Re: [PATCH RFC] io_uring: make signalfd work with io_uring (and aio) POLL

From: Jens Axboe <axboe@kernel.dk>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Jann Horn <jannh@google.com>
Cc: io-uring@vger.kernel.org,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH RFC] io_uring: make signalfd work with io_uring (and aio) POLL
Date: Thu, 14 Nov 2019 08:20:18 -0700	[thread overview]
Message-ID: <6243eb59-3340-deb5-d4b8-08501be01f34@kernel.dk> (raw)
In-Reply-To: <0f74341f-76fa-93ee-c03e-554d02707053@rasmusvillemoes.dk>

On 11/14/19 8:19 AM, Rasmus Villemoes wrote:
> On 14/11/2019 16.09, Jens Axboe wrote:
>> On 11/14/19 7:12 AM, Rasmus Villemoes wrote:
> 
>>> So, I can't really think of anybody that might be relying on inheriting
>>> a signalfd instead of just setting it up in the child, but changing the
>>> semantics of it now seems rather dangerous. Also, I _can_ imagine
>>> threads in a process sharing a signalfd (initial thread sets it up and
>>> blocks the signals, all threads subsequently use that same fd), and for
>>> that case it would be wrong for one thread to dequeue signals directed
>>> at the initial thread. Plus the lifetime problems.
>>
>> What if we just made it specific SFD_CLOEXEC?
> 
> O_CLOEXEC can be set and removed afterwards. Sure, we're far into
> "nobody does that" land, but having signalfd() have wildly different
> semantics based on whether it was initially created with O_CLOEXEC seems
> rather dubious.
> 
>   I don't want to break
>> existing applications, even if the use case is nonsensical, but it is
>> important to allow signalfd to be properly used with use cases that are
>> already in the kernel (aio with IOCB_CMD_POLL, io_uring with
>> IORING_OP_POLL_ADD). Alternatively, if need be, we could add a specific
>> SFD_ flag for this.
> 
> Yeah, if you want another signalfd flavour, adding it via a new SFD_
> flag seems the way to go. Though I can't imagine the resulting code
> would be very pretty.

Well, it's currently _broken_ for the listed in-kernel use cases, so
I think making it work is the first priority here.

-- 
Jens Axboe