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Message-Id: <1235035145.4612.38.camel@laptop>
Date: Thu, 19 Feb 2009 10:19:05 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: krh@...planet.net
Cc: eric@...olt.net, Wang Chen <wangchen@...fujitsu.com>,
dri-devel@...ts.sf.net, linux-kernel@...r.kernel.org,
Kristian Høgsberg <krh@...hat.com>,
Nick Piggin <npiggin@...e.de>,
Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hugh@...itas.com>
Subject: Re: [PATCH] drm: Take mmap_sem up front to avoid lock order
violations.
On Wed, 2009-02-18 at 11:38 -0500, krh@...planet.net wrote:
> From: Kristian Høgsberg <krh@...hat.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@...hat.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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