linux-kernel - Re: [PATCH 1/3] x86/mm/pat: Disable preemption around __flush_tlb

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Date:   Tue, 16 Oct 2018 23:38:59 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 1/3] x86/mm/pat: Disable preemption around
 __flush_tlb_all()

On 2018-10-16 14:25:07 [-0700], Andy Lutomirski wrote:
> > diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
> > index 51a5a69ecac9f..fe6b21f0a6631 100644
> > --- a/arch/x86/mm/pageattr.c
> > +++ b/arch/x86/mm/pageattr.c
> > @@ -2088,7 +2088,9 @@ void __kernel_map_pages(struct page *page, int numpages, int enable)
> >          * We should perform an IPI and flush all tlbs,
> >          * but that can deadlock->flush only current cpu:
> >          */
> > +       preempt_disable();
> >         __flush_tlb_all();
> > +       preempt_enable();
> >
> 
> Depending on your CPU, __flush_tlb_all() is either
> __native_flush_tlb_global() or __native_flush_tlb().  Only
> __native_flush_tlb() could have any problem with preemption, but it
> has a WARN_ON_ONCE(preemptible()); in it.  Can you try to figure out
> why that's not firing for you?

It is firing, it is the warning that was introduced in commit
decab0888e6e (as mention in the commit message; I just noticed it way
later because it popped early in the boot log).

> I suspect that a better fix would be to put preempt_disable() into
> __native_flulsh_tlb(), but I'd still like to understand why the
> warning isn't working.

__native_flulsh_tlb() just had its preempt_disable() removed in
decab0888e6e and __kernel_map_pages() is only called from the debug
code. The other caller of __native_flulsh_tlb() seem to hold a lock or
run with disabled interrupts.

Sebastian