2009-07-31 22:38:29

by Darren Hart

Subject: [PATCH] futex: futex_wait_requeue_pi commentary corrections

futex: futex_wait_requeue_pi commentary corrections

From: Darren Hart <[email protected]>

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 <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Ingo Molnar <[email protected]>
---

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

2009-07-31 23:18:36

by Darren Hart

Subject: Re: [PATCH] futex: futex_wait_requeue_pi commentary corrections

Darren Hart wrote:
> futex: futex_wait_requeue_pi commentary corrections
>
> From: Darren Hart <[email protected]>
>
> 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 <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Steven Rostedt <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> ---
>
> 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 <[email protected]>

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