The Intel documentation recommends that spinlocks should use
loop:
rep;nop;
cmp $0,lock_var
jne loop
ftp://download.intel.com/design/perftool/cbts/appnotes/sse2/w_spinlock.pdf
but the linux spinlock implementation uses
loop:
cmp $0, lock_var
rep; nop;
jne loop.
Why?
According to the Intel documentation, rep nop is a predefined delay that
slows down the cpu to the speed of the memory bus. The linux
implementation will delay again _after_ the lock was released.
What about the attached patch? It also adds 'rep;nop' into the rw
spinlock implementation.
It boots with my Pentium III, but the cpu doesn't support 'rep nop'
(i.e. returns immediately)
-- Manfred
// $Header$ // Kernel Version: // VERSION = 2 // PATCHLEVEL = 4 // SUBLEVEL = 3 // EXTRAVERSION = -ac3 --- 2.4/arch/i386/kernel/semaphore.c Sun Nov 19 02:31:25 2000 +++ build-2.4/arch/i386/kernel/semaphore.c Sat Apr 7 21:26:35 2001 @@ -430,7 +430,8 @@ .globl __write_lock_failed __write_lock_failed: " LOCK "addl $" RW_LOCK_BIAS_STR ",(%eax) -1: cmpl $" RW_LOCK_BIAS_STR ",(%eax) +1: rep; nop + cmpl $" RW_LOCK_BIAS_STR ",(%eax) jne 1b " LOCK "subl $" RW_LOCK_BIAS_STR ",(%eax) @@ -442,7 +443,8 @@ .globl __read_lock_failed __read_lock_failed: lock ; incl (%eax) -1: cmpl $1,(%eax) +1: rep; nop + cmpl $1,(%eax) js 1b lock ; decl (%eax) --- 2.4/include/asm-i386/spinlock.h Sat Apr 7 22:02:23 2001 +++ build-2.4/include/asm-i386/spinlock.h Sat Apr 7 22:01:43 2001 @@ -58,10 +58,10 @@ "js 2f\n" \ ".section .text.lock,\"ax\"\n" \ "2:\t" \ - "cmpb $0,%0\n\t" \ "rep;nop\n\t" \ - "jle 2b\n\t" \ - "jmp 1b\n" \ + "cmpb $0,%0\n\t" \ + "jg 1b\n\t" \ + "jmp 2b\n" \ ".previous" /*
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This archive was generated by hypermail 2b29 : Sun Apr 15 2001 - 21:00:09 EST