6.8-stable review patch. If anyone has any objections, please let me know.
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From: Breno Leitao leitao@debian.org
[ Upstream commit d54e56f31a34fa38fcb5e91df609f9633419a79a ]
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. By reading this counter it can be seen 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 least significant 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 least significant bit of the idt_nmi_seq is set or not, but does the opposite, and checks 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 least significant 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 Signed-off-by: Thomas Gleixner tglx@linutronix.de Reviewed-by: Paul E. McKenney paulmck@kernel.org Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20240207165237.1048837-1-leitao@debian.org Signed-off-by: Sasha Levin sashal@kernel.org --- 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 3082cf24b69e3..6da2cfa23c293 100644 --- a/arch/x86/kernel/nmi.c +++ b/arch/x86/kernel/nmi.c @@ -636,7 +636,7 @@ 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)";