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[PATCH-tip v6 17/22] TP-futex: Enable kernel reader lock stealing

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By default, the TP futexes do not have preference for either readers
or writers. Most reader-writer locks allows users to decide if they
want to prefer readers or writers more.

This patch allows the setting of the prefer-reader mode in the val
argument of the futex system call. If that flag is set, it will
enable kernel reader to steal the lock when the futex is currently
reader-owned and the lock handoff mechanism hasn't been enabled yet.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/futex.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 4139843..cacaaf1 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -197,6 +197,7 @@
 #define FLAGS_CLOCKRT		0x02
 #define FLAGS_HAS_TIMEOUT	0x04
 #define FLAGS_TP_USLOCK		0x08	/* Do the locking in userspace */
+#define FLAGS_TP_PREADER	0x08	/* Prefer readers */
 
 enum futex_type {
 	TYPE_PI = 0,
@@ -4004,6 +4005,20 @@ 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 the handoff
+	 * PID hasn't been set and is in prefer-reader mode.
+	 */
+	if (shared && (flags & FLAGS_TP_PREADER) && !state->handoff_pid) {
+		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);
 
@@ -4219,8 +4234,9 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 #ifdef CONFIG_SMP
 	case FUTEX_LOCK:
 	case FUTEX_LOCK_SHARED:
-		if (val && (cmd == FUTEX_LOCK))
-			flags |= FLAGS_TP_USLOCK;
+		if (val)
+			flags |= (cmd == FUTEX_LOCK) ? FLAGS_TP_USLOCK
+						     : FLAGS_TP_PREADER;
 		return futex_lock(uaddr, flags, timeout,
 				 (cmd == FUTEX_LOCK) ? false : true);
 	case FUTEX_UNLOCK:
-- 
1.8.3.1

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[PATCH-tip v6 00/22] futex: Introducing throughput-optimized (TP) futexes Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 12/22] TP-futex: Return status code on FUTEX_LOCK calls Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 11/22] TP-futex: Implement lock handoff to prevent lock starvation Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 20/22] perf bench: Extend mutex/rwlock futex suite to test TP futexes Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 21/22] sched, TP-futex: Make wake_up_q() return wakeup count Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 02/22] perf bench: New microbenchmark for userspace rwlock performance Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 03/22] futex: Consolidate duplicated timer setup code Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 04/22] futex: Rename futex_pi_state to futex_state Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 22/22] futex: Dump internal futex state via debugfs Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 19/22] TP-futex, doc: Update TP futexes document on shared locking Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 08/22] futex: Allow direct attachment of futex_state objects to hash bucket Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 09/22] futex: Introduce throughput-optimized (TP) futexes Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 01/22] perf bench: New microbenchmark for userspace mutex performance Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 18/22] TP-futex: Group readers together in wait queue Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
    Re: [PATCH-tip v6 18/22] TP-futex: Group readers together in wait  queue kbuild test robot <lkp@intel.com> - 2017-03-24 09:30 +0100
    Re: [PATCH-tip v6 18/22] TP-futex: Group readers together in wait  queue kbuild test robot <lkp@intel.com> - 2017-03-24 09:40 +0100
  [PATCH-tip v6 16/22] TP-futex: Support userspace reader/writer locks Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 13/22] TP-futex: Add timeout support Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 17/22] TP-futex: Enable kernel reader lock stealing Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 05/22] futex: Add helpers to get & cmpxchg futex value without lock Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 10/22] TP-futex: Enable robust handling Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 07/22] futex: Add a new futex type field into futex_state Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 15/22] TP-futex, doc: Add TP futexes documentation Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100
  [PATCH-tip v6 14/22] TP-futex: Optionally return EAGAIN for userspace locking Waiman Long <longman@redhat.com> - 2017-03-22 18:50 +0100

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