Messages in this thread Patch in this message | | | Date | Fri, 28 Feb 2014 12:13:15 +0000 | From | Will Deacon <> | Subject | Re: [PATCH 1/2] asm-generic: rwsem: ensure sem->cnt is only accessed via atomic_long_* |
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On Thu, Feb 27, 2014 at 05:28:24AM +0000, Davidlohr Bueso wrote: > On Fri, 2014-02-21 at 17:22 +0000, Will Deacon wrote: > > The asm-generic rwsem implementation directly acceses sem->cnt when > > performing a __down_read_trylock operation. Whilst this is probably safe > > on all architectures, we should stick to the atomic_long_* API and use > > atomic_long_read instead. > > > > Signed-off-by: Will Deacon <will.deacon@arm.com> > > --- > > include/asm-generic/rwsem.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h > > index bb1e2cdeb9bf..75af612f54f8 100644 > > --- a/include/asm-generic/rwsem.h > > +++ b/include/asm-generic/rwsem.h > > @@ -41,7 +41,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem) > > { > > long tmp; > > > > - while ((tmp = sem->count) >= 0) { > > + while ((tmp = atomic_long_read((atomic_long_t *)&sem->count)) >= 0) { > > That's pretty ugly, how about having infinite look and just do the tmp > assign separately from the conditional? > > It also looks like a cpu_relax() could help here between iterations. > Other than that: > > Reviewed-by: Davidlohr Bueso <davidlohr@hp.com>
Actually, we should make that cmpxchg an atomic_long_cmpxchg, so the extra diff ends up looking like below. It's ugly adding a cpu_relax(), since you only want it when the cmpxchg fails (and we don't have such logic in the asm-generic __atomic_add_unless, for example).
Will
--->8
diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h index 603a0a11e592..2b6401f9e428 100644 --- a/include/asm-generic/rwsem.h +++ b/include/asm-generic/rwsem.h @@ -40,14 +40,16 @@ static inline void __down_read(struct rw_semaphore *sem) static inline int __down_read_trylock(struct rw_semaphore *sem) { long tmp; + atomic_long_t *cnt = (atomic_long_t *)&sem->count; - while ((tmp = atomic_long_read((atomic_long_t *)&sem->count)) >= 0) { - if (tmp == cmpxchg(&sem->count, tmp, - tmp + RWSEM_ACTIVE_READ_BIAS)) { - return 1; - } - } - return 0; + do { + tmp = atomic_long_read(cnt); + if (tmp < 0) + return 0; + } while (tmp != atomic_long_cmpxchg(cnt, tmp, + tmp + RWSEM_ACTIVE_READ_BIAS)); + + return 1; } /*
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