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SubjectRe: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem
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On 11/13/19 5:21 AM, Peter Zijlstra wrote:
> +static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
> + unsigned int mode, int wake_flags,
> + void *key)
> +{
> + struct task_struct *p = get_task_struct(wq_entry->private);
> + bool reader = wq_entry->flags & WQ_FLAG_CUSTOM;
> + struct percpu_rw_semaphore *sem = key;
> +
> + /* concurrent against percpu_down_write(), can get stolen */
> + if (!__percpu_rwsem_trylock(sem, reader))
> + return 1;
> +
> + list_del_init(&wq_entry->entry);
> + smp_store_release(&wq_entry->private, NULL);
> +
> + wake_up_process(p);
> + put_task_struct(p);
> +
> + return !reader; /* wake 'all' readers and 1 writer */
> +}
> +
> +static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader)
> +{
> + DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);
> + bool wait;
> +
> + spin_lock_irq(&sem->waiters.lock);
> + /*
> + * Serialize against the wakeup in percpu_up_write(), if we fail
> + * the trylock, the wakeup must see us on the list.
> + */
> + wait = !__percpu_rwsem_trylock(sem, reader);
> + if (wait) {
> + wq_entry.flags |= WQ_FLAG_EXCLUSIVE | reader * WQ_FLAG_CUSTOM;
> + __add_wait_queue_entry_tail(&sem->waiters, &wq_entry);
> + }
> + spin_unlock_irq(&sem->waiters.lock);
> +
> + while (wait) {
> + set_current_state(TASK_UNINTERRUPTIBLE);
> + if (!smp_load_acquire(&wq_entry.private))
> + break;
> + schedule();
> + }

If I read the function correctly, you are setting the WQ_FLAG_EXCLUSIVE
for both readers and writers and __wake_up() is called with an exclusive
count of one. So only one reader or writer is woken up each time.
However, the comment above said we wake 'all' readers and 1 writer. That
doesn't match the actual code, IMO. To match the comments, you should
have set WQ_FLAG_EXCLUSIVE flag only on writer. In this case, you
probably don't need WQ_FLAG_CUSTOM to differentiate between readers and
writers.

Cheers,
Longman

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