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Commit 756b1df4 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

locking/qspinlock: Rework some comments 



While working my way through the code again; I felt the comments could
use help.

Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: default avatarWill Deacon <will.deacon@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: andrea.parri@amarulasolutions.com
Cc: longman@redhat.com
Link: https://lkml.kernel.org/r/20181003130257.156322446@infradead.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 53bf57fa

kernel/locking/qspinlock.c

+26 −10
Original line number Diff line number Diff line
@@ -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 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) 
	 */



	/*

	 * 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))

	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);



	/*