[PATCH 6/7] smp: Don't yell about IRQs disabled in kgdb_roundup_cpus()

Douglas Anderson
2018-10-29T18:08:38Z

In kgdb_roundup_cpus() we've got code that looks like:
local_irq_enable();
smp_call_function(kgdb_call_nmi_hook, NULL, 0);
local_irq_disable();

In certain cases when we drop into kgdb (like with sysrq-g on a serial
console) we'll get a big yell that looks like:

sysrq: SysRq : DEBUG
------------[ cut here ]------------
DEBUG_LOCKS_WARN_ON(current->hardirq_context)
WARNING: CPU: 0 PID: 0 at .../kernel/locking/lockdep.c:2875 lockdep_hardirqs_on+0xf0/0x160
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.19.0 #27
pstate: 604003c9 (nZCv DAIF +PAN -UAO)
pc : lockdep_hardirqs_on+0xf0/0x160
...
Call trace:
lockdep_hardirqs_on+0xf0/0x160
trace_hardirqs_on+0x188/0x1ac
kgdb_roundup_cpus+0x14/0x3c
kgdb_cpu_enter+0x53c/0x5cc
kgdb_handle_exception+0x180/0x1d4
kgdb_compiled_brk_fn+0x30/0x3c
brk_handler+0x134/0x178
do_debug_exception+0xfc/0x178
el1_dbg+0x18/0x78
kgdb_breakpoint+0x34/0x58
sysrq_handle_dbg+0x54/0x5c
__handle_sysrq+0x114/0x21c
handle_sysrq+0x30/0x3c
qcom_geni_serial_isr+0x2dc/0x30c
...
...
irq event stamp: ...45
hardirqs last enabled at (...44): [...] __do_softirq+0xd8/0x4e4
hardirqs last disabled at (...45): [...] el1_irq+0x74/0x130
softirqs last enabled at (...42): [...] _local_bh_enable+0x2c/0x34
softirqs last disabled at (...43): [...] irq_exit+0xa8/0x100
---[ end trace adf21f830c46e638 ]---

Let's add kgdb to the list of reasons not to warn in
smp_call_function_many(). That will allow us (in a future patch) to
stop calling local_irq_enable() which will get rid of the original
splat.

NOTE: with this change comes the obvious question: will we start
deadlocking more often now when we drop into the debugger. I can't
say that for sure one way or the other, but the fact that we do the
same logic for "oops_in_progress" makes me feel like it shouldn't
happen too often. Also note that the old logic of turning on
interrupts temporarily wasn't exactly safe since (I presume) that
could have violated spin_lock_irqsave() semantics and ended up with a
deadlock of its own.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

kernel/smp.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index 163c451af42e..bb581e58c8dc 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -19,6 +19,7 @@
#include <linux/sched.h>
#include <linux/sched/idle.h>
#include <linux/hypervisor.h>
+#include <linux/kgdb.h>

#include "smpboot.h"

@@ -413,7 +414,8 @@ void smp_call_function_many(const struct cpumask *mask,
* can't happen.
*/
WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
- && !oops_in_progress && !early_boot_irqs_disabled);
+ && !oops_in_progress && !early_boot_irqs_disabled
+ && !in_dbg_master());

/* Try to fastpath. So, what's a CPU they want? Ignoring this one. */
cpu = cpumask_first_and(mask, cpu_online_mask);
--
2.19.1.568.g152ad8e336-goog