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Date: Fri, 22 Apr 2016 16:16:14 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc: "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
"mingo@...nel.org" <mingo@...nel.org>,
"tglx@...utronix.de" <tglx@...utronix.de>,
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Subject: Re: [RFC][PATCH 03/31] locking,arc: Implement
atomic_fetch_{add,sub,and,andnot,or,xor}()
On Fri, Apr 22, 2016 at 10:50:41AM +0000, Vineet Gupta wrote:
> > +#define ATOMIC_FETCH_OP(op, c_op, asm_op) \
> > +static inline int atomic_fetch_##op(int i, atomic_t *v) \
> > +{ \
> > + unsigned int val, result; \
> > + SCOND_FAIL_RETRY_VAR_DEF \
> > + \
> > + /* \
> > + * Explicit full memory barrier needed before/after as \
> > + * LLOCK/SCOND thmeselves don't provide any such semantics \
> > + */ \
> > + smp_mb(); \
> > + \
> > + __asm__ __volatile__( \
> > + "1: llock %[val], [%[ctr]] \n" \
> > + " mov %[result], %[val] \n" \
>
> Calling it result could be a bit confusing, this is meant to be the "orig" value.
> So it indeed "result" of the API, but for atomic operation it is pristine value.
>
> Also we can optimize away that MOV - given there are plenty of regs, so
>
> > + " " #asm_op " %[val], %[val], %[i] \n" \
> > + " scond %[val], [%[ctr]] \n" \
>
> Instead have
>
> + " " #asm_op " %[result], %[val], %[i] \n" \
> + " scond %[result], [%[ctr]] \n" \
>
>
Indeed, how about something like so?
---
Subject: locking,arc: Implement atomic_fetch_{add,sub,and,andnot,or,xor}()
From: Peter Zijlstra <peterz@...radead.org>
Date: Mon Apr 18 01:16:09 CEST 2016
Implement FETCH-OP atomic primitives, these are very similar to the
existing OP-RETURN primitives we already have, except they return the
value of the atomic variable _before_ modification.
This is especially useful for irreversible operations -- such as
bitops (because it becomes impossible to reconstruct the state prior
to modification).
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
arch/arc/include/asm/atomic.h | 69 ++++++++++++++++++++++++++++++++++++++----
1 file changed, 64 insertions(+), 5 deletions(-)
--- a/arch/arc/include/asm/atomic.h
+++ b/arch/arc/include/asm/atomic.h
@@ -102,6 +102,37 @@ static inline int atomic_##op##_return(i
return val; \
}
+#define ATOMIC_FETCH_OP(op, c_op, asm_op) \
+static inline int atomic_fetch_##op(int i, atomic_t *v) \
+{ \
+ unsigned int val, orig; \
+ SCOND_FAIL_RETRY_VAR_DEF \
+ \
+ /* \
+ * Explicit full memory barrier needed before/after as \
+ * LLOCK/SCOND thmeselves don't provide any such semantics \
+ */ \
+ smp_mb(); \
+ \
+ __asm__ __volatile__( \
+ "1: llock %[orig], [%[ctr]] \n" \
+ " " #asm_op " %[val], %[orig], %[i] \n" \
+ " scond %[val], [%[ctr]] \n" \
+ " \n" \
+ SCOND_FAIL_RETRY_ASM \
+ \
+ : [val] "=&r" (val), \
+ [orig] "=&r" (orig) \
+ SCOND_FAIL_RETRY_VARS \
+ : [ctr] "r" (&v->counter), \
+ [i] "ir" (i) \
+ : "cc"); \
+ \
+ smp_mb(); \
+ \
+ return orig; \
+}
+
#else /* !CONFIG_ARC_HAS_LLSC */
#ifndef CONFIG_SMP
@@ -164,23 +196,49 @@ static inline int atomic_##op##_return(i
return temp; \
}
+#define ATOMIC_FETCH_OP(op, c_op, asm_op) \
+static inline int atomic_fetch_##op(int i, atomic_t *v) \
+{ \
+ unsigned long flags; \
+ unsigned long orig; \
+ \
+ /* \
+ * spin lock/unlock provides the needed smp_mb() before/after \
+ */ \
+ atomic_ops_lock(flags); \
+ orig = v->counter; \
+ v->counter c_op i; \
+ atomic_ops_unlock(flags); \
+ \
+ return orig; \
+}
+
#endif /* !CONFIG_ARC_HAS_LLSC */
#define ATOMIC_OPS(op, c_op, asm_op) \
ATOMIC_OP(op, c_op, asm_op) \
- ATOMIC_OP_RETURN(op, c_op, asm_op)
+ ATOMIC_OP_RETURN(op, c_op, asm_op) \
+ ATOMIC_FETCH_OP(op, c_op, asm_op)
ATOMIC_OPS(add, +=, add)
ATOMIC_OPS(sub, -=, sub)
#define atomic_andnot atomic_andnot
-ATOMIC_OP(and, &=, and)
-ATOMIC_OP(andnot, &= ~, bic)
-ATOMIC_OP(or, |=, or)
-ATOMIC_OP(xor, ^=, xor)
+#define atomic_fetch_or atomic_fetch_or
+
+#undef ATOMIC_OPS
+#define ATOMIC_OPS(op, c_op, asm_op) \
+ ATOMIC_OP(op, c_op, asm_op) \
+ ATOMIC_FETCH_OP(op, c_op, asm_op)
+
+ATOMIC_OPS(and, &=, and)
+ATOMIC_OPS(andnot, &= ~, bic)
+ATOMIC_OPS(or, |=, or)
+ATOMIC_OPS(xor, ^=, xor)
#undef ATOMIC_OPS
+#undef ATOMIC_FETCH_OP
#undef ATOMIC_OP_RETURN
#undef ATOMIC_OP
#undef SCOND_FAIL_RETRY_VAR_DEF
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