On Sat, Oct 02, 2021 at 07:28:02PM +0200, Fernando Ramos wrote: > On 21/10/02 09:13AM, Fernando Ramos wrote: > > On 21/10/02 05:30AM, Ville Syrjälä wrote: > > > On Sat, Oct 02, 2021 at 01:05:47AM +0300, Ville Syrjälä wrote: > > > > On Fri, Oct 01, 2021 at 04:48:15PM -0400, Sean Paul wrote: > > > > > On Fri, Oct 01, 2021 at 10:00:50PM +0300, Ville Syrjälä wrote: > > > > > > On Fri, Oct 01, 2021 at 02:36:55PM -0400, Sean Paul wrote: > > > > > > > > > > > > > > Thank you for revising, Fernando! I've pushed the set to drm-misc-next (along > > > > > > > with the necessary drm-tip conflict resolutions). > > > > > > > > > > > > Ugh. Did anyone actually review the locking changes this does? > > > > > > I shot the previous i915 stuff down because the commit messages > > > > > > did not address any of it. > > > > > > > > > > I reviewed the set on 9/17, I didn't see your feedback on that thread. > > > > > > > > It was much earlir than that. > > > > https://lists.freedesktop.org/archives/dri-devel/2021-June/313193.html > > Sorry, I'm new to this and it did not occur to me to search for similar patches > in the mailing list archives in case there were additional comments that applied > to my change set. > > In case I had done that I would have found that, as you mentioned, you had > already raised two issues back in June: > > On Tue, Jun 29, 2021, Ville Syrjälä wrote: > > > > That looks wrong. You're using a private ctx here, but still > > passing dev->mode_config.acquire_ctx to the lower level stuff. > > > > Also DRM_MODESET_LOCK_ALL_{BEGIN,END}() do not seem to be > > equivalent to drm_modeset_{lock,unlock}_all() when it comes to > > mode_config.mutex. So would need a proper review whether we > > actually need that lock or not. > > The first one was pointing out the same error I would later repeat in my patch > series (ups). > > After further inspection of the code it looks to me that changing this: > > intel_modeset_setup_hw_state(dev, dev->mode_config.acquire_ctx); > > ...into this: > > intel_modeset_setup_hw_state(dev, &ctx); > > ...would be enough. Yes. > > Why? The only difference between the old drm_modeset_{lock,unlock}_all() > functions and the new DRM_MODESET_LOCK_ALL_{BEGIN,END}() macros is that the > former use a global context stored in dev->mode_config.acquire_ctx while the > latter depend on a user provided one (typically in the stack). > > In the old (working) code the global context structure is freed in > drm_modeset_unlock_all() thus we are sure no one is holding a reference to it at > that point. This means that as long as no one accesses the global > dev->mode_config.acquire_ctx context in the block that runs between lock/BEGIN > and unlock/END, the code should be equivalent before and after my changes. > > In fact, now that my patch series removes the drm_modeset_{lock,unlock}_all() > functions, the acquire_ctx field of the drm_mode_config structure should be > deleted: > > /** > * @acquire_ctx: > * > * Global implicit acquire context used by atomic drivers for legacy > * IOCTLs. Deprecated, since implicit locking contexts make it > * impossible to use driver-private &struct drm_modeset_lock. Users of > * this must hold @mutex. > */ > struct drm_modeset_acquire_ctx *acquire_ctx; > > If I had done that (ie. removing this field) I would have detected the problem > when compiling. > > There is another place (in the amdgpu driver) where this field is still being > referenced, but before I investigate that I would like to know if you agree that > this is a good path to follow. Yeah, removing the mode_config.acquire_ctx is a good idea if it's no longer needed. > > Regarding the second issue you raised... > > > Also DRM_MODESET_LOCK_ALL_{BEGIN,END}() do not seem to be > > equivalent to drm_modeset_{lock,unlock}_all() when it comes to > > mode_config.mutex. So would need a proper review whether we > > actually need that lock or not. > > ...the only difference regarding mode_config.mutex I see is that in the new > macros the mutex is locked only under this condition: > > if (!drm_drv_uses_atomic_modeset(dev)) > > ...which seems reasonable, right? Is this what you were referring to or is it > something else? In order to eliminate the lock one first has to determine what that lock might be protecting here, and then prove that such protection is not actually needed. -- Ville Syrjälä Intel
