From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will.deacon@arm.com>
Cc: linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, mingo@kernel.org,
boqun.feng@gmail.com, paulmck@linux.vnet.ibm.com,
catalin.marinas@arm.com, Waiman Long <longman@redhat.com>
Subject: Re: [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath
Date: Thu, 5 Apr 2018 19:07:06 +0200 [thread overview]
Message-ID: <20180405170706.GC4082@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <1522947547-24081-3-git-send-email-will.deacon@arm.com>
On Thu, Apr 05, 2018 at 05:58:59PM +0100, Will Deacon wrote:
> The qspinlock locking slowpath utilises a "pending" bit as a simple form
> of an embedded test-and-set lock that can avoid the overhead of explicit
> queuing in cases where the lock is held but uncontended. This bit is
> managed using a cmpxchg loop which tries to transition the uncontended
> lock word from (0,0,0) -> (0,0,1) or (0,0,1) -> (0,1,1).
>
> Unfortunately, the cmpxchg loop is unbounded and lockers can be starved
> indefinitely if the lock word is seen to oscillate between unlocked
> (0,0,0) and locked (0,0,1). This could happen if concurrent lockers are
> able to take the lock in the cmpxchg loop without queuing and pass it
> around amongst themselves.
>
> This patch fixes the problem by unconditionally setting _Q_PENDING_VAL
> using atomic_fetch_or,
Of course, LL/SC or cmpxchg implementations of fetch_or do not in fact
get anything from this ;-)
WARNING: multiple messages have this Message-ID (diff)
From: peterz@infradead.org (Peter Zijlstra)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath
Date: Thu, 5 Apr 2018 19:07:06 +0200 [thread overview]
Message-ID: <20180405170706.GC4082@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <1522947547-24081-3-git-send-email-will.deacon@arm.com>
On Thu, Apr 05, 2018 at 05:58:59PM +0100, Will Deacon wrote:
> The qspinlock locking slowpath utilises a "pending" bit as a simple form
> of an embedded test-and-set lock that can avoid the overhead of explicit
> queuing in cases where the lock is held but uncontended. This bit is
> managed using a cmpxchg loop which tries to transition the uncontended
> lock word from (0,0,0) -> (0,0,1) or (0,0,1) -> (0,1,1).
>
> Unfortunately, the cmpxchg loop is unbounded and lockers can be starved
> indefinitely if the lock word is seen to oscillate between unlocked
> (0,0,0) and locked (0,0,1). This could happen if concurrent lockers are
> able to take the lock in the cmpxchg loop without queuing and pass it
> around amongst themselves.
>
> This patch fixes the problem by unconditionally setting _Q_PENDING_VAL
> using atomic_fetch_or,
Of course, LL/SC or cmpxchg implementations of fetch_or do not in fact
get anything from this ;-)
next prev parent reply other threads:[~2018-04-05 17:07 UTC|newest]
Thread overview: 94+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-05 16:58 [PATCH 00/10] kernel/locking: qspinlock improvements Will Deacon
2018-04-05 16:58 ` Will Deacon
2018-04-05 16:58 ` [PATCH 01/10] locking/qspinlock: Don't spin on pending->locked transition in slowpath Will Deacon
2018-04-05 16:58 ` Will Deacon
2018-04-05 16:58 ` [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath Will Deacon
2018-04-05 16:58 ` Will Deacon
2018-04-05 17:07 ` Peter Zijlstra [this message]
2018-04-05 17:07 ` Peter Zijlstra
2018-04-06 15:08 ` Will Deacon
2018-04-06 15:08 ` Will Deacon
