Re: spin_lock forgets to clobber memory and other smp fixes [was Re: [patch] waitqueue optimization, 2.4.0-test7]

From: Jamie Lokier (
Date: Thu Sep 07 2000 - 11:46:29 EST

Linus Torvalds wrote:
> "volatile" should be equivalent to clobbering memory, although the gcc
> manual pages are certainly not very verbose on the issue.

It isn't. Try the following with/without the memory clobber:

int *p;
int func()
  int x;
  x = *p;
  __asm__ __volatile__ ("" : : : "memory");
  x = *p;
  return x;

With or without, the program reads *p only once. However, with the
clobber it reads _after_ the asm; without, it reads _before_ the asm.

It's ok for the compiler to do that (given we don't know what "volatile"
means anyway :-). But it does have implications for spin_lock:
spin_lock must say that it clobbers memory.

spin_unlock should also say it clobbers memory but I have no test case
to demonstrate the compiler moving reads down past an asm.

-- Jamie
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