This patch looks sensible.
I'd apply either this or my previous patch that adds synchronize_rcu() to
percpu_up_write.
This patch avoids the memory barrier on non-x86 cpus in percpu_up_read, so
it is faster than the previous approach.
Mikulas
On Thu, 18 Oct 2012, Lai Jiangshan wrote:
> ---------------
>
> a very draft example of paired-mb()s is here:
>
>
> diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
> index cf80f7e..84a93c0 100644
> --- a/include/linux/percpu-rwsem.h
> +++ b/include/linux/percpu-rwsem.h
> @@ -12,6 +12,14 @@ struct percpu_rw_semaphore {
> struct mutex mtx;
> };
>
> +#if 1
> +#define light_mb() barrier()
> +#define heavy_mb() synchronize_sched()
> +#else
> +#define light_mb() smp_mb()
> +#define heavy_mb() smp_mb();
> +#endif
> +
> static inline void percpu_down_read(struct percpu_rw_semaphore *p)
> {
> rcu_read_lock();
> @@ -24,22 +32,12 @@ static inline void percpu_down_read(struct
> percpu_rw_semaphore *p)
> }
> this_cpu_inc(*p->counters);
> rcu_read_unlock();
> + light_mb(); /* A, between read of p->locked and read of data, paired
> with D */
> }
>
> static inline void percpu_up_read(struct percpu_rw_semaphore *p)
> {
> - /*
> - * On X86, write operation in this_cpu_dec serves as a memory unlock
> - * barrier (i.e. memory accesses may be moved before the write, but
> - * no memory accesses are moved past the write).
> - * On other architectures this may not be the case, so we need smp_mb()
> - * there.
> - */
> -#if defined(CONFIG_X86) && (!defined(CONFIG_X86_PPRO_FENCE) &&
> !defined(CONFIG_X86_OOSTORE))
> - barrier();
> -#else
> - smp_mb();
> -#endif
> + light_mb(); /* B, between read of the data and write to p->counter,
> paired with C */
> this_cpu_dec(*p->counters);
> }
>
> @@ -61,11 +59,12 @@ static inline void percpu_down_write(struct
> percpu_rw_semaphore *p)
> synchronize_rcu();
> while (__percpu_count(p->counters))
> msleep(1);
> - smp_rmb(); /* paired with smp_mb() in percpu_sem_up_read() */
> + heavy_mb(); /* C, between read of p->counter and write to data, paired
> with B */
> }
>
> static inline void percpu_up_write(struct percpu_rw_semaphore *p)
> {
> + heavy_mb(); /* D, between write to data and write to p->locked, paired
> with A */
> p->locked = false;
> mutex_unlock(&p->mtx);
> }
>
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