Re: What does IOSQE_IO_[HARD]LINK actually mean?

[Jens Axboe <axboe@kernel.dk>]

On 2/1/20 2:18 AM, Andres Freund wrote:
> Hi,
> 
> Reading the manpage from liburing I read:
>        IOSQE_IO_LINK
>               When  this  flag is specified, it forms a link with the next SQE in the submission ring. That next SQE
>               will not be started before this one completes.  This, in effect, forms a chain of SQEs, which  can  be
>               arbitrarily  long. The tail of the chain is denoted by the first SQE that does not have this flag set.
>               This flag has no effect on previous SQE submissions, nor does it impact SQEs that are outside  of  the
>               chain  tail.  This  means  that multiple chains can be executing in parallel, or chains and individual
>               SQEs. Only members inside the chain are serialized. Available since 5.3.
> 
>        IOSQE_IO_HARDLINK
>               Like IOSQE_IO_LINK, but it doesn't sever regardless of the completion result.  Note that the link will
>               still sever if we fail submitting the parent request, hard links are only resilient in the presence of
>               completion results for requests that did submit correctly.  IOSQE_IO_HARDLINK  implies  IOSQE_IO_LINK.
>               Available since 5.5.
> 
> I can make some sense out of that description of IOSQE_IO_LINK without
> looking at kernel code. But I don't think it's possible to understand
> what happens when an earlier chain member fails, and what denotes an
> error.  IOSQE_IO_HARDLINK's description kind of implies that
> IOSQE_IO_LINK will not start the next request if there was a failure,
> but doesn't define failure either.

I won't touch on the rest since Pavel already did, but I did expand the
explanation of when a normal link will sever, and how:

https://git.kernel.dk/cgit/liburing/commit/?id=9416351377f04211f859667f39a58d2a223cbd21

LSFMM will have a session on BPF with io_uring, which we'll need to have
full control of links outside of the basic use cases.

-- 
Jens Axboe