Re: io_uring_prep_openat_direct() and link/drain

[Miklos Szeredi <miklos@szeredi.hu>]

On Wed, 30 Mar 2022 at 17:05, Jens Axboe <axboe@kernel.dk> wrote:
>
> On 3/30/22 8:58 AM, Miklos Szeredi wrote:
> > Next issue:  seems like file slot reuse is not working correctly.
> > Attached program compares reads using io_uring with plain reads of
> > proc files.
> >
> > In the below example it is using two slots alternately but the number
> > of slots does not seem to matter, read is apparently always using a
> > stale file (the prior one to the most recent open on that slot).  See
> > how the sizes of the files lag by two lines:
> >
> > root@kvm:~# ./procreads
> > procreads: /proc/1/stat: ok (313)
> > procreads: /proc/2/stat: ok (149)
> > procreads: /proc/3/stat: read size mismatch 313/150
> > procreads: /proc/4/stat: read size mismatch 149/154
> > procreads: /proc/5/stat: read size mismatch 150/161
> > procreads: /proc/6/stat: read size mismatch 154/171
> > ...
> >
> > Any ideas?
>
> Didn't look at your code yet, but with the current tree, this is the
> behavior when a fixed file is used:
>
> At prep time, if the slot is valid it is used. If it isn't valid,
> assignment is deferred until the request is issued.
>
> Which granted is a bit weird. It means that if you do:
>
> <open fileA into slot 1, slot 1 currently unused><read slot 1>
>
> the read will read from fileA. But for:
>
> <open fileB into slot 1, slot 1 is fileA currently><read slot 1>
>
> since slot 1 is already valid at prep time for the read, the read will
> be from fileA again.
>
> Is this what you are seeing? It's definitely a bit confusing, and the
> only reason why I didn't change it is because it could potentially break
> applications. Don't think there's a high risk of that, however, so may
> indeed be worth it to just bite the bullet and the assignment is
> consistent (eg always done from the perspective of the previous
> dependent request having completed).
>
> Is this what you are seeing?

Right, this explains it.   Then the only workaround would be to wait
for the open to finish before submitting the read, but that would
defeat the whole point of using io_uring for this purpose.

Thanks,
Miklos