Re: [PATCH 4/4] fbdev: Debug knob to register without holding console_lock

Rob Clark
2015-08-25T19:24:08Z

On Tue, Aug 25, 2015 at 9:45 AM, Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> When the usual fbcon legacy options are enabled we have
> ->register_framebuffer
> ->fb notifier chain calls into fbcon
> ->fbcon sets up console on new fbi
> ->fbi->set_par
> ->drm_fb_helper_set_par exercises full kms api
>
> And because of locking inversion hilarity all of register_framebuffer
> is done with the console lock held. Which means that the first time on
> driver load we exercise _all_ the kms code (all probe paths and
> modeset paths for everything connected) is under the console lock.
> That means if anything goes belly-up in that big pile of code nothing
> ever reaches logfiles (and the machine is dead).
>
> Usual tactic to debug that is to temporarily remove those console_lock
> calls to be able to capture backtraces. I'm fed up writing this patch
> and recompiling kernels. Hence this patch here to add an unsafe,
> kernel-taining option to do this at runtime.
>
> Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
> Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
> Cc: linux-fbdev@vger.kernel.org
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

This one was causing me some problems, if I tried to enable
lockless_register_fb. It *looks* like it should work, so I'm not
quite sure what the deal is. But I'm 110% fan of getting something
like this working, because console_lock is pretty much the bane of kms
developer's existence..

I'll have to debug further on a system where I can see more than the
bottom three lines of the second to last backtrace..

BR,
-R

> ---
> drivers/video/fbdev/core/fbmem.c | 14 +++++++++++---
> 1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 0705d8883ede..4e73b6f6b1c0 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1608,6 +1608,11 @@ static int do_remove_conflicting_framebuffers(struct apertures_struct *a,
> return 0;
> }
>
> +static bool lockless_register_fb;
> +module_param_named_unsafe(lockless_register_fb, lockless_register_fb, bool, 0400);
> +MODULE_PARM_DESC(lockless_register_fb,
> + "Lockless framebuffer registration for debugging [default=off]");
> +
> static int do_register_framebuffer(struct fb_info *fb_info)
> {
> int i, ret;
> @@ -1675,15 +1680,18 @@ static int do_register_framebuffer(struct fb_info *fb_info)
> registered_fb[i] = fb_info;
>
> event.info = fb_info;
> - console_lock();
> + if (!lockless_register_fb)
> + console_lock();
> if (!lock_fb_info(fb_info)) {
> - console_unlock();
> + if (!lockless_register_fb)
> + console_unlock();
> return -ENODEV;
> }
>
> fb_notifier_call_chain(FB_EVENT_FB_REGISTERED, &event);
> unlock_fb_info(fb_info);
> - console_unlock();
> + if (!lockless_register_fb)
> + console_unlock();
> return 0;
> }
>
> --
> 1.8.3.1
>
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