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

[Matthew Wilcox <willy@infradead.org>]

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)