[Intel-gfx] [PATCH v2] drm/i915: Fix userptr deadlock with MAP_FIXED
Tvrtko Ursulin
tvrtko.ursulin at linux.intel.com
Tue Jun 30 07:52:52 PDT 2015
On 06/29/2015 04:57 PM, Michał Winiarski wrote:
> On Mon, Jun 29, 2015 at 12:17:33PM +0100, Chris Wilson wrote:
>> Michał Winiarski found a really evil way to trigger a struct_mutex
>> deadlock with userptr. He found that if he allocated a userptr bo and
>> then GTT mmaped another bo, or even itself, at the same address as the
>> userptr using MAP_FIXED, he could then cause a deadlock any time we then
>> had to invalidate the GTT mmappings (so at will).
>>
>> To counter act the deadlock, we make the observation that when the
>> MAP_FIXED is made we would have an invalidate_range event for our
>> object. After that we should no longer alias with the rogue mmapping. If
>> we are then able to mark the object as no longer in use after the first
>> invalidate, we do not need to grab the struct_mutex for the subsequent
>> invalidations.
>>
>> The patch makes one eye-catching change. That is the removal serial=0
>> after detecting a to-be-freed object inside the invalidate walker. I
>> felt setting serial=0 was a questionable pessimisation: it denies us the
>> chance to reuse the current iterator for the next loop (before it is
>> freed) and being explicit makes the reader question the validity of the
>> locking (since the object-free race could occur elsewhere). The
>> serialisation of the iterator is through the spinlock, if the object is
>> freed before the next loop then the notifier.serial will be incremented
>> and we start the walk from the beginning as we detect the invalid cache.
>>
>> v2: Grammar fixes
>>
>> Reported-by: Michał Winiarski <michal.winiarski at intel.com>
>> Testcase: igt/gem_userptr_blits/map-fixed*
>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
>> Cc: Michał Winiarski <michal.winiarski at intel.com>
>> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>> Cc: stable at vger.kernel.org
>> ---
>> drivers/gpu/drm/i915/i915_gem_userptr.c | 43 +++++++++++++++++++++++++++------
>> 1 file changed, 36 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
>> index cb367d9f7909..e1965d8c08c8 100644
>> --- a/drivers/gpu/drm/i915/i915_gem_userptr.c
>> +++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
>> @@ -59,6 +59,7 @@ struct i915_mmu_object {
>> struct interval_tree_node it;
>> struct list_head link;
>> struct drm_i915_gem_object *obj;
>> + bool active;
>> bool is_linear;
>> };
>>
>> @@ -114,7 +115,8 @@ restart:
>>
>> obj = mo->obj;
>>
>> - if (!kref_get_unless_zero(&obj->base.refcount))
>> + if (!mo->active ||
>> + !kref_get_unless_zero(&obj->base.refcount))
>> continue;
>>
>> spin_unlock(&mn->lock);
>> @@ -151,7 +153,8 @@ static void i915_gem_userptr_mn_invalidate_range_start(struct mmu_notifier *_mn,
>> else
>> it = interval_tree_iter_first(&mn->objects, start, end);
>> if (it != NULL) {
>> - obj = container_of(it, struct i915_mmu_object, it)->obj;
>> + struct i915_mmu_object *mo =
>> + container_of(it, struct i915_mmu_object, it);
>>
>> /* The mmu_object is released late when destroying the
>> * GEM object so it is entirely possible to gain a
>> @@ -160,11 +163,9 @@ static void i915_gem_userptr_mn_invalidate_range_start(struct mmu_notifier *_mn,
>> * the struct_mutex - and consequently use it after it
>> * is freed and then double free it.
>> */
>> - if (!kref_get_unless_zero(&obj->base.refcount)) {
>> - spin_unlock(&mn->lock);
>> - serial = 0;
>> - continue;
>> - }
>> + if (mo->active &&
>> + kref_get_unless_zero(&mo->obj->base.refcount))
>> + obj = mo->obj;
>>
>> serial = mn->serial;
>> }
>> @@ -606,6 +607,20 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
>> wake_up_all(&to_i915(dev)->mm.queue);
>> }
>>
>> +static void
>> +__i915_gem_userptr_set_active(struct drm_i915_gem_object *obj,
>> + bool value)
>> +{
>> +#if defined(CONFIG_MMU_NOTIFIER)
>> + if (obj->userptr.mmu_object == NULL)
>> + return;
>> +
>> + spin_lock(&obj->userptr.mmu_object->mn->lock);
>> + obj->userptr.mmu_object->active = value;
>> + spin_unlock(&obj->userptr.mmu_object->mn->lock);
>> +#endif
>> +}
>> +
>> static int
>> i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
>> {
>> @@ -613,6 +628,18 @@ i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
>> struct page **pvec;
>> int pinned, ret;
>>
>> + /* During mm_invalidate_range we need to cancel any userptr that
>> + * overlaps the range being invalidated. Doing so requires the
>> + * struct_mutex, and that risks recursion. In order to cause
>> + * recursion, the user must alias the userptr address space with
>> + * a GTT mmapping (possible with a MAP_FIXED) - then when we have
>> + * to invalidate that mmaping, mm_invalidate_range is called with
>> + * the userptr address *and* the struct_mutex held. To prevent that
>> + * we set a flag under the i915_mmu_notifier spinlock to indicate
>> + * whether this object is valid.
>> + */
>> + __i915_gem_userptr_set_active(obj, true);
>> +
>
> This will set mmu_object to active even if we're exiting early from here
> (because of error handling), creating another possibility for deadlock.
I think this deadlock is reproducible without MAP_FIXED, so commit
message should be probably reworded to allow for the more generic case.
I reproduced it like this:
1. mmap, gem_userptr, munmap
2. Create a normal bo.
3. Loop a bit mmapping the above until it hits the same address as userptr.
4. Write to the BO mmap to set fault_mappable.
5. set_tiling on normal bo handle.
I am still thinking about this active flag in the above scenario.
userptr->get_pages hasn't been called above so active == false. If
between steps 4 and 5 we trigger get_pages, userptr transitions to
active and set_tiling deadlocks. Or I still missing something?
Regards,
Tvrtko
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