Messages in this thread | | | Subject | Re: processes hung after sys_renameat, and 'missing' processes | From | Peter Zijlstra <> | Date | Fri, 08 Jun 2012 17:08:34 +0200 |
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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.
> > > > 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.
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