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authorVineet Gupta <vgupta@synopsys.com>2015-07-03 11:26:22 +0530
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2015-08-10 12:21:57 -0700
commit26121b675728939c515594b2bb866756b7789c16 (patch)
treea7f4fd3e16ba86a1441279f7d4203bc7aa45bedd /arch
parent3092e514c62477fa17401667d4ffb4921b14842c (diff)
ARC: Make ARC bitops "safer" (add anti-optimization)
commit 80f420842ff42ad61f84584716d74ef635f13892 upstream. ARCompact/ARCv2 ISA provide that any instructions which deals with bitpos/count operand ASL, LSL, BSET, BCLR, BMSK .... will only consider lower 5 bits. i.e. auto-clamp the pos to 0-31. ARC Linux bitops exploited this fact by NOT explicitly masking out upper bits for @nr operand in general, saving a bunch of AND/BMSK instructions in generated code around bitops. While this micro-optimization has worked well over years it is NOT safe as shifting a number with a value, greater than native size is "undefined" per "C" spec. So as it turns outm EZChip ran into this eventually, in their massive muti-core SMP build with 64 cpus. There was a test_bit() inside a loop from 63 to 0 and gcc was weirdly optimizing away the first iteration (so it was really adhering to standard by implementing undefined behaviour vs. removing all the iterations which were phony i.e. (1 << [63..32]) | for i = 63 to 0 | X = ( 1 << i ) | if X == 0 | continue So fix the code to do the explicit masking at the expense of generating additional instructions. Fortunately, this can be mitigated to a large extent as gcc has SHIFT_COUNT_TRUNCATED which allows combiner to fold masking into shift operation itself. It is currently not enabled in ARC gcc backend, but could be done after a bit of testing. Fixes STAR 9000866918 ("unsafe "undefined behavior" code in kernel") Reported-by: Noam Camus <noamc@ezchip.com> Signed-off-by: Vineet Gupta <vgupta@synopsys.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'arch')
-rw-r--r--arch/arc/include/asm/bitops.h35
1 files changed, 9 insertions, 26 deletions
diff --git a/arch/arc/include/asm/bitops.h b/arch/arc/include/asm/bitops.h
index f19f0abacffd..dae03e66fa9e 100644
--- a/arch/arc/include/asm/bitops.h
+++ b/arch/arc/include/asm/bitops.h
@@ -50,8 +50,7 @@ static inline void op##_bit(unsigned long nr, volatile unsigned long *m)\
* done for const @nr, but no code is generated due to gcc \
* const prop. \
*/ \
- if (__builtin_constant_p(nr)) \
- nr &= 0x1f; \
+ nr &= 0x1f; \
\
__asm__ __volatile__( \
"1: llock %0, [%1] \n" \
@@ -82,8 +81,7 @@ static inline int test_and_##op##_bit(unsigned long nr, volatile unsigned long *
\
m += nr >> 5; \
\
- if (__builtin_constant_p(nr)) \
- nr &= 0x1f; \
+ nr &= 0x1f; \
\
/* \
* Explicit full memory barrier needed before/after as \
@@ -129,16 +127,13 @@ static inline void op##_bit(unsigned long nr, volatile unsigned long *m)\
unsigned long temp, flags; \
m += nr >> 5; \
\
- if (__builtin_constant_p(nr)) \
- nr &= 0x1f; \
- \
/* \
* spin lock/unlock provide the needed smp_mb() before/after \
*/ \
bitops_lock(flags); \
\
temp = *m; \
- *m = temp c_op (1UL << nr); \
+ *m = temp c_op (1UL << (nr & 0x1f)); \
\
bitops_unlock(flags); \
}
@@ -149,17 +144,14 @@ static inline int test_and_##op##_bit(unsigned long nr, volatile unsigned long *
unsigned long old, flags; \
m += nr >> 5; \
\
- if (__builtin_constant_p(nr)) \
- nr &= 0x1f; \
- \
bitops_lock(flags); \
\
old = *m; \
- *m = old c_op (1 << nr); \
+ *m = old c_op (1UL << (nr & 0x1f)); \
\
bitops_unlock(flags); \
\
- return (old & (1 << nr)) != 0; \
+ return (old & (1UL << (nr & 0x1f))) != 0; \
}
#endif /* CONFIG_ARC_HAS_LLSC */
@@ -174,11 +166,8 @@ static inline void __##op##_bit(unsigned long nr, volatile unsigned long *m) \
unsigned long temp; \
m += nr >> 5; \
\
- if (__builtin_constant_p(nr)) \
- nr &= 0x1f; \
- \
temp = *m; \
- *m = temp c_op (1UL << nr); \
+ *m = temp c_op (1UL << (nr & 0x1f)); \
}
#define __TEST_N_BIT_OP(op, c_op, asm_op) \
@@ -187,13 +176,10 @@ static inline int __test_and_##op##_bit(unsigned long nr, volatile unsigned long
unsigned long old; \
m += nr >> 5; \
\
- if (__builtin_constant_p(nr)) \
- nr &= 0x1f; \
- \
old = *m; \
- *m = old c_op (1 << nr); \
+ *m = old c_op (1UL << (nr & 0x1f)); \
\
- return (old & (1 << nr)) != 0; \
+ return (old & (1UL << (nr & 0x1f))) != 0; \
}
#define BIT_OPS(op, c_op, asm_op) \
@@ -224,10 +210,7 @@ test_bit(unsigned int nr, const volatile unsigned long *addr)
addr += nr >> 5;
- if (__builtin_constant_p(nr))
- nr &= 0x1f;
-
- mask = 1 << nr;
+ mask = 1UL << (nr & 0x1f);
return ((mask & *addr) != 0);
}