Re: [PATCH] pipe: read/write_iter() handler should check for IOCB_NOWAIT

[Jens Axboe <axboe@kernel.dk>]

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