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SubjectRe: Fix unlock_buffer() to work the same way as bit_unlock()
On Tue, 28 Mar 2006, Zoltan Menyhart wrote:

> Why not to use separate bit operations for different purposes?
>
> - e.g. "test_and_set_bit_N_acquire()" for lock acquisition
> - "test_and_set_bit()", "clear_bit()" as they are today
> - "release_N_clear_bit()"...
>

That would force IA64 specifics onto all other architectures.

Could we simply define these smb_mb__*_clear_bit to be noops
and then make the atomic bit ops to have full barriers? That would satisfy
Nick's objections.

Index: linux-2.6.16/include/asm-ia64/bitops.h
===================================================================
--- linux-2.6.16.orig/include/asm-ia64/bitops.h 2006-03-19 21:53:29.000000000 -0800
+++ linux-2.6.16/include/asm-ia64/bitops.h 2006-03-28 15:45:08.000000000 -0800
@@ -45,6 +45,7 @@
old = *m;
new = old | bit;
} while (cmpxchg_acq(m, old, new) != old);
+ smb_mb();
}

/**
@@ -65,7 +66,7 @@
/*
* clear_bit() has "acquire" semantics.
*/
-#define smp_mb__before_clear_bit() smp_mb()
+#define smp_mb__before_clear_bit() do { } while (0)
#define smp_mb__after_clear_bit() do { /* skip */; } while (0)

/**
@@ -92,6 +93,7 @@
old = *m;
new = old & mask;
} while (cmpxchg_acq(m, old, new) != old);
+ smp_mb();
}

/**
@@ -128,6 +130,7 @@
old = *m;
new = old ^ bit;
} while (cmpxchg_acq(m, old, new) != old);
+ smp_mb();
}

/**
@@ -167,6 +170,7 @@
old = *m;
new = old | bit;
} while (cmpxchg_acq(m, old, new) != old);
+ smp_mb();
return (old & bit) != 0;
}

@@ -212,6 +216,7 @@
old = *m;
new = old & mask;
} while (cmpxchg_acq(m, old, new) != old);
+ smp_mb();
return (old & ~mask) != 0;
}

@@ -257,6 +262,7 @@
old = *m;
new = old ^ bit;
} while (cmpxchg_acq(m, old, new) != old);
+ smp_mb();
return (old & bit) != 0;
}

-
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