netdev - rhashtable: Fix walker list corruption

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Message-ID: <20151216084554.GA24395@gondor.apana.org.au>
Date:	Wed, 16 Dec 2015 16:45:54 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Colin Ian King <colin.king@...onical.com>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: rhashtable: Fix walker list corruption

On Fri, Oct 09, 2015 at 11:32:23AM +0100, Colin Ian King wrote:
> 
> I'm hitting a null ptr deference bug when running 2 or more instances of
> the attached reproducer program.  I've bisected this down to the
> following commit:
> 
> commit ba7c95ea3870fe7b847466d39a049ab6f156aa2c
> Author: Herbert Xu <herbert@...dor.apana.org.au>
> Date:   Tue Mar 24 09:53:17 2015 +1100
> 
>     rhashtable: Fix sleeping inside RCU critical section in walk_stop
> 
> 
> Without this commit, the attached reproducer runs fine for hours. With
> the commit, I can oops a 4 core (8 thread) Intel i7-6700 Sharkbay SDP in
> a few seconds.

Thanks Colin.  This commit was indeed bogus, as we end up using
two different locks for the one list.

---8<---
The commit ba7c95ea3870fe7b847466d39a049ab6f156aa2c ("rhashtable:
Fix sleeping inside RCU critical section in walk_stop") introduced
a new spinlock for the walker list.  However, it did not convert
all existing users of the list over to the new spin lock.  Some
continued to use the old mutext for this purpose.  This obviously
led to corruption of the list.

The fix is to use the spin lock everywhere where we touch the list.

This also allows us to do rcu_rad_lock before we take the lock in
rhashtable_walk_start.  With the old mutex this would've deadlocked
but it's safe with the new spin lock.

Fixes: ba7c95ea3870 ("rhashtable: Fix sleeping inside RCU...")
Reported-by: Colin Ian King <colin.king@...onical.com>
Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 1c624db..ed7ba47 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -519,10 +519,10 @@ int rhashtable_walk_init(struct rhashtable *ht, struct rhashtable_iter *iter)
 	if (!iter->walker)
 		return -ENOMEM;
 
-	mutex_lock(&ht->mutex);
+	spin_lock(&ht->lock);
 	iter->walker->tbl = rht_dereference(ht->tbl, ht);
 	list_add(&iter->walker->list, &iter->walker->tbl->walkers);
-	mutex_unlock(&ht->mutex);
+	spin_unlock(&ht->lock);
 
 	return 0;
 }
@@ -536,10 +536,10 @@ EXPORT_SYMBOL_GPL(rhashtable_walk_init);
  */
 void rhashtable_walk_exit(struct rhashtable_iter *iter)
 {
-	mutex_lock(&iter->ht->mutex);
+	spin_lock(&iter->ht->lock);
 	if (iter->walker->tbl)
 		list_del(&iter->walker->list);
-	mutex_unlock(&iter->ht->mutex);
+	spin_unlock(&iter->ht->lock);
 	kfree(iter->walker);
 }
 EXPORT_SYMBOL_GPL(rhashtable_walk_exit);
@@ -563,14 +563,12 @@ int rhashtable_walk_start(struct rhashtable_iter *iter)
 {
 	struct rhashtable *ht = iter->ht;
 
-	mutex_lock(&ht->mutex);
+	rcu_read_lock();
 
+	spin_lock(&ht->lock);
 	if (iter->walker->tbl)
 		list_del(&iter->walker->list);
-
-	rcu_read_lock();
-
-	mutex_unlock(&ht->mutex);
+	spin_unlock(&ht->lock);
 
 	if (!iter->walker->tbl) {
 		iter->walker->tbl = rht_dereference_rcu(ht->tbl, ht);
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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