| Date | Thu, 22 Apr 2010 12:09:33 -0700 | From | Greg KH <> | Subject | [122/197] x86-64, rwsem: Avoid store forwarding hazard in __downgrade_write |
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2.6.32-stable review patch. If anyone has any objections, please let us know.
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From: Avi Kivity <avi@redhat.com>
commit 0d1622d7f526311d87d7da2ee7dd14b73e45d3fc upstream.
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> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
--- arch/x86/include/asm/rwsem.h | 25 +++++-------------------- 1 file changed, 5 insertions(+), 20 deletions(-)
--- a/arch/x86/include/asm/rwsem.h +++ b/arch/x86/include/asm/rwsem.h @@ -232,34 +232,19 @@ static inline void __up_write(struct rw_ */ 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 } /*
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