Check for overflow of the freeoffset version number.

I just thought adding this check in CONFIG_SLUB_DEBUG makes sense. It's
really unlikely that enough interrupt handlers will nest over the slub
fast path, and each of them do about a million alloc/free on 32 bits or
a huge amount of alloc/free on 64 bits, but just in case, it seems good
to warn if we detect we are half-way to a version overflow.

Changelog :
- Mask out the LSB because of alloc fast path. See comment in source.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
---
 mm/slub.c |   40 ++++++++++++++++++++++++++++++++++------
 1 file changed, 34 insertions(+), 6 deletions(-)

Index: linux-2.6-lttng/mm/slub.c
===================================================================
--- linux-2.6-lttng.orig/mm/slub.c	2008-03-04 00:59:01.000000000 -0500
+++ linux-2.6-lttng/mm/slub.c	2008-03-04 01:03:44.000000000 -0500
@@ -1660,7 +1660,7 @@ static __always_inline void *slab_alloc(
  */
 
 #ifdef SLUB_FASTPATH
-	unsigned long freeoffset, newoffset;
+	unsigned long freeoffset, newoffset, resoffset;
 
 	c = get_cpu_slab(s, raw_smp_processor_id());
 	do {
@@ -1682,8 +1682,22 @@ static __always_inline void *slab_alloc(
 		newoffset = freeoffset;
 		newoffset &= ~c->off_mask;
 		newoffset |= (unsigned long)object[c->offset] & c->off_mask;
-	} while (cmpxchg_local(&c->freeoffset, freeoffset, newoffset)
-			!= freeoffset);
+		resoffset = cmpxchg_local(&c->freeoffset, freeoffset,
+				newoffset);
+#ifdef CONFIG_SLUB_DEBUG
+		/*
+		 * Just to be paranoid : warn if we detect that enough free or
+		 * slow paths nested on top of us to get the counter to go
+		 * half-way to overflow. That would be insane to do that much
+		 * allocations/free in interrupt handers, but check it anyway.
+		 * Mask out the LSBs because alloc fast path does not increment
+		 * the sequence number, which may cause the overall values to go
+		 * backward.
+		 */
+		WARN_ON((resoffset & ~c->off_mask)
+			- (freeoffset & ~c->off_mask) > -1UL >> 1);
+#endif
+	} while (resoffset != freeoffset);
 #else
 	unsigned long flags;
 
@@ -1822,7 +1836,7 @@ static __always_inline void slab_free(st
 	struct kmem_cache_cpu *c;
 
 #ifdef SLUB_FASTPATH
-	unsigned long freeoffset, newoffset;
+	unsigned long freeoffset, newoffset, resoffset;
 
 	c = get_cpu_slab(s, raw_smp_processor_id());
 	debug_check_no_locks_freed(object, s->objsize);
@@ -1850,8 +1864,22 @@ static __always_inline void slab_free(st
 		newoffset = freeoffset + c->off_mask + 1;
 		newoffset &= ~c->off_mask;
 		newoffset |= (unsigned long)object & c->off_mask;
-	} while (cmpxchg_local(&c->freeoffset, freeoffset, newoffset)
-			!= freeoffset);
+		resoffset = cmpxchg_local(&c->freeoffset, freeoffset,
+				newoffset);
+#ifdef CONFIG_SLUB_DEBUG
+		/*
+		 * Just to be paranoid : warn if we detect that enough free or
+		 * slow paths nested on top of us to get the counter to go
+		 * half-way to overflow. That would be insane to do that much
+		 * allocations/free in interrupt handers, but check it anyway.
+		 * Mask out the LSBs because alloc fast path does not increment
+		 * the sequence number, which may cause the overall values to go
+		 * backward.
+		 */
+		WARN_ON((resoffset & ~c->off_mask)
+			- (freeoffset & ~c->off_mask) > -1UL >> 1);
+#endif
+	} while (resoffset != freeoffset);
 #else
 	unsigned long flags;
 
-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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