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alistair23-linux/include/linux/atomic.h
Linus Torvalds 105ff3cbf2 atomic: remove all traces of READ_ONCE_CTRL() and atomic*_read_ctrl() 
This seems to be a mis-reading of how alpha memory ordering works, and
is not backed up by the alpha architecture manual.  The helper functions
don't do anything special on any other architectures, and the arguments
that support them being safe on other architectures also argue that they
are safe on alpha.

Basically, the "control dependency" is between a previous read and a
subsequent write that is dependent on the value read.  Even if the
subsequent write is actually done speculatively, there is no way that
such a speculative write could be made visible to other cpu's until it
has been committed, which requires validating the speculation.

Note that most weakely ordered architectures (very much including alpha)
do not guarantee any ordering relationship between two loads that depend
on each other on a control dependency:

    read A
    if (val == 1)
        read B

because the conditional may be predicted, and the "read B" may be
speculatively moved up to before reading the value A.  So we require the
user to insert a smp_rmb() between the two accesses to be correct:

    read A;
    if (A == 1)
        smp_rmb()
        read B

Alpha is further special in that it can break that ordering even if the
*address* of B depends on the read of A, because the cacheline that is
read later may be stale unless you have a memory barrier in between the
pointer read and the read of the value behind a pointer:

    read ptr
    read offset(ptr)

whereas all other weakly ordered architectures guarantee that the data
dependency (as opposed to just a control dependency) will order the two
accesses.  As a result, alpha needs a "smp_read_barrier_depends()" in
between those two reads for them to be ordered.

The coontrol dependency that "READ_ONCE_CTRL()" and "atomic_read_ctrl()"
had was a control dependency to a subsequent *write*, however, and
nobody can finalize such a subsequent write without having actually done
the read.  And were you to write such a value to a "stale" cacheline
(the way the unordered reads came to be), that would seem to lose the
write entirely.

So the things that make alpha able to re-order reads even more
aggressively than other weak architectures do not seem to be relevant
for a subsequent write.  Alpha memory ordering may be strange, but
there's no real indication that it is *that* strange.

Also, the alpha architecture reference manual very explicitly talks
about the definition of "Dependence Constraints" in section 5.6.1.7,
where a preceding read dominates a subsequent write.

Such a dependence constraint admittedly does not impose a BEFORE (alpha
architecture term for globally visible ordering), but it does guarantee
that there can be no "causal loop".  I don't see how you could avoid
such a loop if another cpu could see the stored value and then impact
the value of the first read.  Put another way: the read and the write
could not be seen as being out of order wrt other cpus.

So I do not see how these "x_ctrl()" functions can currently be necessary.

I may have to eat my words at some point, but in the absense of clear
proof that alpha actually needs this, or indeed even an explanation of
how alpha could _possibly_ need it, I do not believe these functions are
called for.

And if it turns out that alpha really _does_ need a barrier for this
case, that barrier still should not be "smp_read_barrier_depends()".
We'd have to make up some new speciality barrier just for alpha, along
with the documentation for why it really is necessary.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul E McKenney <paulmck@us.ibm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2015-11-03 17:22:17 -08:00

