[PATCH 2/6] powerpc/qspinlock: stop queued waiters trying to set lock sleepy

[Nicholas Piggin]

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 */
        }
 }
 
-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