powerpc/crash: Use NMI context for printk when starting to crash

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.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Leonardo Bras <leobras.c@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200512214533.93878-1-leobras.c@gmail.com

diff --git a/arch/powerpc/kexec/crash.c b/arch/powerpc/kexec/crash.c
index d488311efab1f18e5819c6b7454afdf987ba9b0b..c9a889880214eec54114b9f5c4d5b49c26ac7a1a 100644 (file)
--- 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.