On Tue, Feb 04, 2014 at 08:29:36AM -0800, Linus Torvalds wrote:
> On Tue, Feb 4, 2014 at 4:22 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > The below patch assumes the SDM is right (TM), and fixes the atomic_t,
> > cmpxchg() and xchg() implementations by inserting a mf before the
> > cmpxchg.acq (or xchg).
>
> You picked the wrong thing to be right. The SDM is wrong.
Figured, just my luck :-)
> Last time this came up, Tony explained it thus:
>
> >> Worse still - early processor implementations actually just ignored
> >> the acquire/release and did a full fence all the time. Unfortunately
> >> this meant a lot of badly written code that used .acq when they really
> >> wanted .rel became legacy out in the wild - so when we made a cpu
> >> that strictly did the .acq or .rel ... all that code started breaking - so
> >> we had to back-pedal and keep the "legacy" behavior of a full fence :-(
That would make a lovely comment near ia64_cmpxchg().
> and since ia64 is basically on life support as an architecture, we can
> pretty much agree that the SDM is dead, and the only thing that
> matters is implementation.
>
> The above quote was strictly in the context of just cmpxchg, though,
> so it's possible that the "fetchadd" instruction acts differently. I
> would personally expect it to have the same issues, but let's see what
> Tony says.. Tony?
I would suspect it to be a full fence too, let me do the reverse patch.
---
--- a/arch/ia64/include/asm/bitops.h
+++ b/arch/ia64/include/asm/bitops.h
@@ -65,11 +65,8 @@ __set_bit (int nr, volatile void *addr)
*((__u32 *) addr + (nr >> 5)) |= (1 << (nr & 31));
}
-/*
- * clear_bit() has "acquire" semantics.
- */
-#define smp_mb__before_clear_bit() smp_mb()
-#define smp_mb__after_clear_bit() do { /* skip */; } while (0)
+#define smp_mb__before_clear_bit() barrier();
+#define smp_mb__after_clear_bit() barrier();
/**
* clear_bit - Clears a bit in memory
--- a/arch/ia64/include/uapi/asm/cmpxchg.h
+++ b/arch/ia64/include/uapi/asm/cmpxchg.h
@@ -118,6 +118,15 @@ extern long ia64_cmpxchg_called_with_bad
#define cmpxchg_rel(ptr, o, n) \
ia64_cmpxchg(rel, (ptr), (o), (n), sizeof(*(ptr)))
+/*
+ * Worse still - early processor implementations actually just ignored
+ * the acquire/release and did a full fence all the time. Unfortunately
+ * this meant a lot of badly written code that used .acq when they really
+ * wanted .rel became legacy out in the wild - so when we made a cpu
+ * that strictly did the .acq or .rel ... all that code started breaking - so
+ * we had to back-pedal and keep the "legacy" behavior of a full fence :-(
+ */
+
/* for compatibility with other platforms: */
#define cmpxchg(ptr, o, n) cmpxchg_acq((ptr), (o), (n))
#define cmpxchg64(ptr, o, n) cmpxchg_acq((ptr), (o), (n))
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