On Wed, May 13 2009, Miklos Szeredi wrote:
> On Wed, 13 May 2009, Jens Axboe wrote:
> > On Tue, May 12 2009, Andrew Morton wrote:
> > > On Thu, 07 May 2009 15:37:36 +0200 Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> > >
> > > > + for (i = 0; i < spd.nr_pages; i++) {
> > > > + kunmap(pages[i]);
> > >
> > > It is deadlockable if any thread of control holds more than a single
> > > kmap at a time.
> > >
> > > Because there are a finite number of kmaps available, and if one is
> > > unavailable, kmap() waits for one to become free. If the number of
> > > waiting threads equals the number of available slots, nobody makes any
> > > progress.
>
> Ouch.
>
> >
> > Good catch, that will not work reliably. I've applied the below.
>
> Thanks.
>
> The bigger problem is that the default_file_splice_write()
> implementation in the other patch does the same (it calls
> buf->ops->map() on all buffers).
Yep that's even worse, as that should go BUG() pretty much immediately
when the KM_USER0 slot is reused!
> Hmm. Simple solution would be to do a write() for each buffer. But
> this only affects HIGHMEM kernels, so it's a bit pointless to do that
> on all archs. Sigh...
It is unfortunate, we are going to be stuck with that for some time
still...
--
Jens Axboe
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