Not really the comment, but the question...
On 10/22, Mikulas Patocka wrote:
>
> static inline void percpu_down_read(struct percpu_rw_semaphore *p)
> {
> rcu_read_lock();
> @@ -24,22 +27,12 @@ static inline void percpu_down_read(stru
> }
> this_cpu_inc(*p->counters);
> rcu_read_unlock();
> + light_mb(); /* A, between read of p->locked and read of data, paired
> with D */
> }
rcu_read_unlock() (or even preempt_enable) should have compiler barrier
semantics... But I agree, this adds more documentation for free.
> 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 +54,12 @@ static inline void percpu_down_write(str
> 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 */
I _think_ this is correct.
Just I am wondering if this is strongly correct in theory, I would
really like to know what Paul thinks.
Ignoring the current implementation, according to the documentation
synchronize_sched() has all rights to return immediately if there is
no active rcu_read_lock_sched() section. If this were possible, than
percpu_up_read() lacks mb.
So _perhaps_ it makes sense to document that synchronize_sched() also
guarantees that all pending loads/stores on other CPUs should be
completed upon return? Or I misunderstood the patch?
Oleg.
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