[PATCH] futex: futex_wait_requeue_pi commentary corrections

[PATCH] futex: futex_wait_requeue_pi commentary corrections

[Darren Hart]

futex: futex_wait_requeue_pi commentary corrections

From: Darren Hart <dvhltc@us.ibm.com>

The state machine described in the comments wasn't updated with a follow-on
fix.  Address that and cleanup the corresponding commentary in the function.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@elte.hu>
---

 kernel/futex.c |   23 ++++++++++-------------
 1 files changed, 10 insertions(+), 13 deletions(-)


diff --git a/kernel/futex.c b/kernel/futex.c
index 0672ff8..18752ae 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2102,11 +2102,11 @@ int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
  * We call schedule in futex_wait_queue_me() when we enqueue and return there
  * via the following:
  * 1) wakeup on uaddr2 after an atomic lock acquisition by futex_requeue()
- * 2) wakeup on uaddr2 after a requeue and subsequent unlock
- * 3) signal (before or after requeue)
- * 4) timeout (before or after requeue)
+ * 2) wakeup on uaddr2 after a requeue
+ * 3) signal
+ * 4) timeout
  *
- * If 3, we setup a restart_block with futex_wait_requeue_pi() as the function.
+ * If 3, we setup a restart_block and return -ERESTARTNOINTR.
  *
  * If 2, we may then block on trying to take the rt_mutex and return via:
  * 5) successful lock
@@ -2114,7 +2114,7 @@ int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
  * 7) timeout
  * 8) other lock acquisition failure
  *
- * If 6, we setup a restart_block with futex_lock_pi() as the function.
+ * If 6, return -EWOULDBLOCK (restarting the syscall would do the same).
  *
  * If 4 or 7, we cleanup and return with -ETIMEDOUT.
  *
@@ -2232,14 +2232,11 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
 			rt_mutex_unlock(pi_mutex);
 	} else if (ret == -EINTR) {
 		/*
-		 * We've already been requeued, but we have no way to
-		 * restart by calling futex_lock_pi() directly. We
-		 * could restart the syscall, but that will look at
-		 * the user space value and return right away. So we
-		 * drop back with EWOULDBLOCK to tell user space that
-		 * "val" has been changed. That's the same what the
-		 * restart of the syscall would do in
-		 * futex_wait_setup().
+		 * We've already been requeued, but cannot restart by calling
+		 * futex_lock_pi() directly. We could restart this syscall, but
+		 * it would detect that the user space "val" changed and return
+		 * -EWOULDBLOCK.  Save the overhead of the restart and return
+		 * -EWOULDBLOCK directly.
 		 */
 		ret = -EWOULDBLOCK;
 	}
-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team

Re: [PATCH] futex: futex_wait_requeue_pi commentary corrections

[Darren Hart]

Darren Hart wrote:
> futex: futex_wait_requeue_pi commentary corrections
> 
> From: Darren Hart <dvhltc@us.ibm.com>
> 
> The state machine described in the comments wasn't updated with a follow-on
> fix.  Address that and cleanup the corresponding commentary in the 
> function.
> 
> Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ingo Molnar <mingo@elte.hu>
> ---
> 
> kernel/futex.c |   23 ++++++++++-------------
> 1 files changed, 10 insertions(+), 13 deletions(-)
> 
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 0672ff8..18752ae 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -2102,11 +2102,11 @@ int handle_early_requeue_pi_wakeup(struct 
> futex_hash_bucket *hb,
>  * We call schedule in futex_wait_queue_me() when we enqueue and return 
> there
>  * via the following:
>  * 1) wakeup on uaddr2 after an atomic lock acquisition by futex_requeue()
> - * 2) wakeup on uaddr2 after a requeue and subsequent unlock
> - * 3) signal (before or after requeue)
> - * 4) timeout (before or after requeue)
> + * 2) wakeup on uaddr2 after a requeue
> + * 3) signal
> + * 4) timeout
>  *
> - * If 3, we setup a restart_block with futex_wait_requeue_pi() as the 
> function.
> + * If 3, we setup a restart_block and return -ERESTARTNOINTR.

Gah, not we don't.  We removed the restart block altogether.  Sorry.  V2 
en route.

Nacked-by: Darren Hart <dvhltc@us.ibm.com>

-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team