Messages in this thread | | | Date | Mon, 11 Jun 2012 08:17:23 -0400 | From | "J. Bruce Fields" <> | Subject | Re: processes hung after sys_renameat, and 'missing' processes |
| |
On Fri, Jun 08, 2012 at 05:08:34PM +0200, Peter Zijlstra wrote: > On Fri, 2012-06-08 at 10:46 -0400, J. Bruce Fields wrote: > > > > > Sadly, if you get that annotation wrong you can annotate an actual > > > > > deadlock away. > > > > What's a (contrived as you want) example where that happens? > > spinlock_t lock_array[10]; > > void init_array(void) > { > int i; > > for (i = 0; i < ARRAY_SIZE(lock_array); i++) > spin_lock_init(array + i); > } > > void double_lock(int a, int b) > { > spin_lock(lock_array + a); > spin_lock_nested(lock_array + b, SINGLE_DEPTH_NESTING); > } > > The above places all locks in the array in the same class, it then does > a double lock without order, but tells lockdep the nesting is ok. > > A correct version of the double_lock() function would look like: > > void double_lock(int a, int b) > { > if (b < a) > swap(a, b); > > spin_lock(lock_array + a); > spin_lock_nested(lock_array + b, SINGLE_DEPTH_NESTING); > } > > This orders the locks in array order.
Got it, thanks!
--b.
> > > > > > This the reason you have to be very careful when > > > > > annotating stuff. > > > > Or alternatively--what do I need to check before I call > > mutex_lock_nested? > > That the lock order you tell lockdep is ok, is indeed correct. > >
| |