Re: commit "usb: gadget: ffs: add eventfd notification about ffs events"

[Robert Baldyga <r.baldyga@samsung.com>]

Hi Christoph,

On 02/18/2015 06:36 PM, Christoph Hellwig wrote:
> Hi Robert,
> 
> your above commit pokes into kiocb internals it really should not, and
> it breaks my patchset that's been on the -fsdevel list which makes most
> of the kiocb guts private to fs/aio.c.
> 
> First please Cc patches like that to linux-fsdevel.  Second I don't
> think the logic in it is correct.  You should not disable your own
> notifications based on how aio works, but do it unconditionally.  What
> was the reason to disable the eventfd notification if there is an aio
> one as well?
> 

Sometimes we would like to have one endpoint handled in separate thread.
It could have its own eventfd, which notifies only about events on its
own requests. We don't want to notify main event loop in main thread
about these events, because it doesn't make sense.

The another example is using AIO hidden under synchronous API to have
timeout feature. We submit AIO transfer with own eventfd, we select() on
this eventfd with given timeout, and if timeout expired we cancel
transfer. Then we don't want to notify main event loop about async
transfer completion, because from users point of view performed
operation looks like synchronous.

Most of requests will be handled using ffs eventfd, but when we supply
another one to selected transfer, we rather don't want to have
notification on the first one.

Robert