After initialization, futex_hashsize is only used in hash_futex, where we subtract 1 to get the proper mask. So store that mask instead.
Or not: Unfortunately, this causes gcc to emit an 'and with memory op' instead of doing a load which can happen at least somewhat in parallel with the last few instructions computing hash, as this diff shows: -: 48 8b 15 a5 61 e4 00 mov 0xe461a5(%rip),%rdx # ffffffff81eff9f8 <__futex_data+0x8> : 31 c8 xor %ecx,%eax : c1 c9 08 ror $0x8,%ecx : 29 c8 sub %ecx,%eax -: 48 83 ea 01 sub $0x1,%rdx -: 48 21 d0 and %rdx,%rax +: 23 05 9f 61 e4 00 and 0xe4619f(%rip),%eax # ffffffff81eff9f8 <__futex_data+0x8> : 48 c1 e0 06 shl $0x6,%rax -: 48 03 05 84 61 e4 00 add 0xe46184(%rip),%rax # ffffffff81eff9f0 <__futex_data> +: 48 03 05 8c 61 e4 00 add 0xe4618c(%rip),%rax # ffffffff81eff9f0 <__futex_data> : c3 retq Not-signed-off-by: Rasmus Villemoes <[email protected]> --- kernel/futex.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) diff --git a/kernel/futex.c b/kernel/futex.c index c5f33bf78293..22865bcbcae8 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -255,16 +255,16 @@ struct futex_hash_bucket { } ____cacheline_aligned_in_smp; /* - * The base of the bucket array and its size are always used together + * The base of the bucket array and the mask are always used together * (after initialization only in hash_futex()), so ensure that they * reside in the same cacheline. */ static struct { struct futex_hash_bucket *queues; - unsigned long hashsize; + u32 mask; } __futex_data __read_mostly __aligned(16); #define futex_queues (__futex_data.queues) -#define futex_hashsize (__futex_data.hashsize) +#define futex_hashmask (__futex_data.mask) static inline void futex_get_mm(union futex_key *key) @@ -320,7 +320,7 @@ 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, key->both.offset); - return &futex_queues[hash & (futex_hashsize - 1)]; + return &futex_queues[hash & futex_hashmask]; } /* @@ -3035,6 +3035,7 @@ static int __init futex_init(void) { unsigned int futex_shift; unsigned long i; + unsigned long futex_hashsize; #if CONFIG_BASE_SMALL futex_hashsize = 16; @@ -3045,7 +3046,7 @@ static int __init futex_init(void) futex_queues = alloc_large_system_hash("futex", sizeof(*futex_queues), futex_hashsize, 0, futex_hashsize < 256 ? HASH_SMALL : 0, - &futex_shift, NULL, + &futex_shift, &futex_hashmask, futex_hashsize, futex_hashsize); futex_hashsize = 1UL << futex_shift; -- 2.1.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in Please read the FAQ at http://www.tux.org/lkml/
