[rfc][patch] futex: restartable futex_wait?

[Nick Piggin]

Hi Ingo,

I'm seeing an LTP test fail for ltp test sigaction_16_24. Basically,
it tests whether the SA_RESTART flag works for the sem_wait operation.

I see sem_wait is implemented with futex_wait, so I wonder whether we
can make it restartable? Am I going about it the right way? (Seems to
fix the testcase here).

Thanks,
Nick

--
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:
-
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