From: Peter Zijlstra <peterz@infradead.org>
To: Darren Hart <dvhart@infradead.org>
Cc: tglx@linutronix.de, mingo@kernel.org, juri.lelli@arm.com,
rostedt@goodmis.org, xlpang@redhat.com, bigeasy@linutronix.de,
linux-kernel@vger.kernel.org, mathieu.desnoyers@efficios.com,
jdesfossez@efficios.com, bristot@redhat.com
Subject: Re: [PATCH -v6 08/13] futex: Pull rt_mutex_futex_unlock() out from under hb->lock
Date: Thu, 6 Apr 2017 14:42:48 +0200 [thread overview]
Message-ID: <20170406124248.i7ibgne76yojnizh@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20170405235225.GD13494@fury>
On Wed, Apr 05, 2017 at 04:52:25PM -0700, Darren Hart wrote:
> On Wed, Mar 22, 2017 at 11:35:55AM +0100, Peter Zijlstra wrote:
> > There's a number of 'interesting' problems, all caused by holding
> > hb->lock while doing the rt_mutex_unlock() equivalient.
> >
> > Notably:
> >
> > - a PI inversion on hb->lock; and,
> >
> > - a DL crash because of pointer instability.
>
> A DL crash? What is this? Can you elaborate a bit?
See here:
https://lkml.kernel.org/r/20170323145606.480214279@infradead.org
> > @@ -1380,48 +1387,40 @@ static void mark_wake_futex(struct wake_
> > smp_store_release(&q->lock_ptr, NULL);
> > }
> >
> > -static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter,
> > - struct futex_hash_bucket *hb)
> > +/*
> > + * Caller must hold a reference on @pi_state.
> > + */
> > +static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_state)
> > {
> > - struct task_struct *new_owner;
> > - struct futex_pi_state *pi_state = top_waiter->pi_state;
> > u32 uninitialized_var(curval), newval;
> > + struct task_struct *new_owner;
> > + bool deboost = false;
> > DEFINE_WAKE_Q(wake_q);
> > - bool deboost;
>
> Nit: Based on what I've seen from Thomas and others, I ask for declarations in
> decreasing order of line length. So deboost should have stayed where it was.
Hurm, yeah I mostly do that. No idea what went wrong there.
> >
> > /*
> > @@ -2232,7 +2229,8 @@ static int fixup_pi_state_owner(u32 __us
> > /*
> > * We are here either because we stole the rtmutex from the
> > * previous highest priority waiter or we are the highest priority
> > - * waiter but failed to get the rtmutex the first time.
> > + * waiter but have failed to get the rtmutex the first time.
> > + *
> > * We have to replace the newowner TID in the user space variable.
> > * This must be atomic as we have to preserve the owner died bit here.
> > *
> > @@ -2249,7 +2247,7 @@ static int fixup_pi_state_owner(u32 __us
> > if (get_futex_value_locked(&uval, uaddr))
> > goto handle_fault;
> >
> > - while (1) {
> > + for (;;) {
>
> As far as I'm aware, there is no difference and both are used throughout the
> kernel (with the while version having 50% more instances). Is there more to this
> than personal preference?
Nope. Only that. I think I played around with the loop at one point and
this is all that remained of that.
> > newval = (uval & FUTEX_OWNER_DIED) | newtid;
> >
> > if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval))
> > @@ -2345,6 +2343,10 @@ static int fixup_owner(u32 __user *uaddr
> > /*
> > * Got the lock. We might not be the anticipated owner if we
> > * did a lock-steal - fix up the PI-state in that case:
> > + *
> > + * We can safely read pi_state->owner without holding wait_lock
> > + * because we now own the rt_mutex, only the owner will attempt
> > + * to change it.
>
> This seems to contradict the Serialization and lifetime rules:
>
> + * pi_mutex->wait_lock:
> + *
> + * {uval, pi_state}
> + *
> + * (and pi_mutex 'obviously')
>
> It would seem that simply holding pi_mutex is sufficient for serialization on
> pi_state->owner then.
Not a contradiction; just a very specific special case. If current is
the owner of a lock, said owner will not be going anywhere.
> > +
> > + /*
> > + * Grab a reference on the pi_state and drop hb->lock.
> > + *
> > + * The reference ensures pi_state lives, dropping the hb->lock
> > + * is tricky.. wake_futex_pi() will take rt_mutex::wait_lock to
> > + * close the races against futex_lock_pi(), but in case of
> > + * _any_ fail we'll abort and retry the whole deal.
