..2686592 100644 (file)
--- a/fs/proc/proc_misc.c
+++ b/fs/proc/proc_misc.c
@@ -84,10 +84,15 @@ static int loadavg_read_proc(char *page, char **start, off_t off,
 {
        int a, b, c;
        int len;
+       unsigned long seq;
+
+       do {
+               seq = read_seqbegin(&xtime_lock);
+               a = avenrun[0] + (FIXED_1/200);
+               b = avenrun[1] + (FIXED_1/200);
+               c = avenrun[2] + (FIXED_1/200);
+       } while (read_seqretry(&xtime_lock, seq));
 
-       a = avenrun[0] + (FIXED_1/200);
-       b = avenrun[1] + (FIXED_1/200);
-       c = avenrun[2] + (FIXED_1/200);
        len = sprintf(page,"%d.%02d %d.%02d %d.%02d %ld/%d %d\n",
                LOAD_INT(a), LOAD_FRAC(a),
                LOAD_INT(b), LOAD_FRAC(b),