Commit 8d677436 authored Mar 08, 2020 by Thomas Gleixner

futex: Unbreak futex hashing



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: 8019ad13 ("futex: Fix inode life-time issue")
Reported-by: Rong Chen <rong.a.chen@intel.com>
Decoded-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Rong Chen <rong.a.chen@intel.com>
Link: https://lkml.kernel.org/r/87h7yy90ve.fsf@nanos.tec.linutronix.de

parent 8019ad13

kernel/futex.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -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)];
		}

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