The TP-futexes prefer writers as the code path is much simpler for
them. This may not be good for overall system performance especially
if there is a fair amount of reader activities.
This patch enables kernel reader to steal the lock when the futex is
currently reader-owned and the lock handoff mechanism hasn't been
enabled yet. A new field locksteal_disabled is added to the futex
state object for controlling reader lock stealing. So the waiting
reader must retrieve the futex state object first before doing it.
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
kernel/futex.c | 23 +++++++++++++++++++++--
1 file changed, 21 insertions(+), 2 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index ee35264..b690a79 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -237,6 +237,7 @@ struct futex_state {
atomic_t refcount;
u32 handoff_pid; /* For TP futexes only */
+ int locksteal_disabled; /* For TP futexes only */
enum futex_type type;
union futex_key key;
@@ -3436,6 +3437,7 @@ void exit_robust_list(struct task_struct *curr)
state = alloc_futex_state();
state->type = TYPE_TP;
state->key = *key;
+ state->locksteal_disabled = false;
list_add(&state->fs_list, &hb->fs_head);
WARN_ON(atomic_read(&state->refcount) != 1);
@@ -3781,9 +3783,10 @@ static int futex_spin_on_owner(u32 __user *uaddr, const u32 vpid,
WRITE_ONCE(state->handoff_pid, vpid);
/*
- * Disable handoff check.
+ * Disable handoff check and reader lock
+ * stealing.
*/
- handoff_set = true;
+ state->locksteal_disabled = handoff_set = true;
}
}
@@ -3886,6 +3889,7 @@ static int futex_spin_on_owner(u32 __user *uaddr, const u32 vpid,
*/
WRITE_ONCE(state->mutex_owner, NULL);
WRITE_ONCE(state->handoff_pid, 0);
+ state->locksteal_disabled = false;
preempt_enable();
return (ret < 0) ? ret : TP_STATUS_SLEEP(ret, nsleep);
@@ -3966,6 +3970,21 @@ static noinline int futex_lock(u32 __user *uaddr, unsigned int flags,
goto out_put_state_key;
}
+ /*
+ * For reader, we will try to steal the lock here as if it is the
+ * top waiter without taking the serialization mutex if reader
+ * lock stealing isn't disabled even though the FUTEX_WAITERS bit
+ * is set.
+ */
+ if (shared && !state->locksteal_disabled) {
+ ret = futex_trylock(uaddr, vpid, &uval, true);
+ if (ret) {
+ if (ret > 0)
+ ret = TP_LOCK_STOLEN;
+ goto out_put_state_key;
+ }
+ }
+
if (to)
hrtimer_start_expires(&to->timer, HRTIMER_MODE_ABS);
--
1.8.3.1
--
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