From mboxrd@z Thu Jan 1 00:00:00 1970 From: Roland Dreier Subject: Re: [Bug #12491] i915 lockdep warning Date: Wed, 04 Feb 2009 14:37:34 -0800 Message-ID: References: Mime-Version: 1.0 Return-path: In-Reply-To: (Rafael J. Wysocki's message of "Wed, 4 Feb 2009 11:23:57 +0100 (CET)") DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; l=3252; t=1233787057; x=1234651057; c=relaxed/simple; s=sjdkim3002; h=Content-Type:From:Subject:Content-Transfer-Encoding:MIME-Version; d=cisco.com; i=rdreier-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org; z=From:=20Roland=20Dreier=20 |Subject:=20Re=3A=20[Bug=20#12491]=20i915=20lockdep=20warni ng |Sender:=20; bh=V1XMRgs7SmXvep69Utqp1soACu/eFaYEm2PKlNYvRhY=; b=Yv9UmGzc66qCEQF5ceCRYiW2Sbk5ySz08YDNKci3vzIrY4p3WoGcxH4yVi VjI3rSDUniZ8tW/h7/qjUqxU8066oD1RjA/qq6zWvtlLe7zdPYrNQXm3rT9j GpggOqFUQS; Sender: kernel-testers-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: "Rafael J. Wysocki" Cc: Linux Kernel Mailing List , Kernel Testers List , "Brandeburg, Jesse" , drivers_video-dri-ztI5WcYan/vQLgFONoPN62D2FQJk+8+b@public.gmane.org > Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=12491 > Subject : i915 lockdep warning > Submitter : Brandeburg, Jesse > 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 . Reported-by: Jesse Brandeburg Signed-off-by: Roland Dreier --- 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);