The Linux kernel works based on barriers and not on acquire and release
semantics. We cannot expect an acquire or release of clear_bit() to
provide the synchronization behavior expected by core kernel developers.
Not having the proper barriers may introduce subtle breakage.
So change IA64 to provide full barriers for smp_mb__after_clear_bit.
This is also the way that powerpc (which faces similar issues) implements
these macros.
include/asm-powerpc/bitops.h:
/*
* clear_bit doesn't imply a memory barrier
*/
#define smp_mb__before_clear_bit() smp_mb()
#define smp_mb__after_clear_bit() smp_mb()
Signed-off-by: Christoph Lameter <clameter@xxxxxxx>
Index: linux-2.6.16-mm2/include/asm-ia64/bitops.h
===================================================================
--- linux-2.6.16-mm2.orig/include/asm-ia64/bitops.h 2006-03-30 21:21:04.000000000 -0800
+++ linux-2.6.16-mm2/include/asm-ia64/bitops.h 2006-03-31 11:17:53.000000000 -0800
@@ -63,10 +63,13 @@ __set_bit (int nr, volatile void *addr)
}
/*
- * clear_bit() has "acquire" semantics.
+ * The current linux locking paradigm is oriented towards
+ * barriers. We cannot utilize acquire or relase behaviors
+ * to synchronize but must implement full barriers in order
+ * to avoid potential subtle breakage.
*/
#define smp_mb__before_clear_bit() smp_mb()
-#define smp_mb__after_clear_bit() do { /* skip */; } while (0)
+#define smp_mb__after_clear_bit() smp_mb()
/**
* clear_bit - Clears a bit in memory
-
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