Re: New diagnostic routine - mutex_ownable()

[Paul Goyette <paul%whooppee.com@localhost>]

On Sun, 30 Apr 2017, Paul Goyette wrote:

> While working on getting the localcount(9) stuff whipped into shape, I ran 
> across a situation where it is desirable to ensure that the current 
> process/lwp does not already own a mutex.
>
> We cannot use !mutex_owned() since that doesn't return the desired result for 
> a spin mutex, so I'm proposing to add a new routine called mutex_ownable(). 
> This does nothing in normal kernels, but for LOCKDEBUG kernels it does a 
> mutex_enter() followed immediately by mutex_exit(). If the current process 
> already owns the mutex, the system will panic with a "locking against myself" 
> error; otherwise mutex_ownable() just returns 1, enabling its use as
>
> 	KASSERT(mutex_ownable(mtx));
>
> Diffs are attached (including man-page and sets-list updates).
>
> Comments?  Any reason why this cannot be committed?
>
> (Thanks to riastradh@ for the idea and initial review.)

For completeness, it turns out that the following change is needed to 
keep the rump build happy!  This is in addition to the diffs posted 
previously.

Index: locks.c
===================================================================
RCS file: /cvsroot/src/sys/rump/librump/rumpkern/locks.c,v
retrieving revision 1.73
diff -u -p -r1.73 locks.c
--- locks.c	27 Jan 2017 09:50:47 -0000	1.73
+++ locks.c	30 Apr 2017 03:16:58 -0000
@@ -183,6 +183,17 @@ mutex_exit(kmutex_t *mtx)
 __strong_alias(mutex_spin_exit,mutex_exit);

 int
+mutex_ownable(kmutex_t *mtx)
+{
+
+#ifdef RUMP_LOCKDEBUG
+	mutex_enter(mtx);
+	mutex_exit(mtx);
+#endif
+	return 1;
+}
+
+int
 mutex_owned(kmutex_t *mtx)
 {


+------------------+--------------------------+----------------------------+
| Paul Goyette     | PGP Key fingerprint:     | E-mail addresses:          |
| (Retired)        | FA29 0E3B 35AF E8AE 6651 | paul at whooppee dot com   |
| Kernel Developer | 0786 F758 55DE 53BA 7731 | pgoyette at netbsd dot org |
+------------------+--------------------------+----------------------------+