[PATCH] proc: loadavg reading race

[PATCH] proc: loadavg reading race

[Michal Schmidt]

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>
---
 fs/proc/proc_misc.c |   11 ++++++++---
 1 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/proc/proc_misc.c b/fs/proc/proc_misc.c
index e0d064e..10cc9ad 100644
--- a/fs/proc/proc_misc.c
+++ b/fs/proc/proc_misc.c
@@ -83,10 +83,15 @@ 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),
-- 
1.5.3.3