timer statistics: fix race

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>

diff --git a/kernel/time/timer_stats.c b/kernel/time/timer_stats.c
index 868f1bc..fa3d380 100644 (file)
--- a/kernel/time/timer_stats.c
+++ b/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 entry *entry, char *comm)
        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);
@@ -360,6 +364,7 @@ static ssize_t tstats_write(struct file *file, const char __user *buf,
                if (!active) {
                        reset_entries();
                        time_start = ktime_get();
+                       smp_mb();
                        active = 1;
                }
                break;