From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: linux-kernel@vger.kernel.org, netfilter-devel@vger.kernel.org,
netdev@vger.kernel.org, akpm@linux-foundation.org,
mingo@redhat.com, dave@stgolabs.net, manfred@colorfullife.com,
tj@kernel.org, arnd@arndb.de, linux-arch@vger.kernel.org,
will.deacon@arm.com, peterz@infradead.org,
stern@rowland.harvard.edu, parri.andrea@gmail.com,
torvalds@linux-foundation.org
Subject: Re: [PATCH RFC 02/26] task_work: Replace spin_unlock_wait() with lock/unlock pair
Date: Fri, 30 Jun 2017 10:21:01 -0700 [thread overview]
Message-ID: <20170630172101.GA3162@linux.vnet.ibm.com> (raw)
In-Reply-To: <20170630161607.GX2393@linux.vnet.ibm.com>
On Fri, Jun 30, 2017 at 09:16:07AM -0700, Paul E. McKenney wrote:
> On Fri, Jun 30, 2017 at 05:20:10PM +0200, Oleg Nesterov wrote:
> > On 06/30, Paul E. McKenney wrote:
> > >
> > > > > + raw_spin_lock_irq(&task->pi_lock);
> > > > > + raw_spin_unlock_irq(&task->pi_lock);
> > >
> > > I agree that the spin_unlock_wait() implementations would avoid the
> > > deadlock with an acquisition from an interrupt handler, while also
> > > avoiding the need to momentarily disable interrupts. The ->pi_lock is
> > > a per-task lock, so I am assuming (perhaps naively) that contention is
> > > not a problem. So is the overhead of interrupt disabling likely to be
> > > noticeable here?
> >
> > I do not think the overhead will be noticeable in this particular case.
> >
> > But I am not sure I understand why do we want to unlock_wait. Yes I agree,
> > it has some problems, but still...
Well, I tried documenting exactly what it did and did not do, which got
an ack from Peter.
https://marc.info/?l=linux-kernel&m=149575078313105
However, my later pull request spawned a bit of discussion:
https://marc.info/?l=linux-kernel&m=149730349001044
This discussion led me to propose strengthening spin_unlock_wait()
to act as a lock/unlock pair. This can be implemented on x86 as
an smp_mb() followed by a read-only spinloop, as shown on branch
spin_unlock_wait.2017.06.23a on my -rcu tree.
Linus was not amused, and said that if we were going to make
spin_unlock_wait() have the semantics of lock+unlock, we should just
open-code that, especially given that there are way more definitions
of spin_unlock_wait() than there are uses. He also suggested making
spin_unlock_wait() have only acquire semantics (x86 spin loop with
no memory-barrier instructions) and add explicit barriers where
required.
https://marc.info/?l=linux-kernel&m=149860012913036
I did a series for this which may be found on branch
spin_unlock_wait.2017.06.27a on my -rcu tree.
This approach was not loved by others (see later on the above thread), and
Linus's reply (which reiterated his opposition to lock+unlock semantics)
suggested the possibility of removing spin_unlock_wait() entirely.
https://marc.info/?l=linux-kernel&m=149869476911620
So I figured, in for a penny, in for a pound, and therefore did the series
that includes this patch. The most recent update (which does not yet
include your improved version) is on branch spin_unlock_wait.2017.06.30b
of my -rcu tree.
Hey, you asked! ;-)
Thanx, Paul
> > The code above looks strange for me. If we are going to repeat this pattern
> > the perhaps we should add a helper for lock+unlock and name it unlock_wait2 ;)
> >
> > If not, we should probably change this code more:
>
> This looks -much- better than my patch! May I have your Signed-off-by?
>
> Thanx, Paul
>
> > --- a/kernel/task_work.c
> > +++ b/kernel/task_work.c
> > @@ -96,20 +96,16 @@ void task_work_run(void)
> > * work->func() can do task_work_add(), do not set
> > * work_exited unless the list is empty.
> > */
> > + raw_spin_lock_irq(&task->pi_lock);
> > do {
> > work = READ_ONCE(task->task_works);
> > head = !work && (task->flags & PF_EXITING) ?
