Merge branch 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull locking updates from Ingo Molnar:

 - Lots of tidying up changes all across the map for Linux's formal
   memory/locking-model tooling, by Alan Stern, Akira Yokosawa, Andrea
   Parri, Paul E. McKenney and SeongJae Park.

   Notable changes beyond an overall update in the tooling itself is the
   tidying up of spin_is_locked() semantics, which spills over into the
   kernel proper as well.

 - qspinlock improvements: the locking algorithm now guarantees forward
   progress whereas the previous implementation in mainline could starve
   threads indefinitely in cmpxchg() loops. Also other related cleanups
   to the qspinlock code (Will Deacon)

 - misc smaller improvements, cleanups and fixes all across the locking
   subsystem

* 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (51 commits)
  locking/rwsem: Simplify the is-owner-spinnable checks
  tools/memory-model: Add reference for 'Simplifying ARM concurrency'
  tools/memory-model: Update ASPLOS information
  MAINTAINERS, tools/memory-model: Update e-mail address for Andrea Parri
  tools/memory-model: Fix coding style in 'lock.cat'
  tools/memory-model: Remove out-of-date comments and code from lock.cat
  tools/memory-model: Improve mixed-access checking in lock.cat
  tools/memory-model: Improve comments in lock.cat
  tools/memory-model: Remove duplicated code from lock.cat
  tools/memory-model: Flag "cumulativity" and "propagation" tests
  tools/memory-model: Add model support for spin_is_locked()
  tools/memory-model: Add scripts to test memory model
  tools/memory-model: Fix coding style in 'linux-kernel.def'
  tools/memory-model: Model 'smp_store_mb()'
  tools/memory-order: Update the cheat-sheet to show that smp_mb__after_atomic() orders later RMW operations
  tools/memory-order: Improve key for SELF and SV
  tools/memory-model: Fix cheat sheet typo
  tools/memory-model: Update required version of herdtools7
  tools/memory-model: Redefine rb in terms of rcu-fence
  tools/memory-model: Rename link and rcu-path to rcu-link and rb
  ...

1 files changed, 18 insertions, 0 deletions

diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 4894d322d258..1e8a46435838 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -380,6 +380,24 @@ static __always_inline int spin_trylock_irq(spinlock_t *lock)
 	raw_spin_trylock_irqsave(spinlock_check(lock), flags); \
 })
 
+/**
+ * spin_is_locked() - Check whether a spinlock is locked.
+ * @lock: Pointer to the spinlock.
+ *
+ * This function is NOT required to provide any memory ordering
+ * guarantees; it could be used for debugging purposes or, when
+ * additional synchronization is needed, accompanied with other
+ * constructs (memory barriers) enforcing the synchronization.
+ *
+ * Returns: 1 if @lock is locked, 0 otherwise.
+ *
+ * Note that the function only tells you that the spinlock is
+ * seen to be locked, not that it is locked on your CPU.
+ *
+ * Further, on CONFIG_SMP=n builds with CONFIG_DEBUG_SPINLOCK=n,
+ * the return value is always 0 (see include/linux/spinlock_up.h).
+ * Therefore you should not rely heavily on the return value.
+ */
 static __always_inline int spin_is_locked(spinlock_t *lock)
 {
 	return raw_spin_is_locked(&lock->rlock);