[PATCH] ARC: avoid unwanted gcc optimizations in atomic operations

[PATCH] ARC: avoid unwanted gcc optimizations in atomic operations

[Pavel.Kozlov]

From: Pavel Kozlov <pavel.kozlov@synopsys.com>

Notify a compiler about write operations and prevent unwanted
optimizations. Add the "memory" clobber to the clobber list.

An obvious problem with unwanted compiler optimizations appeared after
the cpumask optimization commit 596ff4a09b89 ("cpumask: re-introduce
constant-sized cpumask optimizations").

After this commit the SMP kernels for ARC no longer loads because of
failed assert in the percpu allocator initialization routine:

percpu: BUG: failure at mm/percpu.c:2981/pcpu_build_alloc_info()!

The write operation performed by the scond instruction in the atomic
inline asm code is not properly passed to the compiler. The compiler
cannot correctly optimize a nested loop that runs through the cpumask
in the pcpu_build_alloc_info() function.

Add the "memory" clobber to fix this.

Link: https://github.com/foss-for-synopsys-dwc-arc-processors/linux/issues/135
Cc: <stable@vger.kernel.org> # v6.3+
Signed-off-by: Pavel Kozlov <pavel.kozlov@synopsys.com>
---
 arch/arc/include/asm/atomic-llsc.h    | 6 +++---
 arch/arc/include/asm/atomic64-arcv2.h | 6 +++---
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/arc/include/asm/atomic-llsc.h b/arch/arc/include/asm/atomic-llsc.h
index 1b0ffaeee16d..5258cb81a16b 100644
--- a/arch/arc/include/asm/atomic-llsc.h
+++ b/arch/arc/include/asm/atomic-llsc.h
@@ -18,7 +18,7 @@ static inline void arch_atomic_##op(int i, atomic_t *v)			\
 	: [val]	"=&r"	(val) /* Early clobber to prevent reg reuse */	\
 	: [ctr]	"r"	(&v->counter), /* Not "m": llock only supports reg direct addr mode */	\
 	  [i]	"ir"	(i)						\
-	: "cc");							\
+	: "cc", "memory");						\
 }									\
 
 #define ATOMIC_OP_RETURN(op, asm_op)				\
@@ -34,7 +34,7 @@ static inline int arch_atomic_##op##_return_relaxed(int i, atomic_t *v)	\
 	: [val]	"=&r"	(val)						\
 	: [ctr]	"r"	(&v->counter),					\
 	  [i]	"ir"	(i)						\
-	: "cc");							\
+	: "cc", "memory");						\
 									\
 	return val;							\
 }
@@ -56,7 +56,7 @@ static inline int arch_atomic_fetch_##op##_relaxed(int i, atomic_t *v)	\
 	  [orig] "=&r" (orig)						\
 	: [ctr]	"r"	(&v->counter),					\
 	  [i]	"ir"	(i)						\
-	: "cc");							\
+	: "cc", "memory");						\
 									\
 	return orig;							\
 }
diff --git a/arch/arc/include/asm/atomic64-arcv2.h b/arch/arc/include/asm/atomic64-arcv2.h
index 6b6db981967a..9b5791b85471 100644
--- a/arch/arc/include/asm/atomic64-arcv2.h
+++ b/arch/arc/include/asm/atomic64-arcv2.h
@@ -60,7 +60,7 @@ static inline void arch_atomic64_##op(s64 a, atomic64_t *v)		\
 	"	bnz     1b		\n"				\
 	: "=&r"(val)							\
 	: "r"(&v->counter), "ir"(a)					\
-	: "cc");							\
+	: "cc", "memory");						\
 }									\
 
 #define ATOMIC64_OP_RETURN(op, op1, op2)		        	\
@@ -77,7 +77,7 @@ static inline s64 arch_atomic64_##op##_return_relaxed(s64 a, atomic64_t *v)	\
 	"	bnz     1b		\n"				\
 	: [val] "=&r"(val)						\
 	: "r"(&v->counter), "ir"(a)					\
-	: "cc");	/* memory clobber comes from smp_mb() */	\
+	: "cc", "memory");						\
 									\
 	return val;							\
 }
@@ -99,7 +99,7 @@ static inline s64 arch_atomic64_fetch_##op##_relaxed(s64 a, atomic64_t *v)	\
 	"	bnz     1b		\n"				\
 	: "=&r"(orig), "=&r"(val)					\
 	: "r"(&v->counter), "ir"(a)					\
-	: "cc");	/* memory clobber comes from smp_mb() */	\
+	: "cc", "memory");						\
 									\
 	return orig;							\
 }
-- 
2.25.1


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Re: [PATCH] ARC: avoid unwanted gcc optimizations in atomic operations

[Vineet Gupta]

On 8/15/23 08:11, Pavel.Kozlov@synopsys.com wrote:
> From: Pavel Kozlov<pavel.kozlov@synopsys.com>
>
> Notify a compiler about write operations and prevent unwanted
> optimizations. Add the "memory" clobber to the clobber list.
>
> An obvious problem with unwanted compiler optimizations appeared after
> the cpumask optimization commit 596ff4a09b89 ("cpumask: re-introduce
> constant-sized cpumask optimizations").
>
> After this commit the SMP kernels for ARC no longer loads because of
> failed assert in the percpu allocator initialization routine:
>
> percpu: BUG: failure at mm/percpu.c:2981/pcpu_build_alloc_info()!
>
> The write operation performed by the scond instruction in the atomic
> inline asm code is not properly passed to the compiler. The compiler
> cannot correctly optimize a nested loop that runs through the cpumask
> in the pcpu_build_alloc_info() function.
>
> Add the "memory" clobber to fix this.
>
> Link:https://github.com/foss-for-synopsys-dwc-arc-processors/linux/issues/135
> Cc:<stable@vger.kernel.org>  # v6.3+
> Signed-off-by: Pavel Kozlov<pavel.kozlov@synopsys.com>


Acked-by: Vineet Gupta <vgupta@kernel.org>

Fixes: b64be6836993c431e ("ARC: atomics: implement relaxed variants")

Before that commit, atomic ops could elide memory clobber because the 
trailing smp_mb() did that anyways.
However after that commit, the smp_mb() was optional for relaxed 
variants and thus needs clobber.


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