On Thu, Jul 16, 2020 at 04:14:21PM +0200, Sebastian Andrzej Siewior wrote: > On 2020-07-15 15:14:49 [-0700], Paul E. McKenney wrote: > > > > My concern is that some critical bug will show up at some point > > that requires double-argument kfree_rcu() be invoked while holding > > a raw spinlock. (Single-argument kfree_rcu() must sometimes invoke > > synchronize_rcu(), so it can never be invoked in any state forbidding > > invoking schedule().) > > So you are saying as of today we are good but in near future the > following > synchronize_rcu() -> kfree_rcu() > > may be needed? You lost me on this one. I am instead concerned that something like this might be needed on short notice: raw_spin_lock(&some_lock); kfree_rcu(some_pointer, some_field_offset); In contrast, single-argument kfree_rcu() cannot be invoked from any environment where synchronize_rcu() cannot be invoked. > > Yes, dropping to a plain spinlock would be simple in the here and now, > > but experience indicates that it is only a matter of time, and that when > > that time comes it will come as an emergency. > > Hmmm. I point out the call_rcu() experience. > > One approach would be to replace the "IS_ENABLED(CONFIG_PREEMPT_RT)" > > with some sort of check for being in a context where spinlock acquisition > > is not legal. What could be done along those lines? > > I would rethink the whole concept how this is implemented now and give > it another try. The code does not look pretty and is looking > complicated. The RT covering of this part then just added a simple > return because nothing else seemed to be possible. This patch here > looks like another duct tape attempt to avoid a warning. In addition to the possibility of invocation from BH? Thanx, Paul
