drm/i915: Check for rq->hwsp validity after acquiring RCU lock

Since we allow removing the timeline map at runtime, there is a risk
that rq->hwsp points into a stale page. To control that risk, we hold
the RCU read lock while reading *rq->hwsp, but we missed a couple of
important barriers. First, the unpinning / removal of the timeline map
must be after all RCU readers into that map are complete, i.e. after an
rcu barrier (in this case courtesy of call_rcu()). Secondly, we must
make sure that the rq->hwsp we are about to dereference under the RCU
lock is valid. In this case, we make the rq->hwsp pointer safe during
i915_request_retire() and so we know that rq->hwsp may become invalid
only after the request has been signaled. Therefore is the request is
not yet signaled when we acquire rq->hwsp under the RCU, we know that
rq->hwsp will remain valid for the duration of the RCU read lock.

This is a very small window that may lead to either considering the
request not completed (causing a delay until the request is checked
again, any wait for the request is not affected) or dereferencing an
invalid pointer.

Fixes: 3adac4689f58 ("drm/i915: Introduce concept of per-timeline (context) HWSP")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: <stable@vger.kernel.org> # v5.1+
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201218122421.18344-1-chris@chris-wilson.co.uk
(cherry picked from commit 9bb36cf66091ddf2d8840e5aa705ad3c93a6279b)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210118101755.476744-1-chris@chris-wilson.co.uk

diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
index a24cc1f..0625cbb 100644 (file)
--- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
@@ -134,11 +134,6 @@ static bool remove_signaling_context(struct intel_breadcrumbs *b,
        return true;
 }
 
-static inline bool __request_completed(const struct i915_request *rq)
-{
-       return i915_seqno_passed(__hwsp_seqno(rq), rq->fence.seqno);
-}
-
 __maybe_unused static bool
 check_signal_order(struct intel_context *ce, struct i915_request *rq)
 {
@@ -257,7 +252,7 @@ static void signal_irq_work(struct irq_work *work)
                list_for_each_entry_rcu(rq, &ce->signals, signal_link) {
                        bool release;
 
-                       if (!__request_completed(rq))
+                       if (!__i915_request_is_complete(rq))
                                break;
 
                        if (!test_and_clear_bit(I915_FENCE_FLAG_SIGNAL,
@@ -379,7 +374,7 @@ static void insert_breadcrumb(struct i915_request *rq)
         * straight onto a signaled list, and queue the irq worker for
         * its signal completion.
         */
-       if (__request_completed(rq)) {
+       if (__i915_request_is_complete(rq)) {
                if (__signal_request(rq) &&
                    llist_add(&rq->signal_node, &b->signaled_requests))
                        irq_work_queue(&b->irq_work);

diff --git a/drivers/gpu/drm/i915/gt/intel_timeline.c b/drivers/gpu/drm/i915/gt/intel_timeline.c
index 7ea94d2..8015964 100644 (file)
--- a/drivers/gpu/drm/i915/gt/intel_timeline.c
+++ b/drivers/gpu/drm/i915/gt/intel_timeline.c
@@ -126,6 +126,10 @@ static void __rcu_cacheline_free(struct rcu_head *rcu)
        struct intel_timeline_cacheline *cl =
                container_of(rcu, typeof(*cl), rcu);
 
+       /* Must wait until after all *rq->hwsp are complete before removing */
+       i915_gem_object_unpin_map(cl->hwsp->vma->obj);
+       __idle_hwsp_free(cl->hwsp, ptr_unmask_bits(cl->vaddr, CACHELINE_BITS));
+
        i915_active_fini(&cl->active);
        kfree(cl);
 }
@@ -133,11 +137,6 @@ static void __rcu_cacheline_free(struct rcu_head *rcu)
 static void __idle_cacheline_free(struct intel_timeline_cacheline *cl)
 {
        GEM_BUG_ON(!i915_active_is_idle(&cl->active));
-
-       i915_gem_object_unpin_map(cl->hwsp->vma->obj);
-       i915_vma_put(cl->hwsp->vma);
-       __idle_hwsp_free(cl->hwsp, ptr_unmask_bits(cl->vaddr, CACHELINE_BITS));
-
        call_rcu(&cl->rcu, __rcu_cacheline_free);
 }
 
@@ -179,7 +178,6 @@ cacheline_alloc(struct intel_timeline_hwsp *hwsp, unsigned int cacheline)
                return ERR_CAST(vaddr);
        }
 
-       i915_vma_get(hwsp->vma);
        cl->hwsp = hwsp;
        cl->vaddr = page_pack_bits(vaddr, cacheline);
 

diff --git a/drivers/gpu/drm/i915/i915_request.h b/drivers/gpu/drm/i915/i915_request.h
index 620b6fa..92adfee 100644 (file)
--- a/drivers/gpu/drm/i915/i915_request.h
+++ b/drivers/gpu/drm/i915/i915_request.h
@@ -434,7 +434,7 @@ static inline u32 hwsp_seqno(const struct i915_request *rq)
 
 static inline bool __i915_request_has_started(const struct i915_request *rq)
 {
-       return i915_seqno_passed(hwsp_seqno(rq), rq->fence.seqno - 1);
+       return i915_seqno_passed(__hwsp_seqno(rq), rq->fence.seqno - 1);
 }
 
 /**
@@ -465,11 +465,19 @@ static inline bool __i915_request_has_started(const struct i915_request *rq)
  */
 static inline bool i915_request_started(const struct i915_request *rq)
 {
+       bool result;
+
        if (i915_request_signaled(rq))
                return true;
 
-       /* Remember: started but may have since been preempted! */
-       return __i915_request_has_started(rq);
+       result = true;
+       rcu_read_lock(); /* the HWSP may be freed at runtime */
+       if (likely(!i915_request_signaled(rq)))
+               /* Remember: started but may have since been preempted! */
+               result = __i915_request_has_started(rq);
+       rcu_read_unlock();
+
+       return result;
 }
 
 /**
@@ -482,10 +490,16 @@ static inline bool i915_request_started(const struct i915_request *rq)
  */
 static inline bool i915_request_is_running(const struct i915_request *rq)
 {
+       bool result;
+
        if (!i915_request_is_active(rq))
                return false;
 
-       return __i915_request_has_started(rq);
+       rcu_read_lock();
+       result = __i915_request_has_started(rq) && i915_request_is_active(rq);
+       rcu_read_unlock();
+
+       return result;
 }
 
 /**
@@ -509,12 +523,25 @@ static inline bool i915_request_is_ready(const struct i915_request *rq)
        return !list_empty(&rq->sched.link);
 }
 
+static inline bool __i915_request_is_complete(const struct i915_request *rq)
+{
+       return i915_seqno_passed(__hwsp_seqno(rq), rq->fence.seqno);
+}
+
 static inline bool i915_request_completed(const struct i915_request *rq)
 {
+       bool result;
+
        if (i915_request_signaled(rq))
                return true;
 
-       return i915_seqno_passed(hwsp_seqno(rq), rq->fence.seqno);
+       result = true;
+       rcu_read_lock(); /* the HWSP may be freed at runtime */
+       if (likely(!i915_request_signaled(rq)))
+               result = __i915_request_is_complete(rq);
+       rcu_read_unlock();
+
+       return result;
 }
 
 static inline void i915_request_mark_complete(struct i915_request *rq)