Re: [RFC][PATCH] locking/mcs: Fix ordering for mcs_spin_lock()

Will Deacon
2016-02-01T16:58:18Z

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