Re: possible deadlock in generic_file_write_iter (2)

[Byungchul Park]

On Tue, Dec 05, 2017 at 10:19:07AM +0100, Dmitry Vyukov wrote:
> On Tue, Dec 5, 2017 at 5:58 AM, Byungchul Park <byungchul.park@xxxxxxx> wrote:
> > On 12/4/2017 5:33 PM, Jan Kara wrote:
> >>
> >> Hello,
> >>
> >> adding Peter and Byungchul to CC since the lockdep report just looks
> >> strange and cross-release seems to be involved. Guys, how did #5 get into
> >> the lock chain and what does put_ucounts() have to do with sb_writers
> >> there? Thanks!
> >
> >
> > Hello Jan,
> >
> > In order to get full stack of #5, we have to pass a boot param,
> > "crossrelease_fullstack", to the kernel. Now that it only informs
> > put_ucounts() in the call trace, it's hard to find out what exactly
> > happened at that time, but I can tell #5 shows:
> >
> > When acquire(sb_writers) in put_ucounts(), it was on the way to
> > complete((completion)&req.done) of wait_for_completion() in
> > devtmpfs_create_node().
> >
> > If acquire(sb_writers) in put_ucounts() is stuck, then
> > wait_for_completion() in devtmpfs_create_node() would be also
> > stuck, since complete() being in the context of acquire(sb_writers)
> > cannot be called.
> >
> > This is why cross-release added the lock chain.
> 
> Hi,
> 
> What is cross-release? Is it something new? Should we always enable
> crossrelease_fullstack during testing?

Hello Dmitry,

Yes, it's new one making lockdep track wait_for_completion() as well.

And we should enable crossrelease_fullstack if you don't care system
slowdown but testing.

--
Thanks,
Byungchul