On 2007.06.01 00:59:11 +0200, Ian Kumlien wrote:
> On tor, 2007-05-31 at 17:27 +0200, Björn Steinbrink wrote:
> > On 2007.05.31 17:10:07 +0200, Eric Dumazet wrote:
> > > Well... :) , there is still a memory barrier missing it seems.
> > >
> > > Another cpu might see a bad value if 'active=1' is set before tstat_hash_table is really cleared.
> >
> > Hm, that even makes the assumption in my first try valid ;-)
> > Just for the record, this time I thought that the barrier from the
> > spinlock in timer_stats_update_stats (right before the check for active)
> > would be enough, but that's obviously running on the wrong cpu if we
> > race... *sigh*
> >
>
> Fix the comment below and you can add:
> Signed-off-by: Ian Kumlien <[email protected]>
>
> It's currently been running for the longest period ever, ie, 11 minutes
> =)
Finally! :-)
> I'm gonna leave it running during the night and send a status update
> when the evil daystar reaches it's peak CET. (i haven't been able to
> stop since linus mentioned it... damn it... =))
>
> > @@ -360,6 +364,7 @@ static ssize_t tstats_write(struct file *file, const char __user *buf,
> > if (!active) {
> > reset_entries();
> > time_start = ktime_get();
> > + smb_mb();
>
> smb? you mean smp =)
No idea how I managed to break that after test compiling it... :-/
Thanks for catching it :-)
So unless I messed up something again, here's the final patch.
Thanks,
Björn
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: Björn Steinbrink <[email protected]>
Signed-off-by: Ian Kumlien <[email protected]>
---
diff --git a/kernel/time/timer_stats.c b/kernel/time/timer_stats.c
index 868f1bc..fa3d380 100644
--- 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;
-
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