2018-04-05 17:13 ` Peter Zijlstra
2018-04-05 17:13 ` Peter Zijlstra
2018-04-05 21:16 ` Waiman Long
2018-04-05 21:16 ` Waiman Long
2018-04-06 15:08 ` Will Deacon
2018-04-06 15:08 ` Will Deacon
2018-04-06 20:50 ` Waiman Long
2018-04-06 20:50 ` Waiman Long
2018-04-06 21:09 ` Paul E. McKenney
2018-04-06 21:09 ` Paul E. McKenney
2018-04-07 8:47 ` Peter Zijlstra
2018-04-07 8:47 ` Peter Zijlstra
2018-04-07 23:37 ` Paul E. McKenney
2018-04-07 23:37 ` Paul E. McKenney
2018-04-09 10:58 ` Will Deacon
2018-04-09 10:58 ` Will Deacon
2018-04-07 9:07 ` Peter Zijlstra
2018-04-07 9:07 ` Peter Zijlstra
2018-04-09 10:58 ` Will Deacon
2018-04-09 10:58 ` Will Deacon
2018-04-09 14:54 ` Will Deacon
2018-04-09 14:54 ` Will Deacon
2018-04-09 15:54 ` Peter Zijlstra
2018-04-09 15:54 ` Peter Zijlstra
2018-04-09 17:19 ` Will Deacon
2018-04-09 17:19 ` Will Deacon
2018-04-10 9:35 ` Peter Zijlstra
2018-04-10 9:35 ` Peter Zijlstra
2018-09-20 16:08 ` Peter Zijlstra
2018-09-20 16:08 ` Peter Zijlstra
2018-09-20 16:22 ` Peter Zijlstra
2018-09-20 16:22 ` Peter Zijlstra
2018-04-09 19:33 ` Waiman Long
2018-04-09 19:33 ` Waiman Long
2018-04-09 17:55 ` Waiman Long
2018-04-09 17:55 ` Waiman Long
2018-04-10 13:49 ` Sasha Levin
2018-04-10 13:49 ` Sasha Levin
2018-04-05 16:59 ` [PATCH 03/10] locking/qspinlock: Kill cmpxchg loop when claiming lock from head of queue Will Deacon
2018-04-05 16:59 ` Will Deacon
2018-04-05 17:19 ` Peter Zijlstra
2018-04-05 17:19 ` Peter Zijlstra
2018-04-06 10:54 ` Will Deacon
2018-04-06 10:54 ` Will Deacon
2018-04-05 16:59 ` [PATCH 04/10] locking/qspinlock: Use atomic_cond_read_acquire Will Deacon
2018-04-05 16:59 ` Will Deacon
2018-04-05 16:59 ` [PATCH 05/10] locking/mcs: Use smp_cond_load_acquire() in mcs spin loop Will Deacon
2018-04-05 16:59 ` Will Deacon
2018-04-05 16:59 ` [PATCH 06/10] barriers: Introduce smp_cond_load_relaxed and atomic_cond_read_relaxed Will Deacon
2018-04-05 16:59 ` Will Deacon
2018-04-05 17:22 ` Peter Zijlstra
2018-04-05 17:22 ` Peter Zijlstra
2018-04-06 10:55 ` Will Deacon
2018-04-06 10:55 ` Will Deacon
2018-04-05 16:59 ` [PATCH 07/10] locking/qspinlock: Use smp_cond_load_relaxed to wait for next node Will Deacon
2018-04-05 16:59 ` Will Deacon
2018-04-05 16:59 ` [PATCH 08/10] locking/qspinlock: Merge struct __qspinlock into struct qspinlock Will Deacon
2018-04-05 16:59 ` Will Deacon
2018-04-07 5:23 ` Boqun Feng
2018-04-07 5:23 ` Boqun Feng
2018-04-05 16:59 ` [PATCH 09/10] locking/qspinlock: Make queued_spin_unlock use smp_store_release Will Deacon
2018-04-05 16:59 ` Will Deacon
2018-04-05 16:59 ` [PATCH 10/10] locking/qspinlock: Elide back-to-back RELEASE operations with smp_wmb() Will Deacon
2018-04-05 16:59 ` Will Deacon
2018-04-05 17:28 ` Peter Zijlstra
2018-04-05 17:28 ` Peter Zijlstra
2018-04-06 11:34 ` Will Deacon
2018-04-06 11:34 ` Will Deacon
2018-04-06 13:05 ` Andrea Parri
2018-04-06 13:05 ` Andrea Parri
2018-04-06 15:27 ` Will Deacon
2018-04-06 15:27 ` Will Deacon
2018-04-06 15:49 ` Andrea Parri
2018-04-06 15:49 ` Andrea Parri
2018-04-07 5:47 ` Boqun Feng
2018-04-07 5:47 ` Boqun Feng
2018-04-09 10:47 ` Will Deacon
2018-04-09 10:47 ` Will Deacon
2018-04-06 13:22 ` [PATCH 00/10] kernel/locking: qspinlock improvements Andrea Parri
2018-04-06 13:22 ` Andrea Parri
2018-04-11 10:20 ` Catalin Marinas
2018-04-11 10:20 ` Catalin Marinas
2018-04-11 15:39 ` Andrea Parri
2018-04-11 15:39 ` Andrea Parri
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