Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 7 Feb 2024 10:44:57 -0800 | Subject | Re: [PATCH] x86/nmi: Fix "in NMI handler" check | From | Xin Li <> |
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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@debian.org> > > If someone else is taking this: > > Reviewed-by: Paul E. McKenney <paulmck@kernel.org> > > (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.
> >> --- >> arch/x86/kernel/nmi.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c >> index 17e955ab69fe..6e738ad474dc 100644 >> --- a/arch/x86/kernel/nmi.c >> +++ b/arch/x86/kernel/nmi.c >> @@ -639,7 +639,7 @@ void nmi_backtrace_stall_check(const struct cpumask *btp) >> msgp = nmi_check_stall_msg[idx]; >> if (nsp->idt_ignored_snap != READ_ONCE(nsp->idt_ignored) && (idx & 0x1)) >> modp = ", but OK because ignore_nmis was set"; >> - if (nmi_seq & ~0x1) >> + if (nmi_seq & 0x1) >> msghp = " (CPU currently in NMI handler function)"; >> else if (nsp->idt_nmi_seq_snap + 1 == nmi_seq) >> msghp = " (CPU exited one NMI handler function)"; >> -- >> 2.39.3 >> >
-- Thanks! Xin
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