Messages in this thread Patch in this message | | | Subject | Re: [PATCH] lib: fix data race in rhashtable_rehash_one | From | Eric Dumazet <> | Date | Mon, 21 Sep 2015 16:03:38 -0700 |
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On Tue, 2015-09-22 at 00:25 +0200, Thomas Graf wrote: > On 09/21/15 at 07:51am, Eric Dumazet wrote: > > 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. > > The hash of the chain is part of the NULLS value. Since the > entry might have been moved to a different chain, the NULLS > value must be recalculated to contain the proper hash. > > However, nobody is using the hash today as far as I can > see so we could as well just remove it and use the base > value only for the nulls marker.
What I said is :
In @head you already have the correct nulls value, from hash table.
You do not need to recompute this value, and/or test if hash table chain is empty.
If hash bucket is empty, it contains the appropriate NULLS value.
If you are paranoiac add this debugging check :
if (rht_is_a_nulls(head)) BUG_ON(head != (struct rhash_head *)rht_marker(ht, new_hash));
Therefore, simply fix the bug and unnecessary code with :
diff --git a/lib/rhashtable.c b/lib/rhashtable.c index cc0c69710dcf..a54ff8949f91 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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