[PATCH liburing] man/io_uring_enter.2: document IOSQE_CQE_SKIP_SUCCESS

[PATCH liburing] man/io_uring_enter.2: document IOSQE_CQE_SKIP_SUCCESS

[Pavel Begunkov]

Add a section about IOSQE_CQE_SKIP_SUCCESS describing the behaviour and
use cases.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 man/io_uring_enter.2 | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/man/io_uring_enter.2 b/man/io_uring_enter.2
index 93b97e6..710e84e 100644
--- a/man/io_uring_enter.2
+++ b/man/io_uring_enter.2
@@ -1094,6 +1094,30 @@ are available and this flag is set, then the request will fail with
 as the error code. Once a buffer has been used, it is no longer available in
 the kernel pool. The application must re-register the given buffer again when
 it is ready to recycle it (eg has completed using it). Available since 5.7.
+.TP
+.B IOSQE_CQE_SKIP_SUCCESS
+Instruct to not generate a CQE if the request completes successfully. If the
+request fails an appropriate CQE will be posted as usual and if there is no
+.B IOSQE_IO_HARDLINK,
+CQEs for all linked requests will be omitted. The notion
+of failure/success is opcode specific and is the same as with breaking chains
+of
+.B IOSQE_IO_LINK.
+One special case is when the request has a linked timeout, then the CQE
+generation for the linked timeout is decided solely by whether it has
+.B IOSQE_CQE_SKIP_SUCCESS
+set, regardless whether it timed out or
+was cancelled. In other words, if a linked timeout has the flag set, it's
+guaranteed to not post a CQE.
+
+The semantics is chosen to accommodate several use cases. First, when all but
+last requests of a normal link without linked timeouts are marked with the flag,
+it guarantees to post only one CQE per link. Also, it makes possible to suppress
+CQEs in cases where side effects of a successfully executed operation will be
+enough for the userspace to know the state of the system, e.g. writing to
+a synchronisation file.
+
+Available since 5.17.
 
 .PP
 .I ioprio
-- 
2.34.0

Re: [PATCH liburing] man/io_uring_enter.2: document IOSQE_CQE_SKIP_SUCCESS

[Jens Axboe]

On 11/26/21 6:50 PM, Pavel Begunkov wrote:
> Add a section about IOSQE_CQE_SKIP_SUCCESS describing the behaviour and
> use cases.

Thanks, applied with a few language edits.

-- 
Jens Axboe

Re: [PATCH liburing] man/io_uring_enter.2: document IOSQE_CQE_SKIP_SUCCESS

[Pavel Begunkov]

On 11/27/21 14:08, Jens Axboe wrote:
> On 11/26/21 6:50 PM, Pavel Begunkov wrote:
>> Add a section about IOSQE_CQE_SKIP_SUCCESS describing the behaviour and
>> use cases.
> 
> Thanks, applied with a few language edits.

thanks for tidying it up

-- 
Pavel Begunkov