Re: [PATCH 1/2] break out page allocation warning code

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

On Thu, 2011-04-28 at 16:48 -0700, john stultz wrote:
> On Thu, 2011-04-28 at 15:48 -0700, David Rientjes wrote:
> > On Wed, 27 Apr 2011, john stultz wrote:
> > 
> > > So thinking further, this can be simplified by adding the seqlock first,
> > > and then retaining the task_locking only in the set_task_comm path until
> > > all comm accessors are converted to using get_task_comm.
> > > 
> > 
> > On second thought, I think it would be better to just retain using a 
> > spinlock but instead of using alloc_lock, introduce a new spinlock to 
> > task_struct for the sole purpose of protecting comm.
> > 
> > And, instead, of using get_task_comm() to write into a preallocated 
> > buffer, I think it would be easier in the vast majority of cases that 
> > you'll need to convert to just provide task_comm_lock(p) and 
> > task_comm_unlock(p) so that p->comm can be dereferenced safely.  

Ok.. trying to find a middle ground here by replying to my own
concerns. :)

> So my concern with this is that it means one more lock that could be
> mis-nested. By keeping the locking isolated to the get/set_task_comm, we
> can be sure that won't happen. 
> 
> Also tracking new current->comm references will be easier if we just
> don't allow new ones. Validating that all the comm references are
> correctly locked becomes more difficult if we need locking at each use
> site.

So maybe we still ban current->comm access and instead have a
lightweight get_comm_locked() accessor or something that. Then we can
add debugging options to validate that the lock is properly held
internally.

> Further, since I'm not convinced that we never reference current->comm
> from irq context, if we go with spinlocks, we're going to have to
> disable irqs in the read path as well. seqlocks were nice for that
> aspect.

rwlocks can resolve this concern.


Any other thoughts?

-john