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powerpc: Fix atomic_xxx_return barrier semantics

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The Documentation/memory-barriers.txt document requires that atomic
operations that return a value act as a memory barrier both before
and after the actual atomic operation.

Our current implementation doesn't guarantee this. More specifically,
while a load following the isync can not be issued before stwcx. has
completed, that completion doesn't architecturally means that the
result of stwcx. is visible to other processors (or any previous stores
for that matter) (typically, the other processors L1 caches can still
hold the old value).

This has caused an actual crash in RCU torture testing on Power 7

This fixes it by changing those atomic ops to use new macros instead
of RELEASE/ACQUIRE barriers, called ATOMIC_ENTRY and ATMOIC_EXIT barriers,
which are then defined respectively to lwsync and sync.

I haven't had a chance to measure the performance impact (or rather
what I measured with kernel compiles is in the noise, I yet have to
find a more precise benchmark)

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Benjamin Herrenschmidt 2011-11-15 17:11:27 +00:00
parent a9a8f77ac7
commit b97021f855
4 changed files with 40 additions and 35 deletions
Download patch file Download diff file Expand all files Collapse all files

12
arch/powerpc/include/asm/bitops.h
View file

@ -124,14 +124,14 @@ static __inline__ unsigned long fn( \
return (old & mask); \
}
DEFINE_TESTOP(test_and_set_bits, or, PPC_RELEASE_BARRIER,
PPC_ACQUIRE_BARRIER, 0)
DEFINE_TESTOP(test_and_set_bits, or, PPC_ATOMIC_ENTRY_BARRIER,
PPC_ATOMIC_EXIT_BARRIER, 0)
DEFINE_TESTOP(test_and_set_bits_lock, or, "",
PPC_ACQUIRE_BARRIER, 1)
DEFINE_TESTOP(test_and_clear_bits, andc, PPC_RELEASE_BARRIER,
PPC_ACQUIRE_BARRIER, 0)
DEFINE_TESTOP(test_and_change_bits, xor, PPC_RELEASE_BARRIER,
PPC_ACQUIRE_BARRIER, 0)
DEFINE_TESTOP(test_and_clear_bits, andc, PPC_ATOMIC_ENTRY_BARRIER,
PPC_ATOMIC_EXIT_BARRIER, 0)
DEFINE_TESTOP(test_and_change_bits, xor, PPC_ATOMIC_ENTRY_BARRIER,
PPC_ATOMIC_EXIT_BARRIER, 0)
static __inline__ int test_and_set_bit(unsigned long nr,
volatile unsigned long *addr)