From: Michal Hocko <mhocko@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
LKML <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
"H. Peter Anvin" <hpa@zytor.com>,
"David S. Miller" <davem@davemloft.net>,
Tony Luck <tony.luck@intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Chris Zankel <chris@zankel.net>,
Max Filippov <jcmvbkbc@gmail.com>,
Davidlohr Bueso <dave@stgolabs.net>,
Waiman Long <Waiman.Long@hpe.com>
Subject: Re: [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable
Date: Wed, 11 May 2016 11:02:07 +0200 [thread overview]
Message-ID: <20160511090206.GG16677@dhcp22.suse.cz> (raw)
In-Reply-To: <20160511083512.GG3193@twins.programming.kicks-ass.net>
On Wed 11-05-16 10:35:12, Peter Zijlstra wrote:
> On Wed, May 11, 2016 at 09:23:57AM +0200, Michal Hocko wrote:
> > On Tue 10-05-16 14:38:06, Peter Zijlstra wrote:
>
> > > Also, looking at it again; I think we're forgetting to re-adjust the
> > > BIAS for the cancelled writer.
> >
> > Hmm, __rwsem_down_write_failed_common does
> >
> > /* undo write bias from down_write operation, stop active locking */
> > count = rwsem_atomic_update(-RWSEM_ACTIVE_WRITE_BIAS, sem);
> >
> > which should remove the bias AFAIU.
>
> Right; at this point we're neutral wrt bias.
>
> > Later we do
> >
> > if (waiting) {
> > count = READ_ONCE(sem->count);
> >
> > /*
> > * If there were already threads queued before us and there are
> > * no active writers, the lock must be read owned; so we try to
> > * wake any read locks that were queued ahead of us.
> > */
> > if (count > RWSEM_WAITING_BIAS)
> > sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
> >
> > } else
> > count = rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
> >
> > and that might set RWSEM_WAITING_BIAS but the current holder of the lock
> > should handle that correctly and wake the waiting tasks IIUC. I will go
> > and check the code closer. It is quite easy to get this subtle code
> > wrong..
>
> Subtle; yes.
>
> So if you look at rwsem_try_write_lock() -- traditionally the only way
> to exit this wait loop, you see it does:
>
> if (count == RWSEM_WAITING_BIAS &&
> cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS,
> RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
> if (!list_is_singular(&sem->wait_list))
> rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
> rwsem_set_owner(sem);
> return true;
> }
>
> Which ends up clearing RWSEM_WAITING_BIAS is we were the only waiter --
> or rather, it always clear WAITING, but then tests the list and re-sets
> it if there's more than one waiters on.
>
> Now, the signal break doesn't clear WAITING if we were the only waiter
> on the list; which means any further down_read() will block (I didn't
> look at what a subsequent down_write() would do).
I was staring at this part as well but then I convinced myself that this
is OK because rwsem_down_read_failed does:
if (list_empty(&sem->wait_list))
adjustment += RWSEM_WAITING_BIAS;
list_add_tail(&waiter.list, &sem->wait_list);
/* we're now waiting on the lock, but no longer actively locking */
count = rwsem_atomic_update(adjustment, sem);
/* If there are no active locks, wake the front queued process(es).
*
* If there are no writers and we are first in the queue,
* wake our own waiter to join the existing active readers !
*/
if (count == RWSEM_WAITING_BIAS ||
(count > RWSEM_WAITING_BIAS &&
adjustment != -RWSEM_ACTIVE_READ_BIAS))
sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
__rwsem_do_wake should then see all the readers (including the current
one) and wake them up and set waiter->task to NULL to allow the current
one to break out of the loop as well.
> So I think we needs something like this, to clear WAITING if we leave
> the list empty.
But maybe this is the correct way to go.
> Does that make sense?
I do not see an immediate problem with this. Maybe Tetsuo can try this
out and see if it really makes a difference.
