Messages in this thread | | | Subject | Re: [PATCH] drm: Take mmap_sem up front to avoid lock order violations. | From | Peter Zijlstra <> | Date | Thu, 19 Feb 2009 10:19:05 +0100 |
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On Wed, 2009-02-18 at 11:38 -0500, krh@bitplanet.net wrote: > From: Kristian Høgsberg <krh@redhat.com> > > A number of GEM operations (and legacy drm ones) want to copy data to > or from userspace while holding the struct_mutex lock. However, the > fault handler calls us with the mmap_sem held and thus enforces the > opposite locking order. This patch downs the mmap_sem up front for > those operations that access userspace data under the struct_mutex > lock to ensure the locking order is consistent. > > Signed-off-by: Kristian Høgsberg <krh@redhat.com> > --- > > Here's a different and simpler attempt to fix the locking order > problem. We can just down_read() the mmap_sem pre-emptively up-front, > and the locking order is respected. It's simpler than the > mutex_trylock() game, avoids introducing a new mutex. >
Hell no!
for one, mmap_sem is not a recursive lock, so a pagefault will utterly fail with this in place.
Secondly, holding mmap_sem for no good reason just sucks.
> drivers/gpu/drm/i915/i915_dma.c | 6 +++++- > drivers/gpu/drm/i915/i915_gem.c | 20 +++++++++++++------- > 2 files changed, 18 insertions(+), 8 deletions(-) > > diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c > index 81f1cff..d8b58d9 100644 > --- a/drivers/gpu/drm/i915/i915_dma.c > +++ b/drivers/gpu/drm/i915/i915_dma.c > @@ -642,9 +642,11 @@ static int i915_batchbuffer(struct drm_device *dev, void *data, > sizeof(struct drm_clip_rect))) > return -EFAULT; > > + down_read(¤t->mm->mmap_sem); > mutex_lock(&dev->struct_mutex); > ret = i915_dispatch_batchbuffer(dev, batch); > mutex_unlock(&dev->struct_mutex); > + up_read(¤t->mm->mmap_sem); > > if (sarea_priv) > sarea_priv->last_dispatch = READ_BREADCRUMB(dev_priv); > @@ -674,14 +676,16 @@ static int i915_cmdbuffer(struct drm_device *dev, void *data, > return -EFAULT; > } > > + down_read(¤t->mm->mmap_sem); > mutex_lock(&dev->struct_mutex); > ret = i915_dispatch_cmdbuffer(dev, cmdbuf); > mutex_unlock(&dev->struct_mutex); > + up_read(¤t->mm->mmap_sem); > + > if (ret) { > DRM_ERROR("i915_dispatch_cmdbuffer failed\n"); > return ret; > } > - > if (sarea_priv) > sarea_priv->last_dispatch = READ_BREADCRUMB(dev_priv); > return 0; > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c > index d9cd42f..3dd8b6e 100644 > --- a/drivers/gpu/drm/i915/i915_gem.c > +++ b/drivers/gpu/drm/i915/i915_gem.c > @@ -171,6 +171,7 @@ i915_gem_pread_ioctl(struct drm_device *dev, void *data, > return -EINVAL; > } > > + down_read(¤t->mm->mmap_sem); > mutex_lock(&dev->struct_mutex); > > ret = i915_gem_object_set_cpu_read_domain_range(obj, args->offset, > @@ -196,6 +197,7 @@ i915_gem_pread_ioctl(struct drm_device *dev, void *data, > > drm_gem_object_unreference(obj); > mutex_unlock(&dev->struct_mutex); > + up_read(¤t->mm->mmap_sem); > > return 0; > } > @@ -264,7 +266,7 @@ i915_gem_gtt_pwrite(struct drm_device *dev, struct drm_gem_object *obj, > if (!access_ok(VERIFY_READ, user_data, remain)) > return -EFAULT; > > - > + down_read(¤t->mm->mmap_sem); > mutex_lock(&dev->struct_mutex); > ret = i915_gem_object_pin(obj, 0); > if (ret) { > @@ -315,6 +317,7 @@ i915_gem_gtt_pwrite(struct drm_device *dev, struct drm_gem_object *obj, > fail: > i915_gem_object_unpin(obj); > mutex_unlock(&dev->struct_mutex); > + up_read(¤t->mm->mmap_sem); > > return ret; > } > @@ -328,6 +331,7 @@ i915_gem_shmem_pwrite(struct drm_device *dev, struct drm_gem_object *obj, > loff_t offset; > ssize_t written; > > + down_read(¤t->mm->mmap_sem); > mutex_lock(&dev->struct_mutex); > > ret = i915_gem_object_set_to_cpu_domain(obj, 1); > @@ -350,6 +354,7 @@ i915_gem_shmem_pwrite(struct drm_device *dev, struct drm_gem_object *obj, > } > > mutex_unlock(&dev->struct_mutex); > + up_read(¤t->mm->mmap_sem); > > return 0; > } > @@ -2473,22 +2478,21 @@ i915_gem_execbuffer(struct drm_device *dev, void *data, > goto pre_mutex_err; > } > > + down_read(¤t->mm->mmap_sem); > mutex_lock(&dev->struct_mutex); > > i915_verify_inactive(dev, __FILE__, __LINE__); > > if (dev_priv->mm.wedged) { > DRM_ERROR("Execbuf while wedged\n"); > - mutex_unlock(&dev->struct_mutex); > ret = -EIO; > - goto pre_mutex_err; > + goto mutex_err; > } > > if (dev_priv->mm.suspended) { > DRM_ERROR("Execbuf while VT-switched.\n"); > - mutex_unlock(&dev->struct_mutex); > ret = -EBUSY; > - goto pre_mutex_err; > + goto mutex_err; > } > > /* Look up object handles */ > @@ -2641,8 +2645,6 @@ err: > for (i = 0; i < args->buffer_count; i++) > drm_gem_object_unreference(object_list[i]); > > - 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 *) > @@ -2655,6 +2657,10 @@ err: > args->buffer_count, ret); > } > > +mutex_err: > + mutex_unlock(&dev->struct_mutex); > + up_read(¤t->mm->mmap_sem); > + > pre_mutex_err: > drm_free(object_list, sizeof(*object_list) * args->buffer_count, > DRM_MEM_DRIVER);
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