[patch] futex: restartable futex_wait

[patch] futex: restartable futex_wait

[Nick Piggin]

LTP test sigaction_16_24 fails, because it expects sem_wait to be restarted
if SA_RESTART is set. sem_wait is implemented with futex_wait, that currently
doesn't support being restarted. Ulrich confirms that the call should be
restartable.

Implement a restart_block method to handle the relative timeout, and allow
restarts.

Signed-off-by: Nick Piggin <npiggin@suse.de>

Index: linux-2.6/kernel/futex.c
===================================================================
--- linux-2.6.orig/kernel/futex.c
+++ linux-2.6/kernel/futex.c
@@ -978,6 +978,7 @@ static void unqueue_me_pi(struct futex_q
 	drop_futex_key_refs(&q->key);
 }
 
+static long futex_wait_restart(struct restart_block *restart);
 static int futex_wait(u32 __user *uaddr, u32 val, unsigned long time)
 {
 	struct task_struct *curr = current;
@@ -1077,11 +1078,22 @@ static int futex_wait(u32 __user *uaddr,
 		return 0;
 	if (time == 0)
 		return -ETIMEDOUT;
+
 	/*
 	 * We expect signal_pending(current), but another thread may
 	 * have handled it for us already.
 	 */
-	return -EINTR;
+	if (time == MAX_SCHEDULE_TIMEOUT)
+		return -ERESTARTSYS;
+	else {
+		struct restart_block *restart;
+		restart = &current_thread_info()->restart_block;
+		restart->fn = futex_wait_restart;
+		restart->arg0 = (unsigned long)uaddr;
+		restart->arg1 = (unsigned long)val;
+		restart->arg2 = time;
+		return -ERESTART_RESTARTBLOCK;
+	}
 
  out_unlock_release_sem:
 	queue_unlock(&q, hb);
@@ -1091,6 +1103,17 @@ static int futex_wait(u32 __user *uaddr,
 	return ret;
 }
 
+static long futex_wait_restart(struct restart_block *restart)
+{
+	u32 __user *uaddr = (u32 __user *)restart->arg0;
+	u32 val = (u32)restart->arg1;
+	unsigned long time = restart->arg2;
+
+	restart->fn = do_no_restart_syscall;
+	return (long)futex_wait(uaddr, val, time);
+}
+
+
 /*
  * Userspace tried a 0 -> TID atomic transition of the futex value
  * and failed. The kernel side here does the whole locking operation:

Re: [patch] futex: restartable futex_wait

[Ingo Molnar]

* Nick Piggin <npiggin@suse.de> wrote:

> LTP test sigaction_16_24 fails, because it expects sem_wait to be 
> restarted if SA_RESTART is set. sem_wait is implemented with 
> futex_wait, that currently doesn't support being restarted. Ulrich 
> confirms that the call should be restartable.
> 
> Implement a restart_block method to handle the relative timeout, and 
> allow restarts.
> 
> Signed-off-by: Nick Piggin <npiggin@suse.de>

yeah:

  Acked-by: Ingo Molnar <mingo@elte.hu>

i was totally confused and thought this was the original argument:

> +		restart->arg2 = time;
> +		return -ERESTART_RESTARTBLOCK;

but it's indeed the 'new' relative timeout. (It can still be a tiny bit 
inaccurate because if there are many signals then the signals will delay 
the relative timeout - so absolute timeouts are still better - but your 
fix is correct and needed nevertheless)

	Ingo