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Subject: Re: [PATCH] pipe: read/write_iter() handler should check for
 IOCB_NOWAIT
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <273d8294-2508-a4c2-f96e-a6a394f94166@kernel.dk>
 <20200501035820.GH23230@ZenIV.linux.org.uk>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <210c6b72-179a-993f-1ec8-1960db107174@kernel.dk>
Date:   Thu, 30 Apr 2020 22:14:11 -0600
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On 4/30/20 9:58 PM, Al Viro wrote:
> On Thu, Apr 30, 2020 at 10:24:46AM -0600, Jens Axboe wrote:
>> Pipe read/write only checks for the file O_NONBLOCK flag, but we should
>> also check for IOCB_NOWAIT for whether or not we should handle this read
>> or write in a non-blocking fashion. If we don't, then we will block on
>> data or space for iocbs that explicitly asked for non-blocking
>> operation. This messes up callers that explicitly ask for non-blocking
>> operations.
> 
> Why does io_uring allow setting IOCB_NOWAIT without FMODE_NOWAIT, anyway?

To do per-io non-blocking operations. It's not practical or convenient
to flip the file flag, nor does it work if you have multiple of them
going. If pipes honor the flag for the read/write iter handlers, then
we can handle them a lot more efficiently instead of requiring async
offload.

-- 
Jens Axboe