Re: [PATCH 02/11] locking, rwsem: drop explicit memory barriers

Davidlohr Bueso
2016-04-02T01:17:53Z

On Fri, 01 Apr 2016, Michal Hocko wrote:

>From: Michal Hocko <mhocko@suse.com>
>
>sh and xtensa seem to be the only architectures which use explicit
>memory barriers for rw_semaphore operations even though they are not
>really needed because there is the full memory barrier is always implied
>by atomic_{inc,dec,add,sub}_return resp. cmpxchg. Remove them.

Heh, and sh even defines smp_store_mb() with xchg(), which makes the above
even more so.

>
>Signed-off-by: Michal Hocko <mhocko@suse.com>
>---
> arch/sh/include/asm/rwsem.h | 14 ++------------
> arch/xtensa/include/asm/rwsem.h | 14 ++------------
> 2 files changed, 4 insertions(+), 24 deletions(-)

I think we can actually get rid of these files altogether. While I don't know the archs,
it does not appear to be implementing any kind of workaround/optimization, which is obviously
the whole purpose of defining per-arch rwsem internals, unless the redundant barriers are
actually the purpose. So we could use the generic rwsem.h (which has optimizations and ACQUIRE/
RELEASE semantics, although nops for either sh or xtensa specifically). A first inspection shows
that the code is in fact the same and continue to use 32bit rwsems.

Thanks,
Davidlohr

>diff --git a/arch/sh/include/asm/rwsem.h b/arch/sh/include/asm/rwsem.h
>index a5104bebd1eb..f6c951c7a875 100644
>--- a/arch/sh/include/asm/rwsem.h
>+++ b/arch/sh/include/asm/rwsem.h
>@@ -24,9 +24,7 @@
> */
> static inline void __down_read(struct rw_semaphore *sem)
> {
>- if (atomic_inc_return((atomic_t *)(&sem->count)) > 0)
>- smp_wmb();
>- else
>+ if (atomic_inc_return((atomic_t *)(&sem->count)) <= 0)
> rwsem_down_read_failed(sem);
> }
>
>@@ -37,7 +35,6 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
> while ((tmp = sem->count) >= 0) {
> if (tmp = cmpxchg(&sem->count, tmp,
> tmp + RWSEM_ACTIVE_READ_BIAS)) {
>- smp_wmb();
> return 1;
> }
> }
>@@ -53,9 +50,7 @@ static inline void __down_write(struct rw_semaphore *sem)
>
> tmp = atomic_add_return(RWSEM_ACTIVE_WRITE_BIAS,
> (atomic_t *)(&sem->count));
>- if (tmp = RWSEM_ACTIVE_WRITE_BIAS)
>- smp_wmb();
>- else
>+ if (tmp != RWSEM_ACTIVE_WRITE_BIAS)
> rwsem_down_write_failed(sem);
> }
>
>@@ -65,7 +60,6 @@ static inline int __down_write_trylock(struct rw_semaphore *sem)
>
> tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
> RWSEM_ACTIVE_WRITE_BIAS);
>- smp_wmb();
> return tmp = RWSEM_UNLOCKED_VALUE;
> }
>
>@@ -76,7 +70,6 @@ static inline void __up_read(struct rw_semaphore *sem)
> {
> int tmp;
>
>- smp_wmb();
> tmp = atomic_dec_return((atomic_t *)(&sem->count));
> if (tmp < -1 && (tmp & RWSEM_ACTIVE_MASK) = 0)
> rwsem_wake(sem);
>@@ -87,7 +80,6 @@ static inline void __up_read(struct rw_semaphore *sem)
> */
> static inline void __up_write(struct rw_semaphore *sem)
> {
>- smp_wmb();
> if (atomic_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
> (atomic_t *)(&sem->count)) < 0)
> rwsem_wake(sem);
>@@ -108,7 +100,6 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
> {
> int tmp;
>
>- smp_wmb();
> tmp = atomic_add_return(-RWSEM_WAITING_BIAS, (atomic_t *)(&sem->count));
> if (tmp < 0)
> rwsem_downgrade_wake(sem);
>@@ -119,7 +110,6 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
> */
> static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
> {
>- smp_mb();
> return atomic_add_return(delta, (atomic_t *)(&sem->count));
> }
>
>diff --git a/arch/xtensa/include/asm/rwsem.h b/arch/xtensa/include/asm/rwsem.h
>index 249619e7e7f2..593483f6e1ff 100644
>--- a/arch/xtensa/include/asm/rwsem.h
>+++ b/arch/xtensa/include/asm/rwsem.h
>@@ -29,9 +29,7 @@
> */
> static inline void __down_read(struct rw_semaphore *sem)
> {
>- if (atomic_add_return(1,(atomic_t *)(&sem->count)) > 0)
>- smp_wmb();
>- else
>+ if (atomic_add_return(1,(atomic_t *)(&sem->count)) <= 0)
> rwsem_down_read_failed(sem);
> }
>
>@@ -42,7 +40,6 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
> while ((tmp = sem->count) >= 0) {
> if (tmp = cmpxchg(&sem->count, tmp,
> tmp + RWSEM_ACTIVE_READ_BIAS)) {
>- smp_wmb();
> return 1;
> }
> }
>@@ -58,9 +55,7 @@ static inline void __down_write(struct rw_semaphore *sem)
>
> tmp = atomic_add_return(RWSEM_ACTIVE_WRITE_BIAS,
> (atomic_t *)(&sem->count));
>- if (tmp = RWSEM_ACTIVE_WRITE_BIAS)
>- smp_wmb();
>- else
>+ if (tmp != RWSEM_ACTIVE_WRITE_BIAS)
> rwsem_down_write_failed(sem);
> }
>
>@@ -70,7 +65,6 @@ static inline int __down_write_trylock(struct rw_semaphore *sem)
>
> tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
> RWSEM_ACTIVE_WRITE_BIAS);
>- smp_wmb();
> return tmp = RWSEM_UNLOCKED_VALUE;
> }
>
>@@ -81,7 +75,6 @@ static inline void __up_read(struct rw_semaphore *sem)
> {
> int tmp;
>
>- smp_wmb();
> tmp = atomic_sub_return(1,(atomic_t *)(&sem->count));
> if (tmp < -1 && (tmp & RWSEM_ACTIVE_MASK) = 0)
> rwsem_wake(sem);
>@@ -92,7 +85,6 @@ static inline void __up_read(struct rw_semaphore *sem)
> */
> static inline void __up_write(struct rw_semaphore *sem)
> {
>- smp_wmb();
> if (atomic_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
> (atomic_t *)(&sem->count)) < 0)
> rwsem_wake(sem);
>@@ -113,7 +105,6 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
> {
> int tmp;
>
>- smp_wmb();
> tmp = atomic_add_return(-RWSEM_WAITING_BIAS, (atomic_t *)(&sem->count));
> if (tmp < 0)
> rwsem_downgrade_wake(sem);
>@@ -124,7 +115,6 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
> */
> static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
> {
>- smp_mb();
> return atomic_add_return(delta, (atomic_t *)(&sem->count));
> }
>
>--
>2.8.0.rc3
>