On Tue, Apr 22, 2014 at 03:19:26PM -0700, Davidlohr Bueso wrote:
> ---
> include/linux/rwsem.h | 9 +-
> kernel/locking/rwsem-xadd.c | 213
> +++++++++++++++++++++++++++++++++++++++-----
> kernel/locking/rwsem.c | 31 ++++++-
> 3 files changed, 231 insertions(+), 22 deletions(-)
rwsem-spinlock.c doesn't need changes?
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index cfff143..a911dbf 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -12,6 +12,27 @@
>
> #include <linux/atomic.h>
>
> +#ifdef CONFIG_SMP
> +static inline void rwsem_set_owner(struct rw_semaphore *sem)
> +{
> + sem->owner = current;
> +}
> +
> +static inline void rwsem_clear_owner(struct rw_semaphore *sem)
> +{
> + sem->owner = NULL;
> +}
> +
> +#else
> +static inline void rwsem_set_owner(struct rw_semaphore *sem)
> +{
> +}
> +
> +static inline void rwsem_clear_owner(struct rw_semaphore *sem)
> +{
> +}
> +#endif
> +
> /*
> * lock for reading
> */
> @@ -48,6 +69,7 @@ void __sched down_write(struct rw_semaphore *sem)
> rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
>
> LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
> + rwsem_set_owner(sem);
> }
>
> EXPORT_SYMBOL(down_write);
> @@ -59,8 +81,11 @@ int down_write_trylock(struct rw_semaphore *sem)
> {
> int ret = __down_write_trylock(sem);
>
> - if (ret == 1)
> + if (ret == 1) {
> rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
> + rwsem_set_owner(sem);
> + }
> +
> return ret;
> }
So first acquire lock, then set owner.
> @@ -86,6 +111,7 @@ void up_write(struct rw_semaphore *sem)
> rwsem_release(&sem->dep_map, 1, _RET_IP_);
>
> __up_write(sem);
> + rwsem_clear_owner(sem);
> }
>
> EXPORT_SYMBOL(up_write);
> @@ -100,6 +126,7 @@ void downgrade_write(struct rw_semaphore *sem)
> * dependency.
> */
> __downgrade_write(sem);
> + rwsem_clear_owner(sem);
> }
But here you first release and then clear owner; this is buggy. The
moment you release another acquire can happen and your clear will clear
the new owner, not us.
Or am I missing something obvious here?
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