Commit-ID: 756b1df4c2c82a1cdffeafa9d2aa76c92e7fb405
Gitweb: https://git.kernel.org/tip/756b1df4c2c82a1cdffeafa9d2aa76c92e7fb405
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Wed, 26 Sep 2018 13:01:19 +0200
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 16 Oct 2018 17:33:54 +0200
locking/qspinlock: Rework some comments
While working my way through the code again; I felt the comments could
use help.
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Will Deacon <will.deacon@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: andrea.parri@xxxxxxxxxxxxxxxxxxxx
Cc: longman@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20181003130257.156322446@xxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
kernel/locking/qspinlock.c | 36 ++++++++++++++++++++++++++----------
1 file changed, 26 insertions(+), 10 deletions(-)
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index ec343276f975..47cb99787e4d 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -326,16 +326,23 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
/*
* trylock || pending
*
- * 0,0,0 -> 0,0,1 ; trylock
- * 0,0,1 -> 0,1,1 ; pending
+ * 0,0,* -> 0,1,* -> 0,0,1 pending, trylock
*/
val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
+
/*
- * If we observe any contention; undo and queue.
+ * If we observe contention, there is a concurrent locker.
+ *
+ * Undo and queue; our setting of PENDING might have made the
+ * n,0,0 -> 0,0,0 transition fail and it will now be waiting
+ * on @next to become !NULL.
*/
if (unlikely(val & ~_Q_LOCKED_MASK)) {
+
+ /* Undo PENDING if we set it. */
if (!(val & _Q_PENDING_MASK))
clear_pending(lock);
+
goto queue;
}
@@ -474,16 +481,25 @@ locked:
*/
/*
- * In the PV case we might already have _Q_LOCKED_VAL set.
+ * In the PV case we might already have _Q_LOCKED_VAL set, because
+ * of lock stealing; therefore we must also allow:
*
- * The atomic_cond_read_acquire() call above has provided the
- * necessary acquire semantics required for locking.
+ * n,0,1 -> 0,0,1
+ *
+ * Note: at this point: (val & _Q_PENDING_MASK) == 0, because of the
+ * above wait condition, therefore any concurrent setting of
+ * PENDING will make the uncontended transition fail.
*/
- if (((val & _Q_TAIL_MASK) == tail) &&
- atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL))
- goto release; /* No contention */
+ if ((val & _Q_TAIL_MASK) == tail) {
+ if (atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL))
+ goto release; /* No contention */
+ }
- /* Either somebody is queued behind us or _Q_PENDING_VAL is set */
+ /*
+ * Either somebody is queued behind us or _Q_PENDING_VAL got set
+ * which will then detect the remaining tail and queue behind us
+ * ensuring we'll see a @next.
+ */
set_locked(lock);
/*
|