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x86-64, rwsem: Avoid store forwarding hazard in __downgrade_write

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The Intel Architecture Optimization Reference Manual states that a short
load that follows a long store to the same object will suffer a store
forwading penalty, particularly if the two accesses use different addresses.
Trivially, a long load that follows a short store will also suffer a penalty.

__downgrade_write() in rwsem incurs both penalties:  the increment operation
will not be able to reuse a recently-loaded rwsem value, and its result will
not be reused by any recently-following rwsem operation.

A comment in the code states that this is because 64-bit immediates are
special and expensive; but while they are slightly special (only a single
instruction allows them), they aren't expensive: a test shows that two loops,
one loading a 32-bit immediate and one loading a 64-bit immediate, both take
1.5 cycles per iteration.

Fix this by changing __downgrade_write to use the same add instruction on
i386 and on x86_64, so that it uses the same operand size as all the other
rwsem functions.

Signed-off-by: Avi Kivity <avi@redhat.com>
LKML-Reference: <1266049992-17419-1-git-send-email-avi@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
This commit is contained in:
Avi Kivity 2010-02-13 10:33:12 +02:00 committed by H. Peter Anvin
parent 1838ef1d78
commit 0d1622d7f5
1 changed files with 5 additions and 20 deletions
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25
arch/x86/include/asm/rwsem.h
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@ -232,34 +232,19 @@ static inline void __up_write(struct rw_semaphore *sem)
*/
static inline void __downgrade_write(struct rw_semaphore *sem)
{
#ifdef CONFIG_X86_64
# if RWSEM_WAITING_BIAS != -0x100000000
# error "This code assumes RWSEM_WAITING_BIAS == -2^32"
# endif
/* 64-bit immediates are special and expensive, and not needed here */
asm volatile("# beginning __downgrade_write\n\t"
LOCK_PREFIX "incl 4(%1)\n\t"
/* transitions 0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 */
" jns 1f\n\t"
" call call_rwsem_downgrade_wake\n"
"1:\n\t"
"# ending __downgrade_write\n"
: "+m" (sem->count)
: "a" (sem)
: "memory", "cc");
#else
asm volatile("# beginning __downgrade_write\n\t"
LOCK_PREFIX _ASM_ADD "%2,(%1)\n\t"
/* transitions 0xZZZZ0001 -> 0xYYYY0001 */
/*
* transitions 0xZZZZ0001 -> 0xYYYY0001 (i386)
* 0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 (x86_64)
*/
" jns 1f\n\t"
" call call_rwsem_downgrade_wake\n"
"1:\n\t"
"# ending __downgrade_write\n"
: "+m" (sem->count)
: "a" (sem), "i" (-RWSEM_WAITING_BIAS)
: "a" (sem), "er" (-RWSEM_WAITING_BIAS)
: "memory", "cc");
#endif
}
/*