| From | Greg Kroah-Hartman <> | Subject | [PATCH 4.5 138/200] locking/mcs: Fix mcs_spin_lock() ordering | Date | Mon, 2 May 2016 17:12:17 -0700 |
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4.5-stable review patch. If anyone has any objections, please let me know.
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From: Peter Zijlstra <peterz@infradead.org>
commit 920c720aa5aa3900a7f1689228fdfc2580a91e7e upstream.
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.
Reported-by: Andrea Parri <parri.andrea@gmail.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Will Deacon <will.deacon@arm.com> Fixes: 3552a07a9c4a ("locking/mcs: Use acquire/release semantics") Signed-off-by: Ingo Molnar <mingo@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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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