examples/kdigest: add AF_ALG hash example

[ddiss]

When built with CONFIG_CRYPTO_USER_API_HASH enabled, Linux exposes a
socket based API for hashing data. When coupled with uring
IOSQE_IO_LINK, file hashing can be done in quite an efficient manner,
as demonstrated in this link-cp.c based example.

Signed-off-by: David Disseldorp ddiss@suse.de

[chrisvest]

Looks like the linked pairs be processed out of order, so the hash result is non-deterministic, no?

[ddiss]

Looks like the linked pairs be processed out of order, so the hash result is non-deterministic, no?

Yes, you're right. I'll close this while I rework it. Setting QD to one would be the most obvious fix but will likely hurt performance...

[ddiss]

I think flagging IOSQE_IO_DRAIN on the AF_ALG send I/O would be the simplest fix for hash ordering here... IIUC it'll still permit AF_ALG send and next-offset file read I/Os to run concurrently, but everything else will move in lock step.

[ddiss]

I think flagging IOSQE_IO_DRAIN on the AF_ALG send I/O would be the simplest fix for hash ordering here... IIUC it'll still permit AF_ALG send and next-offset file read I/Os to run concurrently, but everything else will move in lock step.

Looks like my force push has blocked reopening here. Will submit this change as a new PR.