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locking/qspinlock: Clarify xchg_tail() ordering

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While going over the code I noticed that xchg_tail() is a RELEASE but
had no obvious pairing commented.

It pairs with a somewhat unique address dependency through
decode_tail().

So the store-release of xchg_tail() is paired by the address
dependency of the load of xchg_tail followed by the dereference from
the pointer computed from that load.

The @old -> @prev transformation itself is pure, and therefore does
not depend on external state, so that is immaterial wrt. ordering.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <waiman.long@hpe.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2016-06-08 09:12:30 +02:00 committed by Ingo Molnar
parent ae0b5c2f03
commit 8d53fa1904
1 changed files with 13 additions and 2 deletions
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15
kernel/locking/qspinlock.c
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@ -90,7 +90,7 @@ static DEFINE_PER_CPU_ALIGNED(struct mcs_spinlock, mcs_nodes[MAX_NODES]);
* therefore increment the cpu number by one. * therefore increment the cpu number by one.
*/ */
static inline u32 encode_tail(int cpu, int idx) static inline __pure u32 encode_tail(int cpu, int idx)
{ {
u32 tail; u32 tail;
@ -103,7 +103,7 @@ static inline u32 encode_tail(int cpu, int idx)
return tail; return tail;
} }
static inline struct mcs_spinlock *decode_tail(u32 tail) static inline __pure struct mcs_spinlock *decode_tail(u32 tail)
{ {
int cpu = (tail >> _Q_TAIL_CPU_OFFSET) - 1; int cpu = (tail >> _Q_TAIL_CPU_OFFSET) - 1;
int idx = (tail & _Q_TAIL_IDX_MASK) >> _Q_TAIL_IDX_OFFSET; int idx = (tail & _Q_TAIL_IDX_MASK) >> _Q_TAIL_IDX_OFFSET;
@ -455,6 +455,8 @@ queue:
* pending stuff. * pending stuff.
* *
* p,*,* -> n,*,* * p,*,* -> n,*,*
*
* RELEASE, such that the stores to @node must be complete.
*/ */
old = xchg_tail(lock, tail); old = xchg_tail(lock, tail);
next = NULL; next = NULL;
@ -465,6 +467,15 @@ queue:
*/ */
if (old & _Q_TAIL_MASK) { if (old & _Q_TAIL_MASK) {
prev = decode_tail(old); prev = decode_tail(old);
/*
* The above xchg_tail() is also a load of @lock which generates,
* through decode_tail(), a pointer.
*
* The address dependency matches the RELEASE of xchg_tail()
* such that the access to @prev must happen after.
*/
smp_read_barrier_depends();
WRITE_ONCE(prev->next, node); WRITE_ONCE(prev->next, node);
pv_wait_node(node, prev); pv_wait_node(node, prev);