Messages in this thread Patch in this message | | | Date | Mon, 1 Feb 2016 15:37:24 +0100 | From | Peter Zijlstra <> | Subject | [RFC][PATCH] locking/mcs: Fix ordering for mcs_spin_lock() |
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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
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