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/* Atomic operations usable in machine independent code */
#ifndef _LINUX_ATOMIC_H
#define _LINUX_ATOMIC_H
#include <asm/atomic.h>
#include <asm/barrier.h>
/*
* Relaxed variants of xchg, cmpxchg and some atomic operations.
*
* We support four variants:
*
* - Fully ordered: The default implementation, no suffix required.
* - Acquire: Provides ACQUIRE semantics, _acquire suffix.
* - Release: Provides RELEASE semantics, _release suffix.
* - Relaxed: No ordering guarantees, _relaxed suffix.
*
* For compound atomics performing both a load and a store, ACQUIRE
* semantics apply only to the load and RELEASE semantics only to the
* store portion of the operation. Note that a failed cmpxchg_acquire
* does -not- imply any memory ordering constraints.
*
* See Documentation/memory-barriers.txt for ACQUIRE/RELEASE definitions.
*/
#ifndef atomic_read_acquire
#define atomic_read_acquire(v) smp_load_acquire(&(v)->counter)
#endif
#ifndef atomic_set_release
#define atomic_set_release(v, i) smp_store_release(&(v)->counter, (i))
#endif
/*
* The idea here is to build acquire/release variants by adding explicit
* barriers on top of the relaxed variant. In the case where the relaxed
* variant is already fully ordered, no additional barriers are needed.
*/
#define __atomic_op_acquire(op, args...) \
({ \
typeof(op##_relaxed(args)) __ret = op##_relaxed(args); \
smp_mb__after_atomic(); \
__ret; \
})
#define __atomic_op_release(op, args...) \
({ \
smp_mb__before_atomic(); \
op##_relaxed(args); \
})
#define __atomic_op_fence(op, args...) \
({ \
typeof(op##_relaxed(args)) __ret; \
smp_mb__before_atomic(); \
__ret = op##_relaxed(args); \
smp_mb__after_atomic(); \
__ret; \
})
/* atomic_add_return_relaxed */
#ifndef atomic_add_return_relaxed
#define atomic_add_return_relaxed atomic_add_return
#define atomic_add_return_acquire atomic_add_return
#define atomic_add_return_release atomic_add_return
#else /* atomic_add_return_relaxed */
#ifndef atomic_add_return_acquire
#define atomic_add_return_acquire(...) \
__atomic_op_acquire(atomic_add_return, __VA_ARGS__)
#endif
#ifndef atomic_add_return_release
#define atomic_add_return_release(...) \
__atomic_op_release(atomic_add_return, __VA_ARGS__)
#endif
#ifndef atomic_add_return
#define atomic_add_return(...) \
__atomic_op_fence(atomic_add_return, __VA_ARGS__)
#endif
#endif /* atomic_add_return_relaxed */
/* atomic_inc_return_relaxed */
#ifndef atomic_inc_return_relaxed
#define atomic_inc_return_relaxed atomic_inc_return
#define atomic_inc_return_acquire atomic_inc_return
#define atomic_inc_return_release atomic_inc_return
#else /* atomic_inc_return_relaxed */
#ifndef atomic_inc_return_acquire
#define atomic_inc_return_acquire(...) \
__atomic_op_acquire(atomic_inc_return, __VA_ARGS__)
#endif
#ifndef atomic_inc_return_release
#define atomic_inc_return_release(...) \
__atomic_op_release(atomic_inc_return, __VA_ARGS__)
#endif
#ifndef atomic_inc_return
#define atomic_inc_return(...) \
__atomic_op_fence(atomic_inc_return, __VA_ARGS__)
#endif
#endif /* atomic_inc_return_relaxed */
/* atomic_sub_return_relaxed */
#ifndef atomic_sub_return_relaxed
#define atomic_sub_return_relaxed atomic_sub_return
#define atomic_sub_return_acquire atomic_sub_return
#define atomic_sub_return_release atomic_sub_return
#else /* atomic_sub_return_relaxed */
#ifndef atomic_sub_return_acquire
#define atomic_sub_return_acquire(...) \
__atomic_op_acquire(atomic_sub_return, __VA_ARGS__)
#endif
#ifndef atomic_sub_return_release
#define atomic_sub_return_release(...) \
__atomic_op_release(atomic_sub_return, __VA_ARGS__)
#endif
