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

[Christian Brauner <brauner@kernel.org>]

On Tue, Aug 01, 2023 at 02:01:59AM +0100, Matthew Wilcox wrote:
> 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)

And adding a few illustrative examples would be helpful.