x86, kexec: fix hang on i386 when panic occurs while console_sem is held

There's a corner case in 32 bit x86 kdump at the moment.  When the box
panics via nmi, we call bust_spinlocks(1) to disable sensitivity to the
console_sem (allowing us to print to the console in all cases), but we don't
call crash_kexec, until after we call bust_spinlocks(0), which re-enables
console_sem sensitivity.

The result is that, if we get an nmi while the console_sem is held and
kdump is configured, and we try to print something to the console during
kdump shutdown (which we often do) we deadlock the box.  The fix is to
simply do what 64 bit die_nmi does which is to not call bust_spinlocks(0)
until after we call crash_kexec.

Patch below tested successfully by me.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/arch/x86/kernel/dumpstack_32.c b/arch/x86/kernel/dumpstack_32.c
index 1a78180f08d39aa2a87a2b2738e7a6b488a4d5dc..b3614752197b6c6e3c0cf53c7b718dd0167fbdea 100644 (file)
--- a/arch/x86/kernel/dumpstack_32.c
+++ b/arch/x86/kernel/dumpstack_32.c
@@ -405,7 +405,6 @@ die_nmi(char *str, struct pt_regs *regs, int do_panic)
                panic("Non maskable interrupt");
        console_silent();
        spin_unlock(&nmi_print_lock);
-       bust_spinlocks(0);
 
        /*
         * If we are in kernel we are probably nested up pretty bad
@@ -416,6 +415,7 @@ die_nmi(char *str, struct pt_regs *regs, int do_panic)
                crash_kexec(regs);
        }
 
+       bust_spinlocks(0);
        do_exit(SIGSEGV);
 }