Re: [PATCH v2 1/1] powerpc/crash: Use NMI context for printk when starting to crash
From: Nicholas Piggin <npiggin@gmail.com>
Date: 2020-05-13 04:36:29
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Excerpts from Leonardo Bras's message of May 13, 2020 7:45 am:
Currently, if printk lock (logbuf_lock) is held by other thread during crash, there is a chance of deadlocking the crash on next printk, and blocking a possibly desired kdump. At the start of default_machine_crash_shutdown, make printk enter NMI context, as it will use per-cpu buffers to store the message, and avoid locking logbuf_lock.
printk_nmi_enter is used in one other place outside nmi_enter. Is there a different/better way to handle this? What do other architectures do? Other subsystems get put into an nmi-mode when we call nmi_enter (lockdep, ftrace, rcu etc). It seems like those would be useful for similar reasons, so at least explaining why that is not used in a comment would be good. Aside from that, I welcome any effort to make our crashes more reliable so thanks for working on this stuff. Thanks, Nick
quoted hunk ↗ jump to hunk
Suggested-by: Michael Ellerman <mpe@ellerman.id.au> Signed-off-by: Leonardo Bras <redacted> --- Changes since v1: - Added in-code comment explaining the need of context change - Function moved to the start of default_machine_crash_shutdown, to avoid locking any printk on crashing routine. - Title was 'Use NMI context for printk after crashing other CPUs' --- arch/powerpc/kexec/crash.c | 3 +++ 1 file changed, 3 insertions(+)diff --git a/arch/powerpc/kexec/crash.c b/arch/powerpc/kexec/crash.c index d488311efab1..c9a889880214 100644 --- a/arch/powerpc/kexec/crash.c +++ b/arch/powerpc/kexec/crash.c@@ -311,6 +311,9 @@ void default_machine_crash_shutdown(struct pt_regs *regs) unsigned int i; int (*old_handler)(struct pt_regs *regs); + /* Avoid hardlocking with irresponsive CPU holding logbuf_lock */ + printk_nmi_enter(); + /* * This function is only called after the system * has panicked or is otherwise in a critical state.-- 2.25.4