On Mon, 2015-09-21 at 06:31 -0700, Eric Dumazet wrote:
> On Mon, 2015-09-21 at 10:08 +0200, Dmitry Vyukov wrote:
> > rhashtable_rehash_one() uses plain writes to update entry->next,
> > while it is being concurrently accessed by readers.
> > Unfortunately, the compiler is within its rights to (for example) use
> > byte-at-a-time writes to update the pointer, which would fatally confuse
> > concurrent readers.
> >
> This is bogus.
>
> 1) Linux is certainly not working if some arch or compiler is not doing
> single word writes. WRITE_ONCE() would not help at all to enforce this.
>
> 2) If new node is not yet visible, we don't care if we write
> entry->next using any kind of operation.
>
> So the WRITE_ONCE() is not needed at all.
>
>
>
> > + WRITE_ONCE(entry->next, head);
>
>
> The rcu_assign_pointer() immediately following is enough in this case.
>
> We have hundred of similar cases in the kernel.
>
>
The changelog and comment are totally confusing.
Please remove the bogus parts in them, and/or rephrase.
The important part here is that we rehash an item, so we need to make
sure to maintain consistent ->next field, and need to prevent compiler
from using ->next as a temporary variable.
ptr->next = 1UL | ((base + offset) << 1);
Is dangerous because compiler could issue :
ptr->next = (base + offset);
ptr->next <<= 1;
ptr->next += 1UL;
Frankly, all this looks like an oversight in this code.
Not sure why the NULLS value is even recomputed.
diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index cc0c69710dcf..0a29f07ba45a 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -187,10 +187,7 @@ static int rhashtable_rehash_one(struct rhashtable *ht,
unsigned int old_hash)
head = rht_dereference_bucket(new_tbl->buckets[new_hash],
new_tbl, new_hash);
- if (rht_is_a_nulls(head))
- INIT_RHT_NULLS_HEAD(entry->next, ht, new_hash);
- else
- RCU_INIT_POINTER(entry->next, head);
+ RCU_INIT_POINTER(entry->next, head);
rcu_assign_pointer(new_tbl->buckets[new_hash], entry);
spin_unlock(new_bucket_lock);
--
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