Re: [Intel-gfx] [PATCH v2] drm/i915: Check for rq->hwsp validity after acquiring RCU lock

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On 18/12/2020 16:07, Chris Wilson wrote:
Quoting Tvrtko Ursulin (2020-12-18 15:52:05)

On 18/12/2020 12:24, Chris Wilson wrote:
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@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v5.