Re: RFC: hold i_rwsem until aio completes

Christoph Hellwig
2020-01-15T06:56:19Z

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.