On 13.06.2017 18:07, Thierry Reding wrote: > On Tue, Jun 13, 2017 at 05:00:28PM +0300, Dmitry Osipenko wrote: >> On 13.06.2017 16:43, Thierry Reding wrote: >>> On Tue, May 23, 2017 at 03:14:22AM +0300, Dmitry Osipenko wrote: >>>> The framebuffers console fbcon_startup() increments the tegra_drm module >>>> 'use' refcount via try_module_get(), causing an interlock of the DRM subsys >>>> and the tegra_drm modules. In result, the tegra_drm module can't be unloaded >>>> using rmmod. >>>> >>>> Signed-off-by: Dmitry Osipenko <digetx@xxxxxxxxx> >>>> --- >>>> drivers/gpu/drm/tegra/fb.c | 1 - >>>> 1 file changed, 1 deletion(-) >>> >>> That's done on purpose because otherwise you could just rip out the >>> driver from under the framebuffer emulation and things would crash. >>> >>> My understanding is that the right way to unload a module is to unbind >>> the driver first (which will cause the framebuffer to be removed and >>> hence the reference to be dropped) before the rmmod. >>> >>> Thierry >>> >> >> Aha, interesting. I'll try the unbinding and will drop this patch from the >> series, thank you for the clarification. I haven't observed any crashes on a >> module reloading (framebuffer detached/attached just fine), maybe I was lucky then. > > It's possible that it works by accident. The driver removes the > framebuffer as part of the driver removal process, so technically > nothing should crash. However, if, for any reason, anyone was holding > on to a reference to the framebuffer (not sure if that is even possible) > the module needs to stay around long enough as well, otherwise the > function pointers would become dangling. The module reference makes sure > that this doesn't happen (as long as a framebuffer exists, the ops will > stick around). > > So even if there isn't a way to make it crash today, the code is still > correct in taking the reference. Thank you very much for a more detailed explanation. I've successfully unloaded the tegra_drm module after unbinding the VT's, the patch is dropped from the series. echo 0 | tee /sys/devices/virtual/vtconsole/vtcon*/bind -- Dmitry -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
