From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>, mingo@kernel.org, will@kernel.org
Cc: oleg@redhat.com, tglx@linutronix.de,
linux-kernel@vger.kernel.org, bigeasy@linutronix.de,
juri.lelli@redhat.com, williams@redhat.com, bristot@redhat.com,
dave@stgolabs.net, jack@suse.com
Subject: Re: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem
Date: Tue, 19 Nov 2019 08:50:11 -0500 [thread overview]
Message-ID: <ee75fc38-c3c8-3f9e-13ba-5c8312d61325@redhat.com> (raw)
In-Reply-To: <20191113102855.925208237@infradead.org>
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
next prev parent reply other threads:[~2019-11-19 13:50 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-13 10:21 [PATCH 0/5] locking: Percpu-rwsem rewrite Peter Zijlstra
2019-11-13 10:21 ` [PATCH 1/5] locking/percpu-rwsem, lockdep: Make percpu-rwsem use its own lockdep_map Peter Zijlstra
2019-11-15 20:39 ` Davidlohr Bueso
2020-01-08 1:33 ` [PATCH] locking/percpu-rwsem: Add might_sleep() for writer locking Davidlohr Bueso
2020-01-08 1:33 ` Davidlohr Bueso
2020-02-11 12:48 ` [tip: locking/core] " tip-bot2 for Davidlohr Bueso
2019-11-13 10:21 ` [PATCH 2/5] locking/percpu-rwsem: Convert to bool Peter Zijlstra
2019-11-13 10:21 ` [PATCH 3/5] locking/percpu-rwsem: Move __this_cpu_inc() into the slowpath Peter Zijlstra
2019-11-13 10:21 ` [PATCH 4/5] locking/percpu-rwsem: Extract __percpu_down_read_trylock() Peter Zijlstra
2019-11-18 16:28 ` Oleg Nesterov
2019-11-13 10:21 ` [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem Peter Zijlstra
2019-11-18 19:53 ` Davidlohr Bueso
2019-11-18 23:19 ` Davidlohr Bueso
2019-12-17 10:45 ` Peter Zijlstra
2019-12-17 10:35 ` Peter Zijlstra
2019-11-18 21:52 ` Waiman Long
2019-12-17 10:28 ` Peter Zijlstra
2019-11-19 13:50 ` Waiman Long [this message]
2019-11-19 15:58 ` Oleg Nesterov
2019-11-19 16:28 ` Waiman Long
2019-12-17 10:26 ` Peter Zijlstra
2019-12-17 10:28 ` Peter Zijlstra
2019-11-15 17:14 ` [PATCH 0/5] locking: Percpu-rwsem rewrite Juri Lelli
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