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@xxxxxxxxxxxxxx > > > 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
