Re: [patch 134/149] x86, paravirt: Add a global synchronization point for pvclock

Peter Palfrader
2010-07-13T18:23:36Z

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On Tue, 13 Jul 2010, Linus Torvalds wrote:

> On Tue, Jul 13, 2010 at 10:50 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > No, you didn't back-port it wrong. I just didn't fix the right part. I
> > thought the PV code used xchg, not cmpxchg, so I only patched that.
> > But cmpxchg has the exact same issue.
> >
> > Does this fix it?

It appears to, thanks a lot.

[git|v2.6.32.16] weasel@thelma:/scratch/kernel/2.6.32.16$ nm arch/x86/kernel/pvclock.o
0000000000000000 b last_value
..

And it did boot too.

> Btw, this second patch was a bit more aggressive than the first one,
> and actually removes the "memory" clobber entirely, and the fake cast
> of the target type.

Without the cast gcc spews a fair amount of warnings. About four every
time it hits the include file.

Just for completeness' sake I attached the patch on top of 2.6.32.16
that I built with.
--
| .''`. ** Debian GNU/Linux **
Peter Palfrader | : :' : The universal
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diff --git a/arch/x86/include/asm/cmpxchg_64.h b/arch/x86/include/asm/cmpxchg_64.h
index 52de72e..f0551c5 100644
--- a/arch/x86/include/asm/cmpxchg_64.h
+++ b/arch/x86/include/asm/cmpxchg_64.h
@@ -26,26 +26,26 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr,
switch (size) {
case 1:
asm volatile("xchgb %b0,%1"
- : "=q" (x)
- : "m" (*__xg(ptr)), "0" (x)
+ : "=q" (x), "+m" (*__xg(ptr))
+ : "0" (x)
: "memory");
break;
case 2:
asm volatile("xchgw %w0,%1"
- : "=r" (x)
- : "m" (*__xg(ptr)), "0" (x)
+ : "=r" (x), "+m" (*__xg(ptr))
+ : "0" (x)
: "memory");
break;
case 4:
asm volatile("xchgl %k0,%1"
- : "=r" (x)
- : "m" (*__xg(ptr)), "0" (x)
+ : "=r" (x), "+m" (*__xg(ptr))
+ : "0" (x)
: "memory");
break;
case 8:
asm volatile("xchgq %0,%1"
- : "=r" (x)
- : "m" (*__xg(ptr)), "0" (x)
+ : "=r" (x), "+m" (*__xg(ptr))
+ : "0" (x)
: "memory");
break;
}
@@ -66,28 +66,24 @@ static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
unsigned long prev;
switch (size) {
case 1:
- asm volatile(LOCK_PREFIX "cmpxchgb %b1,%2"
- : "=a"(prev)
- : "q"(new), "m"(*__xg(ptr)), "0"(old)
- : "memory");
+ asm volatile(LOCK_PREFIX "cmpxchgb %b2,%1"
+ : "=a"(prev), "+m" (*__xg(ptr))
+ : "q"(new), "0"(old));
return prev;
case 2:
- asm volatile(LOCK_PREFIX "cmpxchgw %w1,%2"
- : "=a"(prev)
- : "r"(new), "m"(*__xg(ptr)), "0"(old)
- : "memory");
+ asm volatile(LOCK_PREFIX "cmpxchgw %w2,%1"
+ : "=a"(prev), "+m" (*__xg(ptr))
+ : "r"(new), "0"(old));
return prev;
case 4:
- asm volatile(LOCK_PREFIX "cmpxchgl %k1,%2"
- : "=a"(prev)
- : "r"(new), "m"(*__xg(ptr)), "0"(old)
- : "memory");
+ asm volatile(LOCK_PREFIX "cmpxchgl %k2,%1"
+ : "=a"(prev), "+m" (*__xg(ptr))
+ : "r"(new), "0"(old));
return prev;
case 8:
- asm volatile(LOCK_PREFIX "cmpxchgq %1,%2"
- : "=a"(prev)
- : "r"(new), "m"(*__xg(ptr)), "0"(old)
- : "memory");
+ asm volatile(LOCK_PREFIX "cmpxchgq %2,%1"
+ : "=a"(prev), "+m" (*__xg(ptr))
+ : "r"(new), "0"(old));
return prev;
}
return old;