On Mon, Oct 16, 2023 at 10:43:01PM +1000, Nicholas Piggin wrote:
> If a queued waiter notices the lock owner or the previous waiter has
> been preempted, it attempts to mark the lock sleepy, but it does this
> as a try-set operation using the original lock value it got when
> queueing, which will become stale as the queue progresses, and the
> try-set will fail. Drop this and just set the sleepy seen clock.
>
> Signed-off-by: Nicholas Piggin <npig...@gmail.com>
> ---
> arch/powerpc/lib/qspinlock.c | 24 ++++++++++--------------
> 1 file changed, 10 insertions(+), 14 deletions(-)
>
> diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
> index 6dd2f46bd3ef..75608ced14c2 100644
> --- a/arch/powerpc/lib/qspinlock.c
> +++ b/arch/powerpc/lib/qspinlock.c
> @@ -247,22 +247,18 @@ static __always_inline void seen_sleepy_lock(void)
> this_cpu_write(sleepy_lock_seen_clock, sched_clock());
> }
>
> -static __always_inline void seen_sleepy_node(struct qspinlock *lock, u32 val)
> +static __always_inline void seen_sleepy_node(void)
> {
> if (pv_sleepy_lock) {
> if (pv_sleepy_lock_interval_ns)
> this_cpu_write(sleepy_lock_seen_clock, sched_clock());
> - if (val & _Q_LOCKED_VAL) {
> - if (!(val & _Q_SLEEPY_VAL))
> - try_set_sleepy(lock, val);
> - }
> + /* Don't set sleepy because we likely have a stale val */
> }
> }
seen_sleepy_lock() and seen_sleepy_node() are now the same
>
> -static struct qnode *get_tail_qnode(struct qspinlock *lock, u32 val)
> +static struct qnode *get_tail_qnode(struct qspinlock *lock, int prev_cpu)
> {
> - int cpu = decode_tail_cpu(val);
> - struct qnodes *qnodesp = per_cpu_ptr(&qnodes, cpu);
> + struct qnodes *qnodesp = per_cpu_ptr(&qnodes, prev_cpu);
> int idx;
>
> /*
> @@ -381,9 +377,8 @@ static __always_inline void propagate_yield_cpu(struct
> qnode *node, u32 val, int
> }
>
> /* Called inside spin_begin() */
> -static __always_inline bool yield_to_prev(struct qspinlock *lock, struct
> qnode *node, u32 val, bool paravirt)
> +static __always_inline bool yield_to_prev(struct qspinlock *lock, struct
> qnode *node, int prev_cpu, bool paravirt)
> {
> - int prev_cpu = decode_tail_cpu(val);
> u32 yield_count;
> int yield_cpu;
> bool preempted = false;
> @@ -412,7 +407,7 @@ static __always_inline bool yield_to_prev(struct
> qspinlock *lock, struct qnode *
> spin_end();
>
> preempted = true;
> - seen_sleepy_node(lock, val);
> + seen_sleepy_node();
>
> smp_rmb();
>
> @@ -436,7 +431,7 @@ static __always_inline bool yield_to_prev(struct
> qspinlock *lock, struct qnode *
> spin_end();
>
> preempted = true;
> - seen_sleepy_node(lock, val);
> + seen_sleepy_node();
>
> smp_rmb(); /* See __yield_to_locked_owner comment */
>
> @@ -587,7 +582,8 @@ static __always_inline void
> queued_spin_lock_mcs_queue(struct qspinlock *lock, b
> * head of the waitqueue.
> */
> if (old & _Q_TAIL_CPU_MASK) {
> - struct qnode *prev = get_tail_qnode(lock, old);
> + int prev_cpu = decode_tail_cpu(old);
> + struct qnode *prev = get_tail_qnode(lock, prev_cpu);
>
> /* Link @node into the waitqueue. */
> WRITE_ONCE(prev->next, node);
> @@ -597,7 +593,7 @@ static __always_inline void
> queued_spin_lock_mcs_queue(struct qspinlock *lock, b
> while (!READ_ONCE(node->locked)) {
> spec_barrier();
>
> - if (yield_to_prev(lock, node, old, paravirt))
> + if (yield_to_prev(lock, node, prev_cpu, paravirt))
> seen_preempted = true;
> }
> spec_barrier();
> --
> 2.42.0
>
