[patch] i386 spinlocks: disable interrupts only if we enabled them

Chuck Ebbert
2006-03-07T23:40:29Z

_raw_spin_lock_flags() is entered with interrupts disabled. If it
cannot obtain a spinlock, it checks the flags that were passed and
re-enables interrupts before spinning if that's how the flags are set.
When the spinlock might be available, it disables interrupts (even if
they are already disabled) before trying to get the lock. Change that
so interrupts are only disabled if they have been enabled. This costs
nine bytes of duplicated spinloop code.

Fastpath before patch:
jle <keep looping> not-taken conditional jump
cli disable interrupts
jmp <try for lock> unconditional jump

Fastpath after patch, if interrupts were not enabled:
jg <try for lock> taken conditional branch

Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>

--- 2.6.16-rc5-d2.orig/include/asm-i386/spinlock.h
+++ 2.6.16-rc5-d2/include/asm-i386/spinlock.h
@@ -35,18 +35,23 @@
#define __raw_spin_lock_string_flags \
"\n1:\t" \
"lock ; decb %0\n\t" \
- "jns 4f\n\t" \
+ "jns 5f\n" \
"2:\t" \
"testl $0x200, %1\n\t" \
- "jz 3f\n\t" \
- "sti\n\t" \
+ "jz 4f\n\t" \
+ "sti\n" \
"3:\t" \
"rep;nop\n\t" \
"cmpb $0, %0\n\t" \
"jle 3b\n\t" \
"cli\n\t" \
"jmp 1b\n" \
- "4:\n\t"
+ "4:\t" \
+ "rep;nop\n\t" \
+ "cmpb $0, %0\n\t" \
+ "jg 1b\n\t" \
+ "jmp 4b\n" \
+ "5:\n\t"

static inline void __raw_spin_lock(raw_spinlock_t *lock)
{
--
Chuck
"Penguins don't come from next door, they come from the Antarctic!"