On 10/24, Paul E. McKenney wrote:
>
> static inline void percpu_up_read(struct percpu_rw_semaphore *p)
> {
> /*
> * Decrement our count, but protected by RCU-sched so that
> * the writer can force proper serialization.
> */
> rcu_read_lock_sched();
> this_cpu_dec(*p->counters);
> rcu_read_unlock_sched();
> }
Yes, the explicit lock/unlock makes the new assumptions about
synchronize_sched && barriers unnecessary. And iiuc this could
even written as
rcu_read_lock_sched();
rcu_read_unlock_sched();
this_cpu_dec(*p->counters);
> Of course, it would be nice to get rid of the extra synchronize_sched().
> One way to do this is to use SRCU, which allows blocking operations in
> its read-side critical sections (though also increasing read-side overhead
> a bit, and also untested):
>
> ------------------------------------------------------------------------
>
> struct percpu_rw_semaphore {
> bool locked;
> struct mutex mtx; /* Could also be rw_semaphore. */
> struct srcu_struct s;
> wait_queue_head_t wq;
> };
but in this case I don't understand
> static inline void percpu_up_write(struct percpu_rw_semaphore *p)
> {
> /* Allow others to proceed, but not yet locklessly. */
> mutex_unlock(&p->mtx);
>
> /*
> * Ensure that all calls to percpu_down_read() that did not
> * start unambiguously after the above mutex_unlock() still
> * acquire the lock, forcing their critical sections to be
> * serialized with the one terminated by this call to
> * percpu_up_write().
> */
> synchronize_sched();
how this synchronize_sched() can help...
Oleg.
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