On 05/11, Peter Zijlstra wrote:
>
> +static inline int __rw_mutex_read_trylock(struct rw_mutex *rw_mutex)
> +{
> + preempt_disable();
> + if (likely(!__rw_mutex_reader_slow(rw_mutex))) {
--- WINDOW ---
> + percpu_counter_mod(&rw_mutex->readers, 1);
> + preempt_enable();
> + return 1;
> + }
> + preempt_enable();
> + return 0;
> +}
>
> [...snip...]
>
> +void rw_mutex_write_lock_nested(struct rw_mutex *rw_mutex, int subclass)
> +{
> [...snip...]
> +
> + /*
> + * block new readers
> + */
> + __rw_mutex_status_set(rw_mutex, RW_MUTEX_READER_SLOW);
> + /*
> + * wait for all readers to go away
> + */
> + wait_event(rw_mutex->wait_queue,
> + (percpu_counter_sum(&rw_mutex->readers) == 0));
> +}
This look a bit suspicious, can't mutex_write_lock() set RW_MUTEX_READER_SLOW
and find percpu_counter_sum() == 0 in that WINDOW above?
> +void rw_mutex_read_unlock(struct rw_mutex *rw_mutex)
> +{
> + rwsem_release(&rw_mutex->dep_map, 1, _RET_IP_);
> +
> + percpu_counter_mod(&rw_mutex->readers, -1);
> + if (unlikely(__rw_mutex_reader_slow(rw_mutex)) &&
> + percpu_counter_sum(&rw_mutex->readers) == 0)
> + wake_up_all(&rw_mutex->wait_queue);
> +}
The same. __rw_mutex_status_set()->wmb() in rw_mutex_write_lock below
is not enough. percpu_counter_mod() doesn't take fbc->lock if < FBC_BATCH,
so we don't have a proper serialization.
write_lock() sets RW_MUTEX_READER_SLOW, finds percpu_counter_sum() != 0,
and sleeps. rw_mutex_read_unlock() decrements cpu-local var, does not
see RW_MUTEX_READER_SLOW and skips wake_up_all().
> +void rw_mutex_write_lock_nested(struct rw_mutex *rw_mutex, int subclass)
> +{
> + might_sleep();
> + rwsem_acquire(&rw_mutex->dep_map, subclass, 0, _RET_IP_);
> +
> + mutex_lock_nested(&rw_mutex->write_mutex, subclass);
> + mutex_lock_nested(&rw_mutex->read_mutex, subclass);
> +
> + /*
> + * block new readers
> + */
> + __rw_mutex_status_set(rw_mutex, RW_MUTEX_READER_SLOW);
> + /*
> + * wait for all readers to go away
> + */
> + wait_event(rw_mutex->wait_queue,
> + (percpu_counter_sum(&rw_mutex->readers) == 0));
> +}
> +
> +void rw_mutex_write_unlock(struct rw_mutex *rw_mutex)
> +{
> + int waiters;
> +
> + rwsem_release(&rw_mutex->dep_map, 1, _RET_IP_);
> +
> + /*
> + * let the readers rip
> + */
> + __rw_mutex_status_set(rw_mutex, RW_MUTEX_READER_FAST);
> + waiters = atomic_read(&rw_mutex->read_waiters);
> + mutex_unlock(&rw_mutex->read_mutex);
> + /*
> + * wait for at least 1 reader to get through
> + */
> + if (waiters) {
> + wait_event(rw_mutex->wait_queue,
> + (atomic_read(&rw_mutex->read_waiters) < waiters));
> + }
> + /*
> + * before we let the writers rip
> + */
> + mutex_unlock(&rw_mutex->write_mutex);
> +}
Looks like we can have only one task on rw_mutex->wait_queue, and it holds
->write_mutex. Can't we use just a "task_struct *write_waiter" instead of
->wait_queue ? This makes rw_mutex smaller.
Oleg.
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