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index df4dcb883b50..7011dd1c286c 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -489,6 +489,8 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
> do {
> if (signal_pending_state(state, current)) {
> raw_spin_lock_irq(&sem->wait_lock);
> + if (list_singular(&sem->wait_list))
> + rwsem_atomic_update(-RWSEM_WAITING_BIAS, sem);
> ret = ERR_PTR(-EINTR);
> goto out;
> }
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2016-05-11 9:02 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-01 11:04 [PATCH 0/11] introduce down_write_killable for rw_semaphore v2 Michal Hocko
2016-04-01 11:04 ` [PATCH 01/11] locking, rwsem: get rid of __down_write_nested Michal Hocko
2016-04-02 0:28 ` Davidlohr Bueso
2016-04-01 11:04 ` [PATCH 02/11] locking, rwsem: drop explicit memory barriers Michal Hocko
2016-04-02 1:17 ` Davidlohr Bueso
2016-04-04 9:03 ` Michal Hocko
2016-04-04 9:06 ` [PATCH 1/2] xtensa, rwsem: drop superfluous arch specific implementation Michal Hocko
2016-04-04 9:06 ` [PATCH 2/2] sh, " Michal Hocko
2016-04-06 9:26 ` Peter Zijlstra
2016-04-06 9:50 ` Geert Uytterhoeven
2016-04-06 10:27 ` Peter Zijlstra
2016-04-04 10:23 ` [PATCH 1/2] xtensa, " Max Filippov
2016-04-06 9:06 ` [PATCH] sparc, " Michal Hocko
2016-04-01 11:04 ` [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable Michal Hocko
2016-04-02 4:41 ` Davidlohr Bueso
2016-04-04 9:17 ` Michal Hocko
2016-04-04 9:21 ` Peter Zijlstra
2016-04-07 6:58 ` Davidlohr Bueso
2016-04-07 7:38 ` Michal Hocko
2016-05-10 10:43 ` Tetsuo Handa
2016-05-10 11:53 ` Michal Hocko
2016-05-10 12:38 ` Peter Zijlstra
2016-05-10 13:57 ` Tetsuo Handa
2016-05-11 7:23 ` Michal Hocko
2016-05-11 8:28 ` Michal Hocko
2016-05-11 8:44 ` Peter Zijlstra
2016-05-11 9:04 ` Michal Hocko
2016-05-11 9:17 ` Peter Zijlstra
2016-05-11 9:31 ` Michal Hocko
2016-05-11 9:41 ` Peter Zijlstra
2016-05-11 13:59 ` Michal Hocko
2016-05-11 18:03 ` Michal Hocko
2016-05-12 11:57 ` [PATCH] locking, rwsem: Fix down_write_killable() Peter Zijlstra
2016-05-12 12:15 ` [tip:locking/rwsem] locking/rwsem: " tip-bot for Peter Zijlstra
2016-05-12 16:59 ` [PATCH] locking, rwsem: " Michal Hocko
2016-05-15 20:57 ` [tip:locking/rwsem] locking/rwsem: " tip-bot for Peter Zijlstra
2016-05-12 12:12 ` [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable Peter Zijlstra
2016-05-12 12:19 ` Michal Hocko
2016-05-12 13:58 ` Peter Zijlstra
2016-05-12 19:42 ` Waiman Long
2016-05-11 8:35 ` Peter Zijlstra
2016-05-11 9:02 ` Michal Hocko [this message]
2016-04-01 11:04 ` [PATCH 04/11] alpha, rwsem: provide __down_write_killable Michal Hocko
2016-04-01 11:04 ` [PATCH 05/11] ia64, " Michal Hocko
2016-04-01 11:04 ` [PATCH 06/11] s390, " Michal Hocko
2016-04-01 11:04 ` [PATCH 07/11] sh, " Michal Hocko
2016-04-01 11:04 ` [PATCH 08/11] sparc, " Michal Hocko
2016-04-01 11:04 ` [PATCH 09/11] xtensa, " Michal Hocko
2016-04-01 11:04 ` [PATCH 10/11] x86, " Michal Hocko
2016-04-06 18:31 ` Peter Zijlstra
2016-04-01 11:04 ` [PATCH 11/11] locking, rwsem: provide down_write_killable Michal Hocko
-- strict thread matches above, loose matches on Subject: below --
2016-02-29 12:58 [PATCH 0/11] introduce down_write_killable for rw_semaphore Michal Hocko
2016-02-29 12:58 ` [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable Michal Hocko
2016-03-30 13:25 ` Peter Zijlstra
2016-03-31 8:33 ` Michal Hocko
2016-03-31 8:44 ` Peter Zijlstra
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