Re: [dm-devel] [patch] dm-raid1.c fix a race bug in __rh_alloc()

["Jun'ichi Nomura" <j-nomura@xxxxxxxxxxxxx>]

Hi,

Jonathan E Brassow wrote:
> I believe this also fixes Jun'ichi's issue (*[dm-devel] [PATCH]
> 2.6.12-rc6: fix __rh_alloc()/rh_update_states() race in dm-raid1.c)

Unfortunately, it doesn't eliminate the possibility of the region
being freed during lock conversion (though the possibility becomes
very very low) and also it will cause other problems.

I modified his patch with fixes below
  - spin_lock should be spin_lock_irq to avoid deadlock,
  - needs to check pending count to avoid moving dirty region to clean list,
and added retry code.

This one should work. How about this?

However, I think we need to find smarter fix not to depend on retry.

Thanks,

Jonathan E Brassow wrote:
> I believe this also fixes Jun'ichi's issue (*[dm-devel] [PATCH] 
> 2.6.12-rc6: fix __rh_alloc()/rh_update_states() race in dm-raid1.c)
>
> brassow
> *
> On Jun 16, 2005, at 9:21 PM, Zhao Qian wrote:
>
>     after write_unlock_irq and just before read_lock, there's a small
>     window which enables a race causing deletion of the region struct in
>     function rh_update_states(). then in rh_dec(), the __rh_lookup()
>     will return null, causing kernel panic.

diff --git a/drivers/md/dm-raid1.c b/drivers/md/dm-raid1.c
--- a/drivers/md/dm-raid1.c
+++ b/drivers/md/dm-raid1.c
@@ -232,6 +232,7 @@ static struct region *__rh_alloc(struct 
 {
 	struct region *reg, *nreg;
 
+retry:
 	read_unlock(&rh->hash_lock);
 	nreg = mempool_alloc(rh->region_pool, GFP_NOIO);
 	nreg->state = rh->log->type->in_sync(rh->log, region, 1) ?
@@ -252,15 +253,19 @@ static struct region *__rh_alloc(struct 
 
 	else {
 		__rh_insert(rh, nreg);
-		if (nreg->state == RH_CLEAN) {
-			spin_lock(&rh->region_lock);
-			list_add(&nreg->list, &rh->clean_regions);
-			spin_unlock(&rh->region_lock);
-		}
 		reg = nreg;
 	}
 	write_unlock_irq(&rh->hash_lock);
 	read_lock(&rh->hash_lock);
+	/* rare, but it's possible the region is freed by other cpu */
+	if (reg != __rh_lookup(rh, region))
+		goto retry;
+	if (reg->state == RH_CLEAN) {
+		spin_lock_irq(&rh->region_lock);
+		if (!atomic_read(&reg->pending) && list_empty(&reg->list))
+			list_add(&reg->list, &rh->clean_regions);
+		spin_unlock_irq(&rh->region_lock);
+	}
 
 	return reg;
 }