On Fri, 19 Oct 2007, Nick Piggin wrote:
> I'm not sure, I had an idea it was relatively expensive on ia64,
> but I didn't really test with a good workload (a microbenchmark
> probably isn't that good because it won't generate too much out
> of order memory traffic that needs to be fenced).
Its expensive on IA64. Is it any less expensive on x86?
> > Where can I find your patchset? I looked through lkml but did not see it.
>
> Infrastructure in -mm, starting at bitops-introduce-lock-ops.patch.
> bit_spin_lock-use-lock-bitops.patch and ia64-lock-bitops.patch are
> ones to look at.
ia64-lock-bitops.patch defines:
static __inline__ void
clear_bit_unlock (int nr, volatile void *addr)
{
__u32 mask, old, new;
volatile __u32 *m;
CMPXCHG_BUGCHECK_DECL
m = (volatile __u32 *) addr + (nr >> 5);
mask = ~(1 << (nr & 31));
do {
CMPXCHG_BUGCHECK(m);
old = *m;
new = old & mask;
} while (cmpxchg_rel(m, old, new) != old);
}
/**
* __clear_bit_unlock - Non-atomically clear a bit with release
*
* This is like clear_bit_unlock, but the implementation may use a non-atomic
* store (this one uses an atomic, however).
*/
#define __clear_bit_unlock clear_bit_unlock
A non atomic store is a misaligned store on IA64. That is not
relevant here. The data is properly aligned. I guess it was intended to
refer to the cmpxchg.
How about this patch? [Works fine on IA64 simulator...]
IA64: Slim down __clear_bit_unlock
__clear_bit_unlock does not need to perform atomic operations on the variable.
Avoid a cmpxchg and simply do a store with release semantics. Add a barrier to
be safe that the compiler does not do funky things.
Signed-off-by: Christoph Lameter <[email protected]>
---
include/asm-ia64/bitops.h | 12 ++++++++++++
1 file changed, 12 insertions(+)
Index: linux-2.6.23-mm1/include/asm-ia64/bitops.h
===================================================================
--- linux-2.6.23-mm1.orig/include/asm-ia64/bitops.h 2007-10-18 19:37:22.000000000 -0700
+++ linux-2.6.23-mm1/include/asm-ia64/bitops.h 2007-10-18 19:50:22.000000000 -0700
@@ -124,10 +124,21 @@ clear_bit_unlock (int nr, volatile void
/**
* __clear_bit_unlock - Non-atomically clear a bit with release
*
- * This is like clear_bit_unlock, but the implementation may use a non-atomic
- * store (this one uses an atomic, however).
+ * This is like clear_bit_unlock, but the implementation uses a store
+ * with release semantics. See also __raw_spin_unlock().
*/
-#define __clear_bit_unlock clear_bit_unlock
+static __inline__ void
+__clear_bit_unlock (int nr, volatile void *addr)
+{
+ __u32 mask, new;
+ volatile __u32 *m;
+
+ m = (volatile __u32 *) addr + (nr >> 5);
+ mask = ~(1 << (nr & 31));
+ new = *m & mask;
+ barrier();
+ asm volatile ("st4.rel.nta [%0] = %1\n\t" :: "r"(m), "r"(new));
+}
/**
* __clear_bit - Clears a bit in memory (non-atomic version)
-
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