Re: [PATCH] x86/nmi: Fix "in NMI handler" check

[Breno Leitao]

On Wed, Feb 07, 2024 at 10:44:57AM -0800, Xin Li wrote:
> On 2/7/2024 10:30 AM, Paul E. McKenney wrote:
> > On Wed, Feb 07, 2024 at 08:52:35AM -0800, Breno Leitao wrote:
> > > Commit 344da544f177 ("x86/nmi: Print reasons why backtrace NMIs are
> > > ignored") creates a super nice framework to diagnose NMIs.
> > > 
> > > Every time nmi_exc() is called, it increments a per_cpu counter
> > > (nsp->idt_nmi_seq). At its exit, it also increments the same counter.
> > > Looking at this counter, you can see how many times that function was
> > > called (dividing by 2), and, if the function is still being executed, by
> > > checking the idt_nmi_seq's last bit.
> > > 
> > > On the check side (nmi_backtrace_stall_check()), that variable is
> > > queried to check if the NMI is still being executed, but, there is a
> > > mistake in the bitwise operation. That code wants to check if the last
> > > bit of the idt_nmi_seq is set or not, but, does the opposite, and check
> > > for all the other bits, which will always be true after the first
> > > exc_nmi() executed successfully.
> > > 
> > > This appends the misleading string to the dump "(CPU currently in NMI
> > > handler function)"
> > > 
> > > Fix it by checking the last bit, and if it is set, append the string.
> > > 
> > > Fixes: 344da544f177 ("x86/nmi: Print reasons why backtrace NMIs are ignored")
> > > Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
> > 
> > If someone else is taking this:
> > 
> > Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > 
> > (I am queueing it for testing in any case.)
> 
> Does this fix need to be backported?
> 
> Commit 344da544f177 has a date on Dec 16 2022.

I would say so, if users are using this detection mechanism and want to
trust the messages being displayed by the kernel.