On Thu, Dec 13, 2018 at 1:36 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote: > > On Thu, Dec 13, 2018 at 11:23 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote: > > > > On Thu, Dec 13, 2018 at 10:58 AM Daniel Vetter <daniel@xxxxxxxx> wrote: > > > > > > On Thu, Dec 13, 2018 at 10:38:14AM +0100, Rafael J. Wysocki wrote: > > > > On Mon, Dec 10, 2018 at 9:47 AM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote: > > > > > > > > > > Drivers might want to remove some sysfs files, which needs the same > > > > > locks and ends up angering lockdep. Relevant snippet of the stack > > > > > trace: > > > > > > > > > > kernfs_remove_by_name_ns+0x3b/0x80 > > > > > bus_remove_driver+0x92/0xa0 > > > > > acpi_video_unregister+0x24/0x40 > > > > > i915_driver_unload+0x42/0x130 [i915] > > > > > i915_pci_remove+0x19/0x30 [i915] > > > > > pci_device_remove+0x36/0xb0 > > > > > device_release_driver_internal+0x185/0x250 > > > > > unbind_store+0xaf/0x180 > > > > > kernfs_fop_write+0x104/0x190 > > > > > > > > Is the acpi_bus_unregister_driver() in acpi_video_unregister() the > > > > source of the lockdep unhappiness? > > > > > > Yeah I guess I cut out too much of the lockdep splat. It complains about > > > kernfs_fop_write and kernfs_remove_by_name_ns acquiring the same lock > > > class. It's ofc not the same lock, so no real deadlock. Getting the > > > device_release_driver outside of the callchain under kernfs_fop_write, > > > which this patch does, "fixes" it. For "fixes" = shut up lockdep. > > > > OK, so the problem really is that the operation is started via sysfs > > which means that this code is running under a lock already. > > > > Which lock does lockdep complain about, exactly? > > mutex_lock(&of->mutex); OK (I thought so) > > > Other options: > > > - Anotate the recursion with the usual lockdep annotations. Potentially > > > results in lockdep not catching real deadlocks (you can still have other > > > loops closing the deadlock, maybe through some subsystem/bus lock). > > > > > > - Rewrite kernfs_fop_write to drop the lock (optionally, for callbacks > > > that know what they're doing), which should be fine if we refcount > > > everything properly (bus, driver & device). > > > > > > - Also note that probably the same bug exists on the bind sysfs interface, > > > but we don't use that, so I don't care :-) > > > > > > - Most of these issues are never visible in normal usage, since normally > > > driver bind/unbind is done from a kthread or model_load/unload, neither > > > of which is running in the context of that kernfs mutex kernfs_fop_write > > > holds. That's why I think the task work is the best solution, since it > > > changes the locking context of the unbind sysfs to match the locking > > > context of module unload and hotunplug. > > > > I think that using a task work here makes sense. There is a drawback, > > which is that the original sysfs write will not wait for the driver to > > actually be released before returning to user space AFAICS, but that > > probably isn't a big deal. > > This would happen with a normal work_struct, which runs on some other > thread eventually. That added asynonchrouns execution uncovered lots > of bugs in our CI (fbcon isn't solid, let's put it that way). Hence > the task work, which will be run before the syscall returns to > userspace, but outside of anything else. Was originally created to > avoid locking inversion on the final fput, where the same "must > complete before returning to userspace, but outside of any other > locking context" issue was causing trouble. I didn't realize that it would run completely before returning to user space, thanks for pointing this out. This isn't an issue then. > > Also please note that the patch changes the code flow slightly, > > because passing a non-NULL parent pointer to > > device_release_driver_internal() potentially has side effects, but > > that should not be a big deal either. > > I can do the old code exactly, but afaict the non-NULL parent just > takes care of the parent bus locking for us, instead of hand-rolling > it in the caller. But if I missed something, I can easily undo that > part. It is different if device links are present, but I'm not worried about that case honestly. :-) > > > Unfortunately that trick doesn't work for the bind sysfs file, since that way we can't thread the errno value back to userspace. > > > > Right. That is unless we wait for the operation to complete and check > > the error left behind by it. That should be doable, but somewhat > > complicated. > > For real deadlocks this doesn't fix anything, it just hides it from > lockdep. cross-release lockdep would still complain. If we want to fix > the bind side _and_ keep reporting the errno from the driver's bind > function, then we need to rework kernfs to and add a callback which > doesn't hold the mutex. Should be doable, just a pile more work. It should be possible to store the error in a variable and export that via a separate attribute for user space to inspect. That would be a significant I/F change, however. Cheers, Rafael _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
