Messages in this thread Patch in this message |  | | Date | Thu, 11 Oct 2001 21:36:55 +0100 | From | Mark Zealey <> | Subject | Question and patch about spinlocks (x86) |
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Just looking through at the spinlock assembly I noticed a few things which I think are bugs:
"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" \ ".previous"
We do the cmp loop as a 'soft' check, as the lock operand locks the whole system bus, stopping the system for a while (as much as 70 cycles, I believe). However, I don't understand why it was put before the 'rep; nop' which just sets the processor to wait for a bit. Surely it would be better to test *after* we have waited, as then we have a better chance of it being correct.
Any comments? Attached is a patch to fix it.
--
Mark Zealey (aka JALH on irc.openprojects.net: #zealos and many more) mark@zealos.org mark@itsolve.co.uk
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(www.geekcode.com) --- include/asm-i386/spinlock.h.old Thu Oct 11 21:28:37 2001 +++ include/asm-i386/spinlock.h Thu Oct 11 21:35:14 2001 @@ -58,8 +58,8 @@ "js 2f\n" \ ".section .text.lock,\"ax\"\n" \ "2:\t" \ - "cmpb $0,%0\n\t" \ "rep;nop\n\t" \ + "cmpb $0,%0\n\t" \ "jle 2b\n\t" \ "jmp 1b\n" \ ".previous" |  |