From: will.deacon@arm•com (Will Deacon)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH v3 05/14] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath
Date: Mon, 30 Apr 2018 09:53:08 +0100 [thread overview]
Message-ID: <20180430085307.GB15504@arm.com> (raw)
In-Reply-To: <20180428124537.GD4082@hirez.programming.kicks-ass.net>
On Sat, Apr 28, 2018 at 02:45:37PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 26, 2018 at 05:55:19PM +0100, Will Deacon wrote:
> > On Thu, Apr 26, 2018 at 05:53:35PM +0200, Peter Zijlstra wrote:
> > > On Thu, Apr 26, 2018 at 11:34:19AM +0100, Will Deacon wrote:
> > > > @@ -290,58 +312,50 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> > > > }
> > > >
> > > > /*
> > > > + * If we observe any contention; queue.
> > > > + */
> > > > + if (val & ~_Q_LOCKED_MASK)
> > > > + goto queue;
> > > > +
> > > > + /*
> > > > * trylock || pending
> > > > *
> > > > * 0,0,0 -> 0,0,1 ; trylock
> > > > * 0,0,1 -> 0,1,1 ; pending
> > > > */
> > > > + val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
> > > > + if (!(val & ~_Q_LOCKED_MASK)) {
> > > > /*
> > > > + * we're pending, wait for the owner to go away.
> > > > + *
> > > > + * *,1,1 -> *,1,0
> > >
> > > Tail must be 0 here, right?
> >
> > Not necessarily. If we're concurrently setting pending with another slowpath
> > locker, they could queue in the tail behind us, so we can't mess with those
> > upper bits.
>
> Could be my brain just entirely stopped working; but I read that as:
>
> !(val & ~0xFF) := !(val & 0xFFFFFF00)
>
> which then pretty much mandates the top bits are empty, no?
Only if there isn't a concurrent locker. For example:
T0:
// fastpath fails to acquire the lock, returns val == _Q_LOCKED_VAL
if (val & ~_Q_LOCKED_MASK)
goto queue;
// Fallthrough
T1:
// fastpath fails to acquire the lock, returns val == _Q_LOCKED_VAL
if (val & ~_Q_LOCKED_MASK)
goto queue;
// Fallthrough
T0:
val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
// val == _Q_LOCKED_VAL
T1:
val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
// val == _Q_PENDING_VAL | _Q_LOCKED_VAL
// Queue into tail
T0:
// Spins for _Q_LOCKED_MASK to go to zero, but tail is *non-zero*
So it's really down to whether the state transitions in the comments refer
to the lockword in memory, or the local "val" variable. I think the former
is more instructive, because the whole thing is concurrent.
Will
next prev parent reply other threads:[~2018-04-30 8:53 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-26 10:34 [PATCH v3 00/14] kernel/locking: qspinlock improvements Will Deacon
2018-04-26 10:34 ` [PATCH v3 01/14] barriers: Introduce smp_cond_load_relaxed and atomic_cond_read_relaxed Will Deacon
2018-04-26 10:34 ` [PATCH v3 02/14] locking/qspinlock: Merge struct __qspinlock into struct qspinlock Will Deacon
2018-04-26 10:34 ` [PATCH v3 03/14] locking/qspinlock: Bound spinning on pending->locked transition in slowpath Will Deacon
2018-04-26 10:34 ` [PATCH v3 04/14] locking/qspinlock/x86: Increase _Q_PENDING_LOOPS upper bound Will Deacon
2018-04-26 10:34 ` [PATCH v3 05/14] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath Will Deacon
2018-04-26 15:53 ` Peter Zijlstra
2018-04-26 16:55 ` Will Deacon
2018-04-28 12:45 ` Peter Zijlstra
2018-04-30 8:53 ` Will Deacon [this message]
2018-04-26 20:16 ` Waiman Long
2018-04-27 10:16 ` Will Deacon
2018-04-27 13:09 ` Waiman Long
2018-04-26 10:34 ` [PATCH v3 06/14] locking/qspinlock: Kill cmpxchg loop when claiming lock from head of queue Will Deacon
2018-04-26 10:34 ` [PATCH v3 07/14] locking/qspinlock: Use atomic_cond_read_acquire Will Deacon
2018-04-26 10:34 ` [PATCH v3 08/14] locking/mcs: Use smp_cond_load_acquire() in mcs spin loop Will Deacon
2018-04-26 10:34 ` [PATCH v3 09/14] locking/qspinlock: Use smp_cond_load_relaxed to wait for next node Will Deacon
2018-04-26 10:34 ` [PATCH v3 10/14] locking/qspinlock: Make queued_spin_unlock use smp_store_release Will Deacon
2018-04-26 10:34 ` [PATCH v3 11/14] locking/qspinlock: Elide back-to-back RELEASE operations with smp_wmb() Will Deacon
2018-04-26 10:34 ` [PATCH v3 12/14] locking/qspinlock: Use try_cmpxchg instead of cmpxchg when locking Will Deacon
2018-04-26 10:34 ` [PATCH v3 13/14] locking/qspinlock: Add stat tracking for pending vs slowpath Will Deacon
2018-04-26 10:34 ` [PATCH v3 14/14] MAINTAINERS: Add myself as a co-maintainer for LOCKING PRIMITIVES Will Deacon
2018-04-26 15:55 ` Peter Zijlstra
2018-04-26 15:54 ` [PATCH v3 00/14] kernel/locking: qspinlock improvements Peter Zijlstra
2018-04-27 9:33 ` Ingo Molnar
2018-04-26 20:18 ` Waiman Long
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