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

Daniel Thompson
2018-10-30T09:42:07Z

On Mon, Oct 29, 2018 at 11:07:06AM -0700, Douglas Anderson wrote:
> 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.

This is part of the code to bring all the cores to a halt and since
the other cores are still running kgdb isn't yet able to use the fact
all the CPUs are halted to bend the rules. It is better for this code
to play by the rules if it can.

Is is possible to get the roundup functions to use a private csd
alongside smp_call_function_single_async()? We could add a helper
function to the debug core to avoid having to add cpu_online loops into
every kgdb_roundup_cpus() implementaton.

Daniel.

>
> 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
>