From: Boqun Feng <boqun.feng@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Waiman Long <waiman.long@hpe.com>, Jason Low <jason.low2@hpe.com>,
Ding Tianhong <dingtianhong@huawei.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Imre Deak <imre.deak@intel.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Davidlohr Bueso <dave@stgolabs.net>,
Tim Chen <tim.c.chen@linux.intel.com>,
Terry Rudd <terry.rudd@hpe.com>,
"Paul E. McKenney" <paulmck@us.ibm.com>,
Jason Low <jason.low2@hp.com>,
Chris Wilson <chris@chris-wilson.co.uk>,
Daniel Vetter <daniel.vetter@ffwll.ch>
Subject: Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop
Date: Mon, 17 Oct 2016 21:45:01 +0800 [thread overview]
Message-ID: <20161017134501.GA27939@tardis.cn.ibm.com> (raw)
In-Reply-To: <20161017132408.GF3157@twins.programming.kicks-ass.net>
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On Mon, Oct 17, 2016 at 03:24:08PM +0200, Peter Zijlstra wrote:
> On Mon, Oct 17, 2016 at 12:44:49PM +0200, Peter Zijlstra wrote:
> > On Thu, Oct 13, 2016 at 04:17:21PM +0100, Will Deacon wrote:
> > > Hi Peter,
> > >
> > > I'm struggling to get my head around the handoff code after this change...
> > >
> > > On Fri, Oct 07, 2016 at 04:52:49PM +0200, Peter Zijlstra wrote:
> > > > --- a/kernel/locking/mutex.c
> > > > +++ b/kernel/locking/mutex.c
> > > > @@ -631,13 +631,21 @@ __mutex_lock_common(struct mutex *lock,
> > > >
> > > > lock_contended(&lock->dep_map, ip);
> > > >
> > > > + set_task_state(task, state);
> > > > for (;;) {
> > > > + /*
> > > > + * Once we hold wait_lock, we're serialized against
> > > > + * mutex_unlock() handing the lock off to us, do a trylock
> > > > + * before testing the error conditions to make sure we pick up
> > > > + * the handoff.
> > > > + */
> > > > if (__mutex_trylock(lock, first))
> > > > - break;
> > > > + goto acquired;
> > > >
> > > > /*
> > > > - * got a signal? (This code gets eliminated in the
> > > > - * TASK_UNINTERRUPTIBLE case.)
> > > > + * Check for signals and wound conditions while holding
> > > > + * wait_lock. This ensures the lock cancellation is ordered
> > > > + * against mutex_unlock() and wake-ups do not go missing.
> > > > */
> > > > if (unlikely(signal_pending_state(state, task))) {
> > > > ret = -EINTR;
> > > > @@ -650,16 +658,27 @@ __mutex_lock_common(struct mutex *lock,
> > > > goto err;
> > > > }
> > > >
> > > > - __set_task_state(task, state);
> > > > spin_unlock_mutex(&lock->wait_lock, flags);
> > > > schedule_preempt_disabled();
> > > > - spin_lock_mutex(&lock->wait_lock, flags);
> > > >
> > > > if (!first && __mutex_waiter_is_first(lock, &waiter)) {
> > > > first = true;
> > > > __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
> > > > }
> > > > +
> > > > + set_task_state(task, state);
> > >
> > > With this change, we no longer hold the lock wit_hen we set the task
> > > state, and it's ordered strictly *after* setting the HANDOFF flag.
> > > Doesn't that mean that the unlock code can see the HANDOFF flag, issue
> > > the wakeup, but then we come in and overwrite the task state?
> > >
> > > I'm struggling to work out whether that's an issue, but it certainly
> > > feels odd and is a change from the previous behaviour.
> >
> > Right, so I think the code is fine, since in that case the
> > __mutex_trylock() must see the handoff and we'll break the loop and
> > (re)set the state to RUNNING.
> >
> > But you're right in that its slightly odd. I'll reorder them and put the
> > set_task_state() above the !first thing.
>
>
> Humm,.. we might actually rely on this order, since the MB implied by
> set_task_state() is the only thing that separates the store of
> __mutex_set_flag() from the load of __mutex_trylock(), and those should
> be ordered I think.
>
But __mutex_set_flag() and __mutex_trylock() actually touch the same
atomic word? So we don't need extra things to order them?
