Re: [PATCH 3/5] io_uring: add support for getdents

From: Matthew Wilcox <willy@infradead.org>
To: Jens Axboe <axboe@kernel.dk>
Cc: "Darrick J. Wong" <djwong@kernel.org>,
	Christian Brauner <brauner@kernel.org>,
	Dave Chinner <david@fromorbit.com>, Hao Xu <hao.xu@linux.dev>,
	io-uring@vger.kernel.org,
	Dominique Martinet <asmadeus@codewreck.org>,
	Pavel Begunkov <asml.silence@gmail.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Stefan Roesch <shr@fb.com>, Clay Harris <bugs@claycon.org>,
	linux-fsdevel@vger.kernel.org, Wanpeng Li <wanpengli@tencent.com>
Subject: Re: [PATCH 3/5] io_uring: add support for getdents
Date: Tue, 1 Aug 2023 02:01:59 +0100	[thread overview]
Message-ID: <ZMhZh2EYPMH1wIXX@casper.infradead.org> (raw)
In-Reply-To: <e2d8e5f1-f794-38eb-cecf-ed30c571206b@kernel.dk>

On Mon, Jul 31, 2023 at 06:49:45PM -0600, Jens Axboe wrote:
> On 7/31/23 6:47?PM, Matthew Wilcox wrote:
> > On Mon, Jul 31, 2023 at 06:28:02PM -0600, Jens Axboe wrote:
> >> It's also not an absolute thing, like memory allocations are. It's
> >> perfectly fine to grab a mutex under NOWAIT issue. What you should not
> >> do is grab a mutex that someone else can grab while waiting on IO. This
> >> kind of extra context is only available in the code in question, not
> >> generically for eg mutex locking.
> > 
> > Is that information documented somewhere?  I didn't know that was the
> > rule, and I wouldn't be surprised if that's news to several of the other
> > people on this thread.
> 
> I've mentioned it in various threads, but would probably be good to
> write down somewhere if that actually makes more people see it. I'm
> dubious, but since it applies to NOWAIT in general, would be a good
> thing to do regardless. And then at least I could point to that rather
> than have to write up a long description every time.

Would be good to include whether it's OK to wait on a mutex that's held
by another thread that's allocating memory without __GFP_NOFS (since
obviously that can block on I/O)