On Thu, Jul 16, 2020 at 05:36:38PM +0200, Sebastian Andrzej Siewior wrote: > On 2020-07-16 08:20:27 [-0700], Paul E. McKenney wrote: > > 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. > > I see. We don't have any kfree() in that context as far as I remember. > We had a few cases in "resize" where you allocate memory, copy content > and free old memory while under the lock but they are gone. True, but we also didn't have any calls to call_rcu() prior to the call to rcu_init() until suddenly we did. (Yeah, I could have put my foot down and prohibited that practice, but the workarounds were quite a bit more complicated than just making call_rcu() work during very early boot.) And last I checked, there really were calls to call_rcu() under raw spinlocks, so the potential or calls to double-argument kfree_rcu() clearly exists and is very real. > > > > 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? > > Invocation from BH should be possible because network would probably be > the first user. I don't remember anything wrong with BH if I remember > correctly. OK, that is reassuring. Here is hoping! Thanx, Paul
