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    Subject[PATCH] drm: Fix lock order reversal between mmap_sem and struct_mutex.
    Date
    The basic problem was
    mmap_sem (do_mmap()) -> struct_mutex (drm_gem_mmap(), i915_gem_fault())
    struct_mutex (i915_gem_execbuffer()) -> mmap_sem (copy_from/to_user())

    We have plenty of places where we want to hold device state the same
    (struct_mutex) while we move a non-trivial amount of data
    (copy_from/to_user()), such as i915_gem_pwrite(). Solve this by moving the
    easy things that needed struct_mutex with mmap_sem held to using a lock to
    cover just those data structures (offset hash and offset manager), and do
    trylock and reschedule in fault.

    Signed-off-by: Eric Anholt <eric@anholt.net>
    ---
    drivers/gpu/drm/drm_gem.c | 8 ++++----
    drivers/gpu/drm/i915/i915_gem.c | 15 ++++++++++++++-
    include/drm/drmP.h | 1 +
    3 files changed, 19 insertions(+), 5 deletions(-)

    diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
    index 88d3368..13a0184 100644
    --- a/drivers/gpu/drm/drm_gem.c
    +++ b/drivers/gpu/drm/drm_gem.c
    @@ -97,6 +97,7 @@ drm_gem_init(struct drm_device *dev)

    dev->mm_private = mm;

    + mutex_init(&mm->offset_mutex);
    if (drm_ht_create(&mm->offset_hash, 19)) {
    drm_free(mm, sizeof(struct drm_gem_mm), DRM_MEM_MM);
    return -ENOMEM;
    @@ -508,10 +509,9 @@ int drm_gem_mmap(struct file *filp, struct vm_area_struct *vma)
    unsigned long prot;
    int ret = 0;

    - mutex_lock(&dev->struct_mutex);
    -
    + mutex_lock(&mm->offset_mutex);
    if (drm_ht_find_item(&mm->offset_hash, vma->vm_pgoff, &hash)) {
    - mutex_unlock(&dev->struct_mutex);
    + mutex_unlock(&mm->offset_mutex);
    return drm_mmap(filp, vma);
    }

    @@ -556,7 +556,7 @@ int drm_gem_mmap(struct file *filp, struct vm_area_struct *vma)
    drm_vm_open_locked(vma);

    out_unlock:
    - mutex_unlock(&dev->struct_mutex);
    + mutex_unlock(&mm->offset_mutex);

    return ret;
    }
    diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
    index ac534c9..da9a2cb 100644
    --- a/drivers/gpu/drm/i915/i915_gem.c
    +++ b/drivers/gpu/drm/i915/i915_gem.c
    @@ -573,8 +573,16 @@ int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
    page_offset = ((unsigned long)vmf->virtual_address - vma->vm_start) >>
    PAGE_SHIFT;

    + /* Get the struct mutex before accessing GEM data structures, but
    + * keep the struct_mutex -> mmap_sem lock ordering so that we don't
    + * need to mangle pwrite/pread to allow mmap_sem -> struct_mutex.
    + */
    + if (!mutex_trylock(&dev->struct_mutex)) {
    + need_resched();
    + return VM_FAULT_NOPAGE;
    + }
    +
    /* Now bind it into the GTT if needed */
    - mutex_lock(&dev->struct_mutex);
    if (!obj_priv->gtt_space) {
    ret = i915_gem_object_bind_to_gtt(obj, obj_priv->gtt_alignment);
    if (ret) {
    @@ -646,6 +654,7 @@ i915_gem_create_mmap_offset(struct drm_gem_object *obj)
    map->size = obj->size;
    map->handle = obj;

    + mutex_lock(&mm->offset_mutex);
    /* Get a DRM GEM mmap offset allocated... */
    list->file_offset_node = drm_mm_search_free(&mm->offset_manager,
    obj->size / PAGE_SIZE, 0, 0);
    @@ -671,12 +680,14 @@ i915_gem_create_mmap_offset(struct drm_gem_object *obj)
    /* By now we should be all set, any drm_mmap request on the offset
    * below will get to our mmap & fault handler */
    obj_priv->mmap_offset = ((uint64_t) list->hash.key) << PAGE_SHIFT;
    + mutex_unlock(&mm->offset_mutex);

    return 0;

    out_free_mm:
    drm_mm_put_block(list->file_offset_node);
    out_free_list:
    + mutex_unlock(&mm->offset_mutex);
    drm_free(list->map, sizeof(struct drm_map_list), DRM_MEM_DRIVER);

    return ret;
    @@ -690,6 +701,7 @@ i915_gem_free_mmap_offset(struct drm_gem_object *obj)
    struct drm_gem_mm *mm = dev->mm_private;
    struct drm_map_list *list;

    + mutex_lock(&mm->offset_mutex);
    list = &obj->map_list;
    drm_ht_remove_item(&mm->offset_hash, &list->hash);

    @@ -704,6 +716,7 @@ i915_gem_free_mmap_offset(struct drm_gem_object *obj)
    }

    obj_priv->mmap_offset = 0;
    + mutex_unlock(&mm->offset_mutex);
    }

    /**
    diff --git a/include/drm/drmP.h b/include/drm/drmP.h
    index e5f4ae9..04f765b 100644
    --- a/include/drm/drmP.h
    +++ b/include/drm/drmP.h
    @@ -570,6 +570,7 @@ struct drm_ati_pcigart_info {
    struct drm_gem_mm {
    struct drm_mm offset_manager; /**< Offset mgmt for buffer objects */
    struct drm_open_hash offset_hash; /**< User token hash table for maps */
    + struct mutex offset_mutex; /**< covers offset_manager and offset_hash */
    };

    /**
    --
    1.5.6.5


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