slub: Fix the lockless code on 32-bit platforms with no 64-bit cmpxchg

The SLUB allocator use of the cmpxchg_double logic was wrong: it
actually needs the irq-safe one.

That happens automatically when we use the native unlocked 'cmpxchg8b'
instruction, but when compiling the kernel for older x86 CPUs that do
not support that instruction, we fall back to the generic emulation
code.

And if you don't specify that you want the irq-safe version, the generic
code ends up just open-coding the cmpxchg8b equivalent without any
protection against interrupts or preemption.  Which definitely doesn't
work for SLUB.

This was reported by Werner Landgraf <w.landgraf@ru.ru>, who saw
instability with his distro-kernel that was compiled to support pretty
much everything under the sun.  Most big Linux distributions tend to
compile for PPro and later, and would never have noticed this problem.

This also fixes the prototypes for the irqsafe cmpxchg_double functions
to use 'bool' like they should.

[ Btw, that whole "generic code defaults to no protection" design just
  sounds stupid - if the code needs no protection, there is no reason to
  use "cmpxchg_double" to begin with.  So we should probably just remove
  the unprotected version entirely as pointless.   - Linus ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reported-and-tested-by: werner <w.landgraf@ru.ru>
Acked-and-tested-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/alpine.LFD.2.02.1105041539050.3005@ionos
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/percpu.h

@@ -948,7 +948,7 @@ do {									\
 	irqsafe_generic_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2)
 # endif
 # define irqsafe_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2)	\
-	__pcpu_double_call_return_int(irqsafe_cpu_cmpxchg_double_, (pcp1), (pcp2), (oval1), (oval2), (nval1), (nval2))
+	__pcpu_double_call_return_bool(irqsafe_cpu_cmpxchg_double_, (pcp1), (pcp2), (oval1), (oval2), (nval1), (nval2))
 #endif
 
 #endif /* __LINUX_PERCPU_H */

mm/slub.c

@@ -1940,7 +1940,7 @@ static __always_inline void *slab_alloc(struct kmem_cache *s,
 		 * Since this is without lock semantics the protection is only against
 		 * code executing on this cpu *not* from access by other cpus.
 		 */
-		if (unlikely(!this_cpu_cmpxchg_double(
+		if (unlikely(!irqsafe_cpu_cmpxchg_double(
 				s->cpu_slab->freelist, s->cpu_slab->tid,
 				object, tid,
 				get_freepointer(s, object), next_tid(tid)))) {
@@ -2145,7 +2145,7 @@ static __always_inline void slab_free(struct kmem_cache *s,
 		set_freepointer(s, object, c->freelist);
 
 #ifdef CONFIG_CMPXCHG_LOCAL
-		if (unlikely(!this_cpu_cmpxchg_double(
+		if (unlikely(!irqsafe_cpu_cmpxchg_double(
 				s->cpu_slab->freelist, s->cpu_slab->tid,
 				c->freelist, tid,
 				object, next_tid(tid)))) {