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Subject: Re: [PATCH] pipe: read/write_iter() handler should check for
 IOCB_NOWAIT
From:   Jens Axboe <axboe@kernel.dk>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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 <d00f0ead-2782-06b3-6e21-559d8c86c461@kernel.dk>
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Date:   Thu, 30 Apr 2020 13:51:58 -0600
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On 4/30/20 12:47 PM, Jens Axboe wrote:
> On 4/30/20 11:58 AM, Matthew Wilcox 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.
>>>
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>
>> Wouldn't this be better?
> 
> Yeah, that's probably a better idea. Care to send a "proper" patch?

I take that back, running into issues going with a whole-sale conversion
like that:

mkdir("/run/dhcpcd", 0755)              = -1 EEXIST (File exists)
openat(AT_FDCWD, "/run/dhcpcd/ens7.pid", O_WRONLY|O_CREAT|O_NONBLOCK|O_CLOEXEC, 0644) = 4
flock(4, LOCK_EX|LOCK_NB)               = 0
getpid()                                = 214
ftruncate(4, 0)                         = 0
lseek(4, 0, SEEK_SET)                   = 0
fstat(4, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
lseek(4, 0, SEEK_CUR)                   = 0
write(4, "214\n", 4)                    = -1 EINVAL (Invalid argument)

which I don't know where is coming from yet, but it's definitely
breakage by auto setting IOCB_NOWAIT if O_NONBLOCK is set.

I'd prefer to go your route, but I also would like this fixed for pipes
for 5.7. So I'd suggest we go with mine, and then investigate why this
is breaking stuff and go with the all-in approach for 5.8 if feasible.

-- 
Jens Axboe