* [RFC][PATCH] locking/mcs: Fix ordering for mcs_spin_lock()
@ 2016-02-01 14:37 Peter Zijlstra
2016-02-01 16:58 ` Will Deacon
0 siblings, 1 reply; 3+ messages in thread
From: Peter Zijlstra @ 2016-02-01 14:37 UTC (permalink / raw)
To: Will Deacon, Paul McKenney
Cc: linux-kernel, Davidlohr Bueso, Ingo Molnar, parri.andrea
Given the below patch; we've now got an unconditional full global
barrier in, does this make the MCS spinlock RCsc ?
The 'problem' is that this barrier can happen before we actually acquire
the lock. That is, if we hit arch_mcs_spin_lock_contended() _that_ will
be the acquire barrier and we end up with a SYNC in between unlock and
lock -- ie. not an smp_mb__after_unlock_lock() equivalent.
---
Subject: locking/mcs: Fix ordering for mcs_spin_lock()
From: Peter Zijlstra <peterz@infradead.org>
Date: Mon Feb 1 15:11:28 CET 2016
Similar to commit b4b29f94856a ("locking/osq: Fix ordering of node
initialisation in osq_lock") the use of xchg_acquire() is
fundamentally broken with MCS like constructs.
Furthermore, it turns out we rely on the global transitivity of this
operation because the unlock path observes the pointer with a
READ_ONCE(), not an smp_load_acquire().
This is non-critical because the MCS code isn't actually used and
mostly serves as documentation, a stepping stone to the more complex
things we've build on top of the idea.
Cc: Will Deacon <will.deacon@arm.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Reported-by: Andrea Parri <parri.andrea@gmail.com>
Fixes: 3552a07a9c4a ("locking/mcs: Use acquire/release semantics")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/locking/mcs_spinlock.h | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
--- a/kernel/locking/mcs_spinlock.h
+++ b/kernel/locking/mcs_spinlock.h
@@ -67,7 +67,13 @@ void mcs_spin_lock(struct mcs_spinlock *
node->locked = 0;
node->next = NULL;
- prev = xchg_acquire(lock, node);
+ /*
+ * We rely on the full barrier with global transitivity implied by the
+ * below xchg() to order the initialization stores above against any
+ * observation of @node. And to provide the ACQUIRE ordering associated
+ * with a LOCK primitive.
+ */
+ prev = xchg(lock, node);
if (likely(prev == NULL)) {
/*
* Lock acquired, don't need to set node->locked to 1. Threads
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [RFC][PATCH] locking/mcs: Fix ordering for mcs_spin_lock()
2016-02-01 14:37 [RFC][PATCH] locking/mcs: Fix ordering for mcs_spin_lock() Peter Zijlstra
@ 2016-02-01 16:58 ` Will Deacon
2016-02-01 17:24 ` Peter Zijlstra
0 siblings, 1 reply; 3+ messages in thread
From: Will Deacon @ 2016-02-01 16:58 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Paul McKenney, linux-kernel, Davidlohr Bueso, Ingo Molnar, parri.andrea
Hi Peter,
On Mon, Feb 01, 2016 at 03:37:24PM +0100, Peter Zijlstra wrote:
> Given the below patch; we've now got an unconditional full global
> barrier in, does this make the MCS spinlock RCsc ?
>
> The 'problem' is that this barrier can happen before we actually acquire
> the lock. That is, if we hit arch_mcs_spin_lock_contended() _that_ will
> be the acquire barrier and we end up with a SYNC in between unlock and
> lock -- ie. not an smp_mb__after_unlock_lock() equivalent.
In which case, I don't think the lock will be RCsc with this change;
you'd need an smp_mb__after_unlock_lock() after
arch_mcs_spin_lock_contended(...) if you wanted the thing to be RCsc.
> Subject: locking/mcs: Fix ordering for mcs_spin_lock()
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Mon Feb 1 15:11:28 CET 2016
>
> Similar to commit b4b29f94856a ("locking/osq: Fix ordering of node
> initialisation in osq_lock") the use of xchg_acquire() is
> fundamentally broken with MCS like constructs.
>
> Furthermore, it turns out we rely on the global transitivity of this
> operation because the unlock path observes the pointer with a
> READ_ONCE(), not an smp_load_acquire().
>
> This is non-critical because the MCS code isn't actually used and
> mostly serves as documentation, a stepping stone to the more complex
> things we've build on top of the idea.
>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Reported-by: Andrea Parri <parri.andrea@gmail.com>
> Fixes: 3552a07a9c4a ("locking/mcs: Use acquire/release semantics")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> kernel/locking/mcs_spinlock.h | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
Acked-by: Will Deacon <will.deacon@arm.com>
Although I wonder how useful this is as a documentation aid now that we
have the osq_lock.
Will
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [RFC][PATCH] locking/mcs: Fix ordering for mcs_spin_lock()
2016-02-01 16:58 ` Will Deacon
@ 2016-02-01 17:24 ` Peter Zijlstra
0 siblings, 0 replies; 3+ messages in thread
From: Peter Zijlstra @ 2016-02-01 17:24 UTC (permalink / raw)
To: Will Deacon
Cc: Paul McKenney, linux-kernel, Davidlohr Bueso, Ingo Molnar, parri.andrea
On Mon, Feb 01, 2016 at 04:58:13PM +0000, Will Deacon wrote:
> Hi Peter,
>
> On Mon, Feb 01, 2016 at 03:37:24PM +0100, Peter Zijlstra wrote:
> > Given the below patch; we've now got an unconditional full global
> > barrier in, does this make the MCS spinlock RCsc ?
> >
> > The 'problem' is that this barrier can happen before we actually acquire
> > the lock. That is, if we hit arch_mcs_spin_lock_contended() _that_ will
> > be the acquire barrier and we end up with a SYNC in between unlock and
> > lock -- ie. not an smp_mb__after_unlock_lock() equivalent.
>
> In which case, I don't think the lock will be RCsc with this change;
> you'd need an smp_mb__after_unlock_lock() after
> arch_mcs_spin_lock_contended(...) if you wanted the thing to be RCsc.
Right, I think it works for TSO, but in general it makes my head hurt.
> > This is non-critical because the MCS code isn't actually used and
> > mostly serves as documentation, a stepping stone to the more complex
> > things we've build on top of the idea.
>
> Although I wonder how useful this is as a documentation aid now that we
> have the osq_lock.
So the OSQ thing is horribly complex, pure MCS is a nice step-stone.
^ permalink raw reply [flat|nested] 3+ messages in thread
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