#ifndef atomic_sub_return
#define atomic_sub_return(...) \
__atomic_op_fence(atomic_sub_return, __VA_ARGS__)
#endif
#endif /* atomic_sub_return_relaxed */
/* atomic_dec_return_relaxed */
#ifndef atomic_dec_return_relaxed
#define atomic_dec_return_relaxed atomic_dec_return
#define atomic_dec_return_acquire atomic_dec_return
#define atomic_dec_return_release atomic_dec_return
#else /* atomic_dec_return_relaxed */
#ifndef atomic_dec_return_acquire
#define atomic_dec_return_acquire(...) \
__atomic_op_acquire(atomic_dec_return, __VA_ARGS__)
#endif
#ifndef atomic_dec_return_release
#define atomic_dec_return_release(...) \
__atomic_op_release(atomic_dec_return, __VA_ARGS__)
#endif
#ifndef atomic_dec_return
#define atomic_dec_return(...) \
__atomic_op_fence(atomic_dec_return, __VA_ARGS__)
#endif
#endif /* atomic_dec_return_relaxed */
/* atomic_xchg_relaxed */
#ifndef atomic_xchg_relaxed
#define atomic_xchg_relaxed atomic_xchg
#define atomic_xchg_acquire atomic_xchg
#define atomic_xchg_release atomic_xchg
#else /* atomic_xchg_relaxed */
#ifndef atomic_xchg_acquire
#define atomic_xchg_acquire(...) \
__atomic_op_acquire(atomic_xchg, __VA_ARGS__)
#endif
#ifndef atomic_xchg_release
#define atomic_xchg_release(...) \
__atomic_op_release(atomic_xchg, __VA_ARGS__)
#endif
#ifndef atomic_xchg
#define atomic_xchg(...) \
__atomic_op_fence(atomic_xchg, __VA_ARGS__)
#endif
#endif /* atomic_xchg_relaxed */
/* atomic_cmpxchg_relaxed */
#ifndef atomic_cmpxchg_relaxed
#define atomic_cmpxchg_relaxed atomic_cmpxchg
#define atomic_cmpxchg_acquire atomic_cmpxchg
#define atomic_cmpxchg_release atomic_cmpxchg
#else /* atomic_cmpxchg_relaxed */
#ifndef atomic_cmpxchg_acquire
#define atomic_cmpxchg_acquire(...) \
__atomic_op_acquire(atomic_cmpxchg, __VA_ARGS__)
#endif
#ifndef atomic_cmpxchg_release
#define atomic_cmpxchg_release(...) \
__atomic_op_release(atomic_cmpxchg, __VA_ARGS__)
#endif
#ifndef atomic_cmpxchg
#define atomic_cmpxchg(...) \
__atomic_op_fence(atomic_cmpxchg, __VA_ARGS__)
#endif
#endif /* atomic_cmpxchg_relaxed */
#ifndef atomic64_read_acquire
#define atomic64_read_acquire(v) smp_load_acquire(&(v)->counter)
#endif
#ifndef atomic64_set_release
#define atomic64_set_release(v, i) smp_store_release(&(v)->counter, (i))
#endif
/* atomic64_add_return_relaxed */
#ifndef atomic64_add_return_relaxed
#define atomic64_add_return_relaxed atomic64_add_return
#define atomic64_add_return_acquire atomic64_add_return
#define atomic64_add_return_release atomic64_add_return
#else /* atomic64_add_return_relaxed */
#ifndef atomic64_add_return_acquire
#define atomic64_add_return_acquire(...) \
__atomic_op_acquire(atomic64_add_return, __VA_ARGS__)
#endif
#ifndef atomic64_add_return_release
#define atomic64_add_return_release(...) \
__atomic_op_release(atomic64_add_return, __VA_ARGS__)
#endif
#ifndef atomic64_add_return
#define atomic64_add_return(...) \
__atomic_op_fence(atomic64_add_return, __VA_ARGS__)
#endif
#endif /* atomic64_add_return_relaxed */
/* atomic64_inc_return_relaxed */
#ifndef atomic64_inc_return_relaxed
#define atomic64_inc_return_relaxed atomic64_inc_return
#define atomic64_inc_return_acquire atomic64_inc_return
#define atomic64_inc_return_release atomic64_inc_return
#else /* atomic64_inc_return_relaxed */
#ifndef atomic64_inc_return_acquire
#define atomic64_inc_return_acquire(...) \
__atomic_op_acquire(atomic64_inc_return, __VA_ARGS__)
#endif
#ifndef atomic64_inc_return_release
#define atomic64_inc_return_release(...) \
__atomic_op_release(atomic64_inc_return, __VA_ARGS__)
#endif
#ifndef atomic64_inc_return
#define atomic64_inc_return(...) \
__atomic_op_fence(atomic64_inc_return, __VA_ARGS__)
#endif
