Re: [PATCH -v3 3/4] lib, Make gen_pool memory allocator lockless

Paul E. McKenney
2011-04-15T17:43:24Z

On Fri, Apr 15, 2011 at 12:46:29PM -0400, Mathieu Desnoyers wrote:
> * Huang Ying (ying.huang@intel.com) wrote:
> > On 04/14/2011 05:07 AM, Mathieu Desnoyers wrote:
> > > * Huang Ying (ying.huang@intel.com) wrote:
> > > [...]
> > >> + * rcu_read_lock and rcu_read_unlock is not used int gen_pool_alloc,
> > >> + * gen_pool_free, gen_pool_avail and gen_pool_size etc, because chunks
> > >> + * are only added into pool, not deleted from pool unless the pool
> > >> + * itself is destroyed. If chunk will be deleted from pool,
> > >> + * rcu_read_lock and rcu_read_unlock should be uses in these
> > >> + * functions.
> > >
> > > So how do you protect between pool destruction and adding chunks into
> > > the pool ?
> >
> > Because the pool itself will be freed when destruction, we need some
> > mechanism outside of pool. For example, if gen_pool_add() is called via
> > device file IOCTL, we must un-register the device file first, and
> > destroy the pool after the last reference to device has gone.
>
> I am concerned about the list_for_each_entry_rcu() (and thus
> rcu_dereference_raw()) used outside of rcu_read_lock/unlock pairs.
> Validation infrastructure as recently been added to RCU: it triggers
> warnings when these situations are encountered in some RCU debugging
> configurations. The case of RCU list iteration is not covered by the
> checks, but it would make sense to be aware of it.
>
> So although it seems like your code does not require rcu read lock
> critical sections, I'd prefer to let Paul McKenney have a look.

As long as you add elements and never remove them, then you can get
away with using list_for_each_entry_rcu() outside of an RCU read-side
critical section. But please comment this -- it is all too easy
for someone to decide later to start deleting elements without also
inserting the needed rcu_read_lock() and rcu_read_unlock() pairs.

But I have lost the thread -- what code am I supposed to look at?

Thanx, Paul