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SubjectRe: [PATCH 1/2] asm-generic: rwsem: ensure sem->cnt is only accessed via atomic_long_*
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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