#endif /* atomic64_inc_return_relaxed */
/* atomic64_sub_return_relaxed */
#ifndef atomic64_sub_return_relaxed
#define atomic64_sub_return_relaxed atomic64_sub_return
#define atomic64_sub_return_acquire atomic64_sub_return
#define atomic64_sub_return_release atomic64_sub_return
#else /* atomic64_sub_return_relaxed */
#ifndef atomic64_sub_return_acquire
#define atomic64_sub_return_acquire(...) \
__atomic_op_acquire(atomic64_sub_return, __VA_ARGS__)
#endif
#ifndef atomic64_sub_return_release
#define atomic64_sub_return_release(...) \
__atomic_op_release(atomic64_sub_return, __VA_ARGS__)
#endif
#ifndef atomic64_sub_return
#define atomic64_sub_return(...) \
__atomic_op_fence(atomic64_sub_return, __VA_ARGS__)
#endif
#endif /* atomic64_sub_return_relaxed */
/* atomic64_dec_return_relaxed */
#ifndef atomic64_dec_return_relaxed
#define atomic64_dec_return_relaxed atomic64_dec_return
#define atomic64_dec_return_acquire atomic64_dec_return
#define atomic64_dec_return_release atomic64_dec_return
#else /* atomic64_dec_return_relaxed */
#ifndef atomic64_dec_return_acquire
#define atomic64_dec_return_acquire(...) \
__atomic_op_acquire(atomic64_dec_return, __VA_ARGS__)
#endif
#ifndef atomic64_dec_return_release
#define atomic64_dec_return_release(...) \
__atomic_op_release(atomic64_dec_return, __VA_ARGS__)
#endif
#ifndef atomic64_dec_return
#define atomic64_dec_return(...) \
__atomic_op_fence(atomic64_dec_return, __VA_ARGS__)
#endif
#endif /* atomic64_dec_return_relaxed */
/* atomic64_xchg_relaxed */
#ifndef atomic64_xchg_relaxed
#define atomic64_xchg_relaxed atomic64_xchg
#define atomic64_xchg_acquire atomic64_xchg
#define atomic64_xchg_release atomic64_xchg
#else /* atomic64_xchg_relaxed */
#ifndef atomic64_xchg_acquire
#define atomic64_xchg_acquire(...) \
__atomic_op_acquire(atomic64_xchg, __VA_ARGS__)
#endif
#ifndef atomic64_xchg_release
#define atomic64_xchg_release(...) \
__atomic_op_release(atomic64_xchg, __VA_ARGS__)
#endif
#ifndef atomic64_xchg
#define atomic64_xchg(...) \
__atomic_op_fence(atomic64_xchg, __VA_ARGS__)
#endif
#endif /* atomic64_xchg_relaxed */
/* atomic64_cmpxchg_relaxed */
#ifndef atomic64_cmpxchg_relaxed
#define atomic64_cmpxchg_relaxed atomic64_cmpxchg
#define atomic64_cmpxchg_acquire atomic64_cmpxchg
#define atomic64_cmpxchg_release atomic64_cmpxchg
#else /* atomic64_cmpxchg_relaxed */
#ifndef atomic64_cmpxchg_acquire
#define atomic64_cmpxchg_acquire(...) \
__atomic_op_acquire(atomic64_cmpxchg, __VA_ARGS__)
#endif
#ifndef atomic64_cmpxchg_release
#define atomic64_cmpxchg_release(...) \
__atomic_op_release(atomic64_cmpxchg, __VA_ARGS__)
#endif
#ifndef atomic64_cmpxchg
#define atomic64_cmpxchg(...) \
__atomic_op_fence(atomic64_cmpxchg, __VA_ARGS__)
#endif
#endif /* atomic64_cmpxchg_relaxed */
/* cmpxchg_relaxed */
#ifndef cmpxchg_relaxed
#define cmpxchg_relaxed cmpxchg
#define cmpxchg_acquire cmpxchg
#define cmpxchg_release cmpxchg
#else /* cmpxchg_relaxed */
#ifndef cmpxchg_acquire
#define cmpxchg_acquire(...) \
__atomic_op_acquire(cmpxchg, __VA_ARGS__)
#endif
#ifndef cmpxchg_release
#define cmpxchg_release(...) \
__atomic_op_release(cmpxchg, __VA_ARGS__)
#endif
#ifndef cmpxchg
#define cmpxchg(...) \
__atomic_op_fence(cmpxchg, __VA_ARGS__)
#endif
#endif /* cmpxchg_relaxed */
/* cmpxchg64_relaxed */
#ifndef cmpxchg64_relaxed
#define cmpxchg64_relaxed cmpxchg64
#define cmpxchg64_acquire cmpxchg64
#define cmpxchg64_release cmpxchg64
#else /* cmpxchg64_relaxed */
#ifndef cmpxchg64_acquire
#define cmpxchg64_acquire(...) \
__atomic_op_acquire(cmpxchg64, __VA_ARGS__)
#endif
#ifndef cmpxchg64_release
#define cmpxchg64_release(...) \
