> Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=12491 > Subject : i915 lockdep warning > Submitter : Brandeburg, Jesse <jesse.brandeburg@xxxxxxxxx> > Date : 2009-01-13 23:17 (23 days old) > References : http://marc.info/?l=linux-kernel&m=123188898423532&w=4 Looking at the code, it seems that the issue is that the DRM struct_mutex must be taken inside mmap_sem (because struct_mutex is taken in drm_vm_open(), which is called with mmap_sem already held), but i915_gem_execbuffer() does a copy_to_user() while holding struct_mutex, and if this copy faults, then the VM tries to acquire mmap_sem -- ie lockdep identifies correctly a potential AB/BA deadlock. I don't pretend to fully understand the DRM or GEM, but a possible fix is below -- would be worth it to test and review, and get into 2.6.29 if it is a correct fix: --- i915: Fix potential AB-BA deadlock in i915_gem_execbuffer() Lockdep warns that i915_gem_execbuffer() can trigger a page fault (which takes mmap_sem) while holding dev->struct_mutex, while drm_vm_open() (which is called with mmap_sem already held) takes dev->struct_mutex. So this is a potential AB-BA deadlock. The way that i915_gem_execbuffer() triggers a page fault is by doing copy_to_user() when returning new buffer offsets back to userspace; however there is no reason to hold the struct_mutex when doing this copy, since what is being copied is a private array anyway. So we can fix the potential deadlock (and get rid of the lockdep warning) by simply moving the copy_to_user() outside of where struct_mutex is held. This fixes <http://bugzilla.kernel.org/show_bug.cgi?id=12491>. Reported-by: Jesse Brandeburg <jesse.brandeburg@xxxxxxxxx> Signed-off-by: Roland Dreier <rolandd@xxxxxxxxx> --- drivers/gpu/drm/i915/i915_gem.c | 21 ++++++++++++--------- 1 files changed, 12 insertions(+), 9 deletions(-) diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c index debad5c..23aad8c 100644 --- a/drivers/gpu/drm/i915/i915_gem.c +++ b/drivers/gpu/drm/i915/i915_gem.c @@ -2610,15 +2610,6 @@ i915_gem_execbuffer(struct drm_device *dev, void *data, i915_verify_inactive(dev, __FILE__, __LINE__); - /* Copy the new buffer offsets back to the user's exec list. */ - ret = copy_to_user((struct drm_i915_relocation_entry __user *) - (uintptr_t) args->buffers_ptr, - exec_list, - sizeof(*exec_list) * args->buffer_count); - if (ret) - DRM_ERROR("failed to copy %d exec entries " - "back to user (%d)\n", - args->buffer_count, ret); err: for (i = 0; i < pinned; i++) i915_gem_object_unpin(object_list[i]); @@ -2628,6 +2619,18 @@ err: mutex_unlock(&dev->struct_mutex); + if (!ret) { + /* Copy the new buffer offsets back to the user's exec list. */ + ret = copy_to_user((struct drm_i915_relocation_entry __user *) + (uintptr_t) args->buffers_ptr, + exec_list, + sizeof(*exec_list) * args->buffer_count); + if (ret) + DRM_ERROR("failed to copy %d exec entries " + "back to user (%d)\n", + args->buffer_count, ret); + } + pre_mutex_err: drm_free(object_list, sizeof(*object_list) * args->buffer_count, DRM_MEM_DRIVER); -- To unsubscribe from this list: send the line "unsubscribe kernel-testers" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
