Re: [patch] splice mmap_sem deadlock

[Jens Axboe <jens.axboe@oracle.com>]

On Sat, Sep 29 2007, Nick Piggin wrote:
> On Fri, Sep 28, 2007 at 01:02:50PM -0700, Linus Torvalds wrote:
> > 
> > 
> > On Fri, 28 Sep 2007, Jens Axboe wrote:
> > > 
> > > Hmm, part of me doesn't like this patch, since we now end up beating on
> > > mmap_sem for each part of the vec. It's fine for a stable patch, but how
> > > about
> > > 
> > > - prefaulting the iovec
> > > - using __get_user()
> > > - only dropping/regrabbing the lock if we have to fault
> > 
> > "__get_user()" doesn't help any. But we should do the same thing we do for 
> > generic_file_write(), or whatever - probe it while in an atomic region.
> > 
> > So something like the appended might work. Untested.
> 
> I got an idea for getting rid of mmap_sem from here completely. Which
> is why I was looking at these callers in the first place.
> 
> It would be really convenient and help me play with the idea if mmap_sem
> is wrapped closely around get_user_pages where possible...

Well, move it back there in your first patch? Not a big deal, surely :-)

> If you're really worried about mmap_sem batching here, can you just
> avoid this complexity and do all the get_user()s up-front, before taking
> mmap_sem at all? You only have to save PIPE_BUFFERS number of
> them.

Sure, that is easily doable at the cost of some stack. I have other
patches that grow PIPE_BUFFERS dynamically in the pipeline, so I'd
prefer not to since that'll then turn into a dynamic allocation.

-- 
Jens Axboe

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