__atomic_op_release(cmpxchg64, __VA_ARGS__)
#endif
#ifndef cmpxchg64
#define cmpxchg64(...) \
__atomic_op_fence(cmpxchg64, __VA_ARGS__)
#endif
#endif /* cmpxchg64_relaxed */
/* xchg_relaxed */
#ifndef xchg_relaxed
#define xchg_relaxed xchg
#define xchg_acquire xchg
#define xchg_release xchg
#else /* xchg_relaxed */
#ifndef xchg_acquire
#define xchg_acquire(...) __atomic_op_acquire(xchg, __VA_ARGS__)
#endif
#ifndef xchg_release
#define xchg_release(...) __atomic_op_release(xchg, __VA_ARGS__)
#endif
#ifndef xchg
#define xchg(...) __atomic_op_fence(xchg, __VA_ARGS__)
#endif
#endif /* xchg_relaxed */
/**
* atomic_add_unless - add unless the number is already a given value
* @v: pointer of type atomic_t
* @a: the amount to add to v...
* @u: ...unless v is equal to u.
*
* Atomically adds @a to @v, so long as @v was not already @u.
* Returns non-zero if @v was not @u, and zero otherwise.
*/
static inline int atomic_add_unless(atomic_t *v, int a, int u)
{
return __atomic_add_unless(v, a, u) != u;
}
/**
* atomic_inc_not_zero - increment unless the number is zero
* @v: pointer of type atomic_t
*
* Atomically increments @v by 1, so long as @v is non-zero.
* Returns non-zero if @v was non-zero, and zero otherwise.
*/
#ifndef atomic_inc_not_zero
#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
#endif
#ifndef atomic_andnot
static inline void atomic_andnot(int i, atomic_t *v)
{
atomic_and(~i, v);
}
#endif
static inline __deprecated void atomic_clear_mask(unsigned int mask, atomic_t *v)
{
atomic_andnot(mask, v);
}
static inline __deprecated void atomic_set_mask(unsigned int mask, atomic_t *v)
{
atomic_or(mask, v);
}
/**
* atomic_inc_not_zero_hint - increment if not null
* @v: pointer of type atomic_t
* @hint: probable value of the atomic before the increment
*
* This version of atomic_inc_not_zero() gives a hint of probable
* value of the atomic. This helps processor to not read the memory
* before doing the atomic read/modify/write cycle, lowering
* number of bus transactions on some arches.
*
* Returns: 0 if increment was not done, 1 otherwise.
*/
#ifndef atomic_inc_not_zero_hint
static inline int atomic_inc_not_zero_hint(atomic_t *v, int hint)
{
int val, c = hint;
/* sanity test, should be removed by compiler if hint is a constant */
if (!hint)
return atomic_inc_not_zero(v);
do {
val = atomic_cmpxchg(v, c, c + 1);
if (val == c)
return 1;
c = val;
} while (c);
return 0;
}
#endif
#ifndef atomic_inc_unless_negative
static inline int atomic_inc_unless_negative(atomic_t *p)
{
int v, v1;
for (v = 0; v >= 0; v = v1) {
v1 = atomic_cmpxchg(p, v, v + 1);
if (likely(v1 == v))
return 1;
}
return 0;
}
#endif
#ifndef atomic_dec_unless_positive
static inline int atomic_dec_unless_positive(atomic_t *p)
{
int v, v1;
for (v = 0; v <= 0; v = v1) {
v1 = atomic_cmpxchg(p, v, v - 1);
if (likely(v1 == v))
return 1;
}
return 0;
}
#endif
/*
* atomic_dec_if_positive - decrement by 1 if old value positive
* @v: pointer of type atomic_t
*
* The function returns the old value of *v minus 1, even if
* the atomic variable, v, was not decremented.
*/
#ifndef atomic_dec_if_positive
static inline int atomic_dec_if_positive(atomic_t *v)
{
int c, old, dec;
c = atomic_read(v);
for (;;) {
dec = c - 1;
if (unlikely(dec < 0))
break;
old = atomic_cmpxchg((v), c, dec);
if (likely(old == c))
break;
c = old;
}
return dec;
}
#endif
#ifdef CONFIG_GENERIC_ATOMIC64
#include <asm-generic/atomic64.h>
#endif
#ifndef atomic64_andnot
static inline void atomic64_andnot(long long i, atomic64_t *v)
{
atomic64_and(~i, v);
}
#endif
#include <asm-generic/atomic-long.h>
#endif /* _LINUX_ATOMIC_H */
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