On Thu, Nov 24, 2022 at 11:18:53AM +0100, Jan Kara wrote: > On Wed 23-11-22 16:21:28, Paul E. McKenney wrote: > > On Wed, Nov 23, 2022 at 12:46:45PM +0100, Jan Kara wrote: > > > Hello! > > > > > > We were pondering with Amir about some issues with fsnotify subsystem and > > > as a building block we would need a mechanism to make sure write(2) has > > > completed. For simplicity we could imagine it like a sequence > > > > > > write(2) > > > START > > > do stuff to perform write > > > END > > > > > > and we need a mechanism to wait for all processes that already passed START > > > to reach END. Ideally without blocking new writes while we wait for the > > > pending ones. Now this seems like a good task for SRCU. We could do: > > > > > > write(2) > > > srcu_read_lock(&sb->s_write_rcu); > > > do stuff to perform write > > > srcu_read_unlock(&sb->s_write_rcu); > > > > > > and use synchronize_srcu(&sb->s_write_rcu) for waiting. > > > > > > But the trouble with writes is there are things like aio or io_uring where > > > the part with srcu_read_lock() happens from one task (the submitter) while > > > the part with srcu_read_unlock() happens from another context (usually worker > > > thread triggered by IRQ reporting that the HW has finished the IO). > > > > > > Is there any chance to make SRCU work in a situation like this? It seems to > > > me in principle it should be possible to make this work but maybe there are > > > some implementation constraints I'm missing... > > > > The srcu_read_lock_notrace() and srcu_read_unlock_notrace() functions > > will work for this, though that is not their intended purpose. Plus you > > might want to trace these functions, which, as their names indicate, is > > not permitted. I assume that you do not intend to use these functions > > from NMI handlers, though that really could be accommodated. (But why > > would you need that?) > > > > So how about srcu_down_read() and srcu_up_read(), as shown in the > > (untested) patch below? > > Great, that looks exactly like what we need! Thanks for the quick reply! > > > Note that you do still need to pass the return value from srcu_down_read() > > into srcu_up_read(). I am guessing that io_uring has a convenient place > > that this value can be placed. No idea about aio. > > Sure, we know about the need to pass the return value but we can handle > that :) Very good, and looking forward to seeing a Tested-by. ;-) Thanx, Paul
