From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <llong@redhat.com>
Cc: mingo@redhat.com, boqun.feng@gmail.com, will@kernel.org,
linux-kernel@vger.kernel.org, yanfei.xu@windriver.com
Subject: Re: [RFC][PATCH 3/4] locking/mutex: Introduce __mutex_trylock_or_handoff()
Date: Wed, 30 Jun 2021 20:04:34 +0200 [thread overview]
Message-ID: <YNyyMvPYBmyexlmC@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <9c47a47c-0c95-4f8f-8b62-a1aff10be748@redhat.com>
On Wed, Jun 30, 2021 at 12:30:43PM -0400, Waiman Long wrote:
> On 6/30/21 11:35 AM, Peter Zijlstra wrote:
> > Yanfei reported that it is possible to loose HANDOFF when we race with
> > mutex_unlock() and end up setting HANDOFF on an unlocked mutex. At
> > that point anybody can steal it, loosing HANDOFF in the process.
> >
> > If this happens often enough, we can in fact starve the top waiter.
> >
> > Solve this by folding the 'set HANDOFF' operation into the trylock
> > operation, such that either we acquire the lock, or it gets HANDOFF
> > set. This avoids having HANDOFF set on an unlocked mutex.
> >
> > Reported-by: Yanfei Xu <yanfei.xu@windriver.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > ---
> > kernel/locking/mutex.c | 58 +++++++++++++++++++++++++++++--------------------
> > 1 file changed, 35 insertions(+), 23 deletions(-)
> >
> > --- a/kernel/locking/mutex.c
> > +++ b/kernel/locking/mutex.c
> > @@ -91,10 +91,7 @@ static inline unsigned long __owner_flag
> > return owner & MUTEX_FLAGS;
> > }
> > -/*
> > - * Trylock variant that returns the owning task on failure.
> > - */
> > -static inline struct task_struct *__mutex_trylock_or_owner(struct mutex *lock)
> > +static inline struct task_struct *__mutex_trylock_common(struct mutex *lock, bool handoff)
> > {
> > unsigned long owner, curr = (unsigned long)current;
> > @@ -104,39 +101,56 @@ static inline struct task_struct *__mute
> > unsigned long task = owner & ~MUTEX_FLAGS;
> > if (task) {
> > - if (likely(task != curr))
> > + if (flags & MUTEX_FLAG_PICKUP) {
> > + if (task != curr)
> > + break;
> > + flags &= ~MUTEX_FLAG_HANDOFF;
>
> I think you mean "flags &= ~MUTEX_FLAG_PICKUP". Right:-)
Duh, yes. That's what you get trying to write patches with a kid in your
lap.. :-)
next prev parent reply other threads:[~2021-06-30 18:05 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-30 15:35 [RFC][PATCH 0/4] locking/mutex: Some HANDOFF fixes Peter Zijlstra
2021-06-30 15:35 ` [RFC][PATCH 1/4] locking/mutex: Use try_cmpxchg() Peter Zijlstra
2021-07-05 11:59 ` Xu, Yanfei
2021-07-05 14:00 ` Peter Zijlstra
2021-07-05 14:52 ` Xu, Yanfei
2021-07-05 15:07 ` [PATCH] Documentation/atomic_t: Document cmpxchg() vs try_cmpxchg() Peter Zijlstra
2021-07-05 15:21 ` Will Deacon
2021-07-05 15:25 ` Xu, Yanfei
2021-07-05 17:12 ` Peter Zijlstra
2021-07-08 8:42 ` [tip: locking/core] " tip-bot2 for Peter Zijlstra
2021-07-08 8:42 ` [tip: locking/core] locking/mutex: Use try_cmpxchg() tip-bot2 for Peter Zijlstra
2021-06-30 15:35 ` [RFC][PATCH 2/4] locking/mutex: Fix HANDOFF condition Peter Zijlstra
2021-07-08 8:42 ` [tip: locking/core] " tip-bot2 for Peter Zijlstra
2021-06-30 15:35 ` [RFC][PATCH 3/4] locking/mutex: Introduce __mutex_trylock_or_handoff() Peter Zijlstra
2021-06-30 16:30 ` Waiman Long
2021-06-30 18:04 ` Peter Zijlstra [this message]
2021-07-08 8:42 ` [tip: locking/core] " tip-bot2 for Peter Zijlstra
2021-06-30 15:35 ` [RFC][PATCH 4/4] locking/mutex: Add MUTEX_WARN_ON Peter Zijlstra
2021-07-08 8:42 ` [tip: locking/core] " tip-bot2 for Peter Zijlstra
2021-06-30 19:03 ` [RFC][PATCH 0/4] locking/mutex: Some HANDOFF fixes Waiman Long
2021-07-01 2:11 ` Xu, Yanfei
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