Messages in this thread | | | Date | Mon, 1 Feb 2016 16:58:13 +0000 | From | Will Deacon <> | Subject | Re: [RFC][PATCH] locking/mcs: Fix ordering for mcs_spin_lock() |
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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
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