std: linux uring kernel interfaces #2525
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std/os.zig
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| // kernel has the matching store barrier for the update. The | ||
| // kernel also ensures that previous stores to CQEs are ordered | ||
| // with the tail update. | ||
| @fence(.SeqCst); |
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I'm not entirely sure what sort of fence these should be. The upstream examples use these definitions: http://git.kernel.dk/cgit/liburing/tree/src/barrier.h
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It has acquire semantics for loads and release semantics for stores, see here.
On x86 that means you don't need synchronization due to its memory model but on e.g. arm64 you'd insert ldar and stlr instructions.
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After seeing some commits
2019-04-30 | io_uring: remove unnecessary barrier after unsetting IORING_SQ_NEED_WAKEUP | Stefan Bühler
2019-04-30 | io_uring: remove unnecessary barrier after incrementing dropped counter | Stefan Bühler
2019-04-30 | io_uring: remove unnecessary barrier before reading SQ tail | Stefan Bühler
2019-04-30 | io_uring: remove unnecessary barrier after updating SQ head | Stefan Bühler
2019-04-30 | io_uring: remove unnecessary barrier before reading cq head | Stefan Bühler
2019-04-30 | io_uring: remove unnecessary barrier before wq_has_sleeper | Stefan Bühler
I'm not sure if that comment is still true? @stbuehler ?
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Hi. The standard way would be using acquire on cq.khead here, which would order the load on cq.khead before all other loads following, especially those reading the CQE.
A SeqCst fence is basically a full memory barrier afaik, and should work of course; an Acquire fence should work too, or a simple read memory barrier.
I didn't bother fixing liburing, because I'm not that crazy about C, and liburing doesn't have atomic/fences - it only knows the barriers you linked (it probably should use C11 atomics instead, and then it should be easy).
You can see my proof-of-concept code for rust here (for now): https://github.com/stbuehler/rust-io-uring/blob/wip/io-uring/src/lib.rs
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I've taken the wrapper out of this PR, I'll send it as another PR soon. |
This branch contains the start of a uring based command buffer structure.Currently only implemented for linux (and needs kernel 5.1+); I have plans to modify/extend the interface to other operating systems and older linux versions.
Early commits on the branch are fixes for other areas, please consider cherry-picking them now even if the final commit is too much to review right away!