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futex: Unbreak futex hashing

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commit 8d67743653dce5a0e7aa500fcccb237cde7ad88e upstream.

The recent futex inode life time fix changed the ordering of the futex key
union struct members, but forgot to adjust the hash function accordingly,

As a result the hashing omits the leading 64bit and even hashes beyond the
futex key causing a bad hash distribution which led to a ~100% performance
regression.

Hand in the futex key pointer instead of a random struct member and make
the size calculation based of the struct offset.

Fixes: 8019ad13ef7f ("futex: Fix inode life-time issue")
Reported-by: Rong Chen <[email protected]>
Decoded-by: Linus Torvalds <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Rong Chen <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>
This commit is contained in:
Thomas Gleixner 2020-03-08 19:07:17 +01:00 • committed by Greg Kroah-Hartman
commit 163489b643
1 changed files with 2 additions and 2 deletions
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4
kernel/futex.c
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@ -385,9 +385,9 @@ static inline int hb_waiters_pending(struct futex_hash_bucket *hb)
*/
static struct futex_hash_bucket *hash_futex(union futex_key *key)
{
u32 hash = jhash2((u32*)&key->both.word,
(sizeof(key->both.word)+sizeof(key->both.ptr))/4,
u32 hash = jhash2((u32 *)key, offsetof(typeof(*key), both.offset) / 4,
key->both.offset);
return &futex_queues[hash & (futex_hashsize - 1)];
}