>
> s/fail/failure/
I don't think that survives the patch-set. That is, I cannot find it in
the current code.
next prev parent reply other threads:[~2017-04-06 12:43 UTC|newest]
Thread overview: 60+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-22 10:35 [PATCH -v6 00/13] The arduous story of FUTEX_UNLOCK_PI Peter Zijlstra
2017-03-22 10:35 ` [PATCH -v6 01/13] futex: Cleanup variable names for futex_top_waiter() Peter Zijlstra
2017-03-23 18:19 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-03-24 21:11 ` [PATCH -v6 01/13] " Darren Hart
2017-03-22 10:35 ` [PATCH -v6 02/13] futex: Use smp_store_release() in mark_wake_futex() Peter Zijlstra
2017-03-23 18:19 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-03-24 21:16 ` [PATCH -v6 02/13] " Darren Hart
2017-03-22 10:35 ` [PATCH -v6 03/13] futex: Remove rt_mutex_deadlock_account_*() Peter Zijlstra
2017-03-23 18:20 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-03-24 21:29 ` [PATCH -v6 03/13] " Darren Hart
2017-03-24 21:31 ` Darren Hart
2017-03-22 10:35 ` [PATCH -v6 04/13] futex,rt_mutex: Provide futex specific rt_mutex API Peter Zijlstra
2017-03-23 18:20 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-03-25 0:37 ` [PATCH -v6 04/13] " Darren Hart
2017-04-06 12:15 ` Peter Zijlstra
2017-04-06 17:02 ` Darren Hart
2017-04-05 15:02 ` Darren Hart
2017-04-06 12:17 ` Peter Zijlstra
2017-04-06 17:08 ` Darren Hart
2017-03-22 10:35 ` [PATCH -v6 05/13] futex: Change locking rules Peter Zijlstra
2017-03-23 18:21 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-04-05 21:18 ` [PATCH -v6 05/13] " Darren Hart
2017-04-06 12:28 ` Peter Zijlstra
2017-04-06 15:58 ` Joe Perches
2017-04-06 17:21 ` Darren Hart
2017-03-22 10:35 ` [PATCH -v6 06/13] futex: Cleanup refcounting Peter Zijlstra
2017-03-23 18:21 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-04-05 21:29 ` [PATCH -v6 06/13] " Darren Hart
2017-03-22 10:35 ` [PATCH -v6 07/13] futex: Rework inconsistent rt_mutex/futex_q state Peter Zijlstra
2017-03-23 18:22 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-04-05 21:58 ` [PATCH -v6 07/13] " Darren Hart
2017-03-22 10:35 ` [PATCH -v6 08/13] futex: Pull rt_mutex_futex_unlock() out from under hb->lock Peter Zijlstra
2017-03-23 18:22 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-04-05 23:52 ` [PATCH -v6 08/13] " Darren Hart
2017-04-06 12:42 ` Peter Zijlstra [this message]
2017-04-06 17:42 ` Darren Hart
2017-03-22 10:35 ` [PATCH -v6 09/13] futex,rt_mutex: Introduce rt_mutex_init_waiter() Peter Zijlstra
2017-03-23 18:23 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-04-05 23:57 ` [PATCH -v6 09/13] " Darren Hart
2017-03-22 10:35 ` [PATCH -v6 10/13] futex,rt_mutex: Restructure rt_mutex_finish_proxy_lock() Peter Zijlstra
2017-03-23 18:23 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-04-07 23:30 ` [PATCH -v6 10/13] " Darren Hart
2017-04-07 23:35 ` Darren Hart
2017-03-22 10:35 ` [PATCH -v6 11/13] futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock() Peter Zijlstra
2017-03-23 18:24 ` [tip:locking/core] " tip-bot for Peter Zijlstra
2017-04-08 0:55 ` [PATCH -v6 11/13] " Darren Hart
2017-04-10 15:51 ` alexander.levin
2017-04-10 16:03 ` Thomas Gleixner
2017-04-14 9:30 ` [tip:locking/core] futex: Avoid freeing an active timer tip-bot for Thomas Gleixner
2017-03-22 10:35 ` [PATCH -v6 12/13] futex: futex_unlock_pi() determinism Peter Zijlstra
2017-03-23 18:24 ` [tip:locking/core] futex: Futex_unlock_pi() determinism tip-bot for Peter Zijlstra
2017-04-08 1:27 ` [PATCH -v6 12/13] futex: futex_unlock_pi() determinism Darren Hart
2017-03-22 10:36 ` [PATCH -v6 13/13] futex: futex_lock_pi() vs PREEMPT_RT_FULL Peter Zijlstra
2017-03-23 18:25 ` [tip:locking/core] futex: Drop hb->lock before enqueueing on the rtmutex tip-bot for Peter Zijlstra
2017-04-08 2:26 ` [PATCH -v6 13/13] futex: futex_lock_pi() vs PREEMPT_RT_FULL Darren Hart
2017-04-08 5:22 ` Mike Galbraith
2017-04-10 8:43 ` Sebastian Andrzej Siewior
2017-04-10 9:08 ` Peter Zijlstra
2017-04-10 16:05 ` Darren Hart
2017-03-24 1:45 ` [PATCH -v6 00/13] The arduous story of FUTEX_UNLOCK_PI Darren Hart
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