> > &work_exited : NULL;
> > } while (cmpxchg(&task->task_works, work, head) != work);
> > + raw_spin_unlock_irq(&task->pi_lock);
> >
> > if (!work)
> > break;
> > - /*
> > - * Synchronize with task_work_cancel(). It can't remove
> > - * the first entry == work, cmpxchg(task_works) should
> > - * fail, but it can play with *work and other entries.
> > - */
> > - raw_spin_unlock_wait(&task->pi_lock);
> >
> > do {
> > next = work->next;
> >
> > performance-wise this is almost the same, and if we do not really care about
> > overhead we can simplify the code: this way it is obvious that we can't race
> > with task_work_cancel().
> >
> > Oleg.
> >
next prev parent reply other threads:[~2017-06-30 17:21 UTC|newest]
Thread overview: 122+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-29 23:59 [PATCH RFC 0/26] Remove spin_unlock_wait() Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 01/26] netfilter: Replace spin_unlock_wait() with lock/unlock pair Paul E. McKenney
[not found] ` <a6642feb-2f3a-980f-5ed6-2deb79563e6b@colorfullife.com>
2017-07-02 2:00 ` Paul E. McKenney
2017-07-03 14:39 ` Alan Stern
2017-07-03 17:14 ` Paul E. McKenney
2017-07-03 19:01 ` Manfred Spraul
2017-07-03 19:57 ` Alan Stern
2017-07-06 18:43 ` Manfred Spraul
2017-07-03 20:04 ` Alan Stern
2017-07-03 20:53 ` Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 02/26] task_work: " Paul E. McKenney
2017-06-30 11:04 ` Oleg Nesterov
2017-06-30 12:50 ` Paul E. McKenney
2017-06-30 15:20 ` Oleg Nesterov
2017-06-30 16:16 ` Paul E. McKenney
2017-06-30 17:21 ` Paul E. McKenney [this message]
2017-06-30 19:21 ` Oleg Nesterov
2017-06-30 19:50 ` Alan Stern
2017-06-30 20:04 ` Paul E. McKenney
2017-06-30 20:02 ` Paul E. McKenney
2017-06-30 20:19 ` Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 03/26] sched: " Paul E. McKenney
2017-06-30 10:31 ` Arnd Bergmann
2017-06-30 12:35 ` Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 04/26] completion: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 05/26] exit: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 07/26] drivers/ata: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 08/26] locking: Remove spin_unlock_wait() generic definitions Paul E. McKenney
2017-06-30 9:19 ` Will Deacon
2017-06-30 12:38 ` Paul E. McKenney
2017-06-30 13:13 ` Will Deacon
2017-06-30 22:18 ` Paul E. McKenney
2017-07-03 13:15 ` Will Deacon
2017-07-03 16:18 ` Paul E. McKenney
2017-07-03 16:40 ` Linus Torvalds
2017-07-03 17:13 ` Will Deacon
2017-07-03 22:30 ` Paul E. McKenney
2017-07-03 22:49 ` Linus Torvalds
2017-07-04 0:39 ` Paul E. McKenney
2017-07-04 0:54 ` Paul E. McKenney
2017-07-03 21:10 ` Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 09/26] alpha: Remove spin_unlock_wait() arch-specific definitions Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 10/26] arc: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 11/26] arm: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 12/26] arm64: " Paul E. McKenney
2017-06-30 9:20 ` Will Deacon
2017-06-30 17:29 ` Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 13/26] blackfin: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 14/26] hexagon: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 15/26] ia64: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 16/26] m32r: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 18/26] mips: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 19/26] mn10300: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 20/26] parisc: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 21/26] powerpc: " Paul E. McKenney
