On Mon, Nov 26, 2018 at 07:51:31PM -0800, Douglas Anderson wrote:
> When I had lockdep turned on and dropped into kgdb I got a nice splat
> on my system. Specifically it hit:
> DEBUG_LOCKS_WARN_ON(current->hardirq_context)
>
> Specifically it looked like this:
> 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 ]---
>
> Looking closely at it, it seems like a really bad idea to be calling
> local_irq_enable() in kgdb_roundup_cpus(). If nothing else that seems
> like it could violate spinlock semantics and cause a deadlock.
>
> Instead, let's use a private csd alongside
> smp_call_function_single_async() to round up the other CPUs. Using
> smp_call_function_single_async() doesn't require interrupts to be
> enabled so we can remove the offending bit of code.
>
> In order to avoid duplicating this across all the architectures that
> use the default kgdb_roundup_cpus(), we'll add a "weak" implementation
> to debug_core.c.
>
> Looking at all the people who previously had copies of this code,
> there were a few variants. I've attempted to keep the variants
> working like they used to. Specifically:
> * For arch/arc we passed NULL to kgdb_nmicallback() instead of
> get_irq_regs().
> * For arch/mips there was a bit of extra code around
> kgdb_nmicallback()
>
> NOTE: In this patch we will still get into trouble if we try to round
> up a CPU that failed to round up before. We'll try to round it up
> again and potentially hang when we try to grab the csd lock. That's
> not new behavior but we'll still try to do better in a future patch.
>
> Suggested-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
>
> Changes in v5:
> - Add a comment about get_irq_regs().
> - get_cpu() => raw_smp_processor_id() in kgdb_roundup_cpus().
> - for_each_cpu() => for_each_online_cpu()
> - Error check smp_call_function_single_async()
For the arm64 bit:
> diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
> index 12c339ff6e75..da880247c734 100644
> --- a/arch/arm64/kernel/kgdb.c
> +++ b/arch/arm64/kernel/kgdb.c
> @@ -284,18 +284,6 @@ static struct step_hook kgdb_step_hook = {
> .fn = kgdb_step_brk_fn
> };
>
> -static void kgdb_call_nmi_hook(void *ignored)
> -{
> - kgdb_nmicallback(raw_smp_processor_id(), get_irq_regs());
> -}
> -
> -void kgdb_roundup_cpus(void)
> -{
> - local_irq_enable();
> - smp_call_function(kgdb_call_nmi_hook, NULL, 0);
> - local_irq_disable();
> -}
> -
Acked-by: Will Deacon <will.deacon@xxxxxxx>
Will
