[patch 27/54] timer statistics: fix race

Chris Wright
2007-06-08T07:56:15Z

[-- Attachment #1: timer-statistics-fix-race.patch --]
[-- Type: text/plain, Size: 2704 bytes --]

-stable review patch. If anyone has any objections, please let us know.
---------------------

From: Bjorn Steinbrink <B.Steinbrink@gmx.de>

Fix two races in the timer stats lookup code. One by ensuring that the
initialization of a new entry is finished upon insertion of that entry.
The other by cleaning up the hash table when the entries array is cleared,
so that we don't have any "pre-inserted" entries.

Thanks to Eric Dumazet for reminding me of the memory barriers.

Signed-off-by: Bjorn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: Ian Kumlien <pomac@vapor.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: Eric Dumazet <dada1@cosmosbay.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>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
kernel/time/timer_stats.c | 37 +++++++++++++++++++++----------------
1 file changed, 21 insertions(+), 16 deletions(-)

--- linux-2.6.21.4.orig/kernel/time/timer_stats.c
+++ linux-2.6.21.4/kernel/time/timer_stats.c
@@ -117,21 +117,6 @@ static struct entry entries[MAX_ENTRIES]

static atomic_t overflow_count;

-static void reset_entries(void)
-{
- nr_entries = 0;
- memset(entries, 0, sizeof(entries));
- atomic_set(&overflow_count, 0);
-}
-
-static struct entry *alloc_entry(void)
-{
- if (nr_entries >= MAX_ENTRIES)
- return NULL;
-
- return entries + nr_entries++;
-}
-
/*
* The entries are in a hash-table, for fast lookup:
*/
@@ -149,6 +134,22 @@ static struct entry *alloc_entry(void)

static struct entry *tstat_hash_table[TSTAT_HASH_SIZE] __read_mostly;

+static void reset_entries(void)
+{
+ nr_entries = 0;
+ memset(entries, 0, sizeof(entries));
+ memset(tstat_hash_table, 0, sizeof(tstat_hash_table));
+ atomic_set(&overflow_count, 0);
+}
+
+static struct entry *alloc_entry(void)
+{
+ if (nr_entries >= MAX_ENTRIES)
+ return NULL;
+
+ return entries + nr_entries++;
+}
+
static int match_entries(struct entry *entry1, struct entry *entry2)
{
return entry1->timer == entry2->timer &&
@@ -202,12 +203,15 @@ static struct entry *tstat_lookup(struct
if (curr) {
*curr = *entry;
curr->count = 0;
+ curr->next = NULL;
memcpy(curr->comm, comm, TASK_COMM_LEN);
+
+ smp_mb(); /* Ensure that curr is initialized before insert */
+
if (prev)
prev->next = curr;
else
*head = curr;
- curr->next = NULL;
}
out_unlock:
spin_unlock(&table_lock);
@@ -364,6 +368,7 @@ static ssize_t tstats_write(struct file
if (!active) {
reset_entries();
time_start = ktime_get();
+ smp_mb();
active = 1;
}
break;

--