Messages in this thread | | | Date | Fri, 28 Feb 2014 07:50:06 -0500 | From | Peter Hurley <> | Subject | Re: [PATCH 1/2] asm-generic: rwsem: ensure sem->cnt is only accessed via atomic_long_* |
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On 02/28/2014 07:13 AM, Will Deacon wrote: > 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.
This is the read trylock so no cpu_relax().
>> 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;
The shared rwsem failure paths (kernel/locking/rwsem_xadd.c) peek at sem->count as long type, so this isn't really necessary.
> > - 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; > }
Regards, Peter Hurley
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