Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate

Luis Chamberlain
2021-10-27T14:28:10Z

On Wed, Oct 27, 2021 at 01:57:40PM +0200, Miroslav Benes wrote:
> On Tue, 26 Oct 2021, Luis Chamberlain wrote:
>
> > On Tue, Oct 26, 2021 at 11:37:30PM +0800, Ming Lei wrote:
> > > OK, then Luis shouldn't consider livepatching as one such issue to solve
> > > with one generic solution.
> >
> > It's not what I was told when the deadlock was found with zram, so I was
> > informed quite the contrary.
>
> From my perspective, it is quite easy to get it wrong due to either a lack
> of generic support, or missing rules/documentation.

Indeed. I agree some level of guidence is needed, even if subtle, rather
than tribal knowledge. I'll start off with the test_sysfs demo'ing what
not to do and documenting this there. I don't think it makes sense to
formalize yet documentation for "though shalt not do this" generically
until a full depth search is done with Coccinelle.

> So if this thread
> leads to "do not share locks between a module removal and a sysfs
> operation" strict rule, it would be at least something.

I think that's where we are at. I'll wait to complete my coccinelle
deadlock hunt patch to complete the full search, and that could be
useful to *warn* aboute new use cases, so to prevent this deadlock
in the future. Until then I agree that the complexity introduced is
not worth it given the evidence of users, but the full evidence of
actual users still remains to be determined. A perfect job left to
advances with Coccinelle.

> In the same
> manner as Luis proposed to document try_module_get() expectations.

Right and so sysfs ops using try_module_get() *still* remains safe,
and so will keep that patch in my next iteration because there *are*
*many* uses cases for that.

Luis