On 3/30/22 9:12 AM, Miklos Szeredi wrote: > On Wed, 30 Mar 2022 at 17:05, Jens Axboe <axboe@xxxxxxxxx> 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. Honestly, I think we should just change it during this round, making it consistent with the "slot is unused" use case. The old use case is more more of a "it happened to work" vs the newer consistent behavior of "we always assign the file when execution starts on the request". Let me spin a patch, would be great if you could test. -- Jens Axboe
