linux-kernel - Re: [PATCH] pipe: read/write_iter() handler should check for IOCB

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Date:   Thu, 30 Apr 2020 13:56:55 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pipe: read/write_iter() handler should check for
 IOCB_NOWAIT

On 4/30/20 1:51 PM, Jens Axboe wrote:
> 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@...nel.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.

OK, it's the old classic in generic_write_checks(), is my guess:

        if ((iocb->ki_flags & IOCB_NOWAIT) && !(iocb->ki_flags & IOCB_DIRECT))  
                return -EINVAL;

so we definitely can't just flip the switch on O_NONBLOCK -> IOCB_NOWAIT
in general, at least not for writes.

-- 
Jens Axboe