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SubjectRe: [PATCH] hugetlb: select PREEMPT_COUNT if HUGETLB_PAGE for in_atomic use
On Thu 11-03-21 12:36:51, Peter Zijlstra wrote:
> On Thu, Mar 11, 2021 at 12:09:15PM +0100, Michal Hocko wrote:
>
> > Sorry for being dense but I do not follow. You have provided the
> > following example
> > spin_lock(&A);
> > <IRQ>
> > spin_lock(&A);
> >
> > if A == hugetlb_lock then we should never reenter with
> > free_huge_page
>
> What I'm saying is that if irq_disabled(), the that interrupt cannot
> happen, so the second spin_lock cannot happen, so the deadlock cannot
> happen.
>
> So: '!irqs_disabled() && in_atomic()' is sufficient to avoid the IRQ
> recursion deadlock.

OK, then I understand your point now. I thought you were arguing
an actual deadlock scenario. As I've said irq_disabled check would be
needed for sleeping operations that we already do.

> Also, Linus hates constructs like this:
>
> https://lkml.kernel.org/r/CAHk-=wht7kAeyR5xEW2ORj7m0hibVxZ3t+2ie8vNHLQfdbN2_g@mail.gmail.com
>
> > From the code simplicity POV (and hugetlb has grown a lot of complexity)
> > it would be really easiest to make sure __free_huge_page to be called
> > from a non-atomic process context. There are few ways to do that
> > - defer each call to a WQ - user visible which sucks
> > - defer from atomic or otherwise non-sleeping contextx - requires
> > reliable in_atomic AFAICS
> > - defer sleeping operations - makes the code flow more complex and it
> > would be again user visible in some cases.
> >
> > So I would say we are in "pick your own poison" kind of situation.
>
> Just to be clear:
>
> NAK on this patch and any and all ductape crap. Fix it properly, make
> hugetlb_lock, spool->lock IRQ-safe, move the workqueue into the CMA
> thing.
>
> The code really doesn't look _that_ complicated.

Fair enough. As I've said I am not a great fan of this patch either
but it is a quick fix for a likely long term problem. If reworking the
hugetlb locking is preferable then be it.
--
Michal Hocko
SUSE Labs

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