Regards,
Boqun
> Argh, completely messed up my brain. I'll not touch it and think on this
> again tomorrow.
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next prev parent reply other threads:[~2016-10-17 13:45 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-10-07 14:52 [PATCH -v4 0/8] locking/mutex: Rewrite basic mutex Peter Zijlstra
2016-10-07 14:52 ` [PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra
2016-10-07 15:43 ` Peter Zijlstra
2016-10-07 15:58 ` Linus Torvalds
2016-10-07 16:13 ` Peter Zijlstra
2016-10-07 21:58 ` Waiman Long
2016-10-08 11:58 ` Thomas Gleixner
2016-10-08 14:01 ` Peter Zijlstra
2016-10-08 14:11 ` Thomas Gleixner
2016-10-08 16:42 ` Peter Zijlstra
2016-11-09 10:38 ` Peter Zijlstra
2016-10-18 12:48 ` Peter Zijlstra
2016-10-18 12:57 ` Peter Zijlstra
2016-11-11 11:22 ` Daniel Vetter
2016-11-11 11:38 ` Peter Zijlstra
2016-11-12 10:58 ` Ingo Molnar
2016-11-14 14:04 ` Peter Zijlstra
2016-11-14 14:27 ` Ingo Molnar
2016-10-18 12:57 ` Chris Wilson
2016-10-07 14:52 ` [PATCH -v4 2/8] locking/mutex: Rework mutex::owner Peter Zijlstra
2016-10-12 17:59 ` Davidlohr Bueso
2016-10-12 19:52 ` Jason Low
2016-10-13 15:18 ` Will Deacon
2016-10-07 14:52 ` [PATCH -v4 3/8] locking/mutex: Kill arch specific code Peter Zijlstra
2016-10-07 14:52 ` [PATCH -v4 4/8] locking/mutex: Allow MUTEX_SPIN_ON_OWNER when DEBUG_MUTEXES Peter Zijlstra
2016-10-07 14:52 ` [PATCH -v4 5/8] locking/mutex: Add lock handoff to avoid starvation Peter Zijlstra
2016-10-13 15:14 ` Will Deacon
2016-10-17 9:22 ` Peter Zijlstra
2016-10-17 18:45 ` Waiman Long
2016-10-17 19:07 ` Waiman Long
2016-10-18 13:02 ` Peter Zijlstra
2016-10-18 12:36 ` Peter Zijlstra
2016-12-27 13:55 ` Chris Wilson
2017-01-09 11:52 ` [PATCH] locking/mutex: Clear mutex-handoff flag on interrupt Chris Wilson
2017-01-11 16:43 ` Peter Zijlstra
2017-01-11 16:57 ` Chris Wilson
2017-01-12 20:58 ` Chris Wilson
2016-10-07 14:52 ` [PATCH -v4 6/8] locking/mutex: Restructure wait loop Peter Zijlstra
2016-10-13 15:17 ` Will Deacon
2016-10-17 10:44 ` Peter Zijlstra
2016-10-17 13:24 ` Peter Zijlstra
2016-10-17 13:45 ` Boqun Feng [this message]
2016-10-17 15:49 ` Peter Zijlstra
2016-10-19 17:34 ` Peter Zijlstra
2016-10-24 1:57 ` ciao set_task_state() (was Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop) Davidlohr Bueso
2016-10-24 13:26 ` Kent Overstreet
2016-10-24 14:27 ` Kent Overstreet
2016-10-25 16:55 ` Eric Wheeler
2016-10-25 17:45 ` Kent Overstreet
2016-10-17 23:16 ` [PATCH -v4 6/8] locking/mutex: Restructure wait loop Waiman Long
2016-10-18 13:14 ` Peter Zijlstra
2016-10-07 14:52 ` [PATCH -v4 7/8] locking/mutex: Simplify some ww_mutex code in __mutex_lock_common() Peter Zijlstra
2016-10-07 14:52 ` [PATCH -v4 8/8] locking/mutex: Enable optimistic spinning of woken waiter Peter Zijlstra
2016-10-13 15:28 ` Will Deacon
2016-10-17 9:32 ` Peter Zijlstra
2016-10-17 23:21 ` Waiman Long
2016-10-18 12:19 ` Peter Zijlstra
2016-10-07 15:20 ` [PATCH -v4 0/8] locking/mutex: Rewrite basic mutex Linus Torvalds
2016-10-11 18:42 ` Jason Low
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