[RESEND] [BUG] futex_unlock_pi() hurts my brain and may cause application deadlock

[john stultz <johnstul@us.ibm.com>]

On Wed, 2007-05-30 at 17:49 -0700, john stultz wrote:
> All,
> 	So we've been seeing PI mutex deadlocks with a few of our applications
> using the -rt kernel. After narrowing things down, we were finding that
> the applications were indirectly calling futex_unlock_pi(), which on
> occasion would return -EFAULT, which is promptly ignored by glibc. This
> would cause the application to continue on as if it has unlocked the
> mutex, until it tried to reacquire it and deadlock.
> 
> In looking into why the kernel was returning -EFAULT, I found the
> following:
> 
> ...
> retry_locked:
> 	/*
> 	 * To avoid races, try to do the TID -> 0 atomic transition
> 	 * again. If it succeeds then we can return without waking
> 	 * anyone else up:
> 	 */
> 	if (!(uval & FUTEX_OWNER_DIED)) {
> 		pagefault_disable();
> 		uval = futex_atomic_cmpxchg_inatomic(uaddr, current->pid, 0);
> 		pagefault_enable();
> 	}
> 
> 	if (unlikely(uval == -EFAULT))
> 		goto pi_faulted;
> ...[snip]...
> pi_faulted:
> 	/*
> 	 * We have to r/w  *(int __user *)uaddr, but we can't modify it
> 	 * non-atomically.  Therefore, if get_user below is not
> 	 * enough, we need to handle the fault ourselves, while
> 	 * still holding the mmap_sem.
> 	 */
> 	if (attempt++) {
> 		ret = futex_handle_fault((unsigned long)uaddr, fshared,
> 					 attempt);
> 		if (ret)
> 			goto out_unlock;
> 		goto retry_locked;
> 	}
> 
> 
> Should we fault through normal causes, on the second round we call
> futex_handle_fault, which faults in the address, and we then jump back
> to retry_locked. However, since uval is -EFAULT from the last cmpxchg,
> it &s w/ FUTEX_OWNER_DIED so we don't enter the first conditional to try
> to cmpxchg again. So since uval is still -EFAULT, we loop back to
> pi_faulted! This will loop until futex_handle_fault() bombs out because
> attempt is too big and we return -EFAULT.
> 
> I *think* this is a possible quick fix here, but I'm no futex guru, so I
> wanted to run it by folks for review.
> 
> Big thanks to Sripathi and Angela Lin for helping debug this, and Steven
> for suggesting a cleaner fix then what I first tried.


Hey Ingo,
	I sent this fix for this back in the 2.6.21-rt9 era, and I thought to
picked it up, but I think it arrived amid some futex churn and
apparently has been dropped or was just lost.

Some of our testers have been reporting futex deadlocks and this fix
resolves it, so I believe it to still be needed.

thanks
-john

Avoid futex_unlock_pi returning -EFAULT (which results in deadlock), by
clearing uval before jumping to retry_locked.

Signed-off-by: John Stultz <johnstul@us.ibm.com>
Index: 2.6-rt/kernel/futex.c
===================================================================
--- 2.6-rt.orig/kernel/futex.c	2007-07-31 16:48:41.000000000 -0700
+++ 2.6-rt/kernel/futex.c	2007-07-31 16:48:48.000000000 -0700
@@ -1673,6 +1673,7 @@ pi_faulted:
 					 attempt);
 		if (ret)
 			goto out;
+		uval = 0;
 		goto retry_unlocked;
 	}