2017-07-02 3:58 ` Boqun Feng
2017-07-05 23:57 ` Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 22/26] s390: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 23/26] sh: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 24/26] sparc: " Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 25/26] tile: " Paul E. McKenney
2017-06-30 0:06 ` Linus Torvalds
2017-06-30 0:09 ` Paul E. McKenney
2017-06-30 0:14 ` Paul E. McKenney
2017-06-30 0:10 ` Linus Torvalds
2017-06-30 0:24 ` Paul E. McKenney
2017-06-30 0:01 ` [PATCH RFC 26/26] xtensa: " Paul E. McKenney
[not found] ` <1498780894-8253-6-git-send-email-paulmck@linux.vnet.ibm.com>
2017-07-01 19:23 ` [PATCH RFC 06/26] ipc: Replace spin_unlock_wait() with lock/unlock pair Manfred Spraul
2017-07-02 3:16 ` Paul E. McKenney
2017-07-05 23:29 ` [PATCH v2 0/9] Remove spin_unlock_wait() Paul E. McKenney
2017-07-05 23:31 ` [PATCH v2 1/9] net/netfilter/nf_conntrack_core: Fix net_conntrack_lock() Paul E. McKenney
2017-07-06 18:45 ` Manfred Spraul
2017-07-06 20:26 ` Paul E. McKenney
2017-07-05 23:31 ` [PATCH v2 2/9] task_work: Replace spin_unlock_wait() with lock/unlock pair Paul E. McKenney
2017-07-05 23:31 ` [PATCH v2 3/9] sched: " Paul E. McKenney
2017-07-05 23:31 ` [PATCH v2 4/9] completion: " Paul E. McKenney
2017-07-05 23:31 ` [PATCH v2 5/9] exit: " Paul E. McKenney
2017-07-05 23:31 ` [PATCH v2 6/9] ipc: " Paul E. McKenney
2017-07-05 23:31 ` [PATCH v2 7/9] drivers/ata: " Paul E. McKenney
2017-07-05 23:31 ` [PATCH v2 8/9] locking: Remove spin_unlock_wait() generic definitions Paul E. McKenney
2017-07-05 23:31 ` [PATCH v2 9/9] arch: Remove spin_unlock_wait() arch-specific definitions Paul E. McKenney
2017-07-06 14:12 ` [PATCH v2 0/9] Remove spin_unlock_wait() David Laight
2017-07-06 15:21 ` Paul E. McKenney
2017-07-06 16:10 ` Peter Zijlstra
2017-07-06 16:24 ` Paul E. McKenney
2017-07-06 16:41 ` Peter Zijlstra
2017-07-06 17:03 ` Paul E. McKenney
2017-07-06 16:49 ` Alan Stern
2017-07-06 16:54 ` Peter Zijlstra
2017-07-06 19:37 ` Alan Stern
2017-07-06 16:05 ` Peter Zijlstra
2017-07-06 16:20 ` Paul E. McKenney
2017-07-06 16:50 ` Peter Zijlstra
2017-07-06 17:08 ` Will Deacon
2017-07-06 17:29 ` Paul E. McKenney
2017-07-06 17:18 ` Paul E. McKenney
2017-07-07 8:31 ` Ingo Molnar
2017-07-07 8:44 ` Peter Zijlstra
2017-07-07 10:33 ` Ingo Molnar
2017-07-07 11:23 ` Peter Zijlstra
2017-07-07 14:41 ` Paul E. McKenney
2017-07-08 8:43 ` Ingo Molnar
2017-07-08 11:41 ` Paul E. McKenney
2017-07-07 17:47 ` Manfred Spraul
2017-07-08 8:35 ` Ingo Molnar
2017-07-08 11:39 ` Paul E. McKenney
2017-07-08 12:30 ` Ingo Molnar
2017-07-08 14:45 ` Paul E. McKenney
2017-07-08 16:21 ` Alan Stern
2017-07-10 17:22 ` Manfred Spraul
2017-07-07 8:06 ` Ingo Molnar
2017-07-07 9:32 ` Ingo Molnar
2017-07-07 19:27 ` [PATCH v3 " Paul E. McKenney
2017-07-07 19:28 ` [PATCH v3 1/9] net/netfilter/nf_conntrack_core: Fix net_conntrack_lock() Paul E. McKenney
2017-07-07 19:28 ` [PATCH v3 2/9] task_work: Replace spin_unlock_wait() with lock/unlock pair Paul E. McKenney
2017-07-07 19:28 ` [PATCH v3 3/9] sched: " Paul E. McKenney
2017-07-07 19:28 ` [PATCH v3 4/9] completion: " Paul E. McKenney
2017-07-07 19:28 ` [PATCH v3 5/9] exit: " Paul E. McKenney
2017-07-07 19:28 ` [PATCH v3 6/9] ipc: " Paul E. McKenney
2017-07-07 19:28 ` [PATCH v3 7/9] drivers/ata: " Paul E. McKenney
2017-07-07 19:28 ` [PATCH v3 8/9] locking: Remove spin_unlock_wait() generic definitions Paul E. McKenney
2017-07-07 19:28 ` [PATCH v3 9/9] arch: Remove spin_unlock_wait() arch-specific definitions Paul E. McKenney
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