On Wed, Jan 15, 2020 at 07:56:14AM +0100, Christoph Hellwig wrote: > On Tue, Jan 14, 2020 at 03:27:00PM -0400, Jason Gunthorpe wrote: > > I've seen similar locking patterns quite a lot, enough I've thought > > about having a dedicated locking primitive to do it. It really wants > > to be a rwsem, but as here the rwsem rules don't allow it. > > > > The common pattern I'm looking at looks something like this: > > > > 'try begin read'() // aka down_read_trylock() > > > > /* The lockdep release hackery you describe, > > the rwsem remains read locked */ > > 'exit reader'() > > > > .. delegate unlock to work queue, timer, irq, etc .. > > > > in the new context: > > > > 're_enter reader'() // Get our lockdep tracking back > > > > 'end reader'() // aka up_read() > > > > vs a typical write side: > > > > 'begin write'() // aka down_write() > > > > /* There is no reason to unlock it before kfree of the rwsem memory. > > Somehow the user prevents any new down_read_trylock()'s */ > > 'abandon writer'() // The object will be kfree'd with a locked writer > > kfree() > > > > The typical goal is to provide an object destruction path that can > > serialize and fence all readers wherever they may be before proceeding > > to some synchronous destruction. > > > > Usually this gets open coded with some atomic/kref/refcount and a > > completion or wait queue. Often implemented wrongly, lacking the write > > favoring bias in the rwsem, and lacking any lockdep tracking on the > > naked completion. > > > > Not to discourage your patch, but to ask if we can make the solution > > more broadly applicable? > > Your requirement seems a little different, and in fact in many ways > similar to the percpu_ref primitive. I was interested because you are talking about allowing the read/write side of a rw sem to be held across a return to user space/etc, which is the same basic problem. precpu refcount looks more like a typical refcount with a release that is called by whatever context does the final put. The point above is to basically move the release of a refcount into a synchrnous path by introducing some barrier to wait for the refcount to go to zero. In the above the barrier is the down_write() as it is really closer to a rwsem than a refcount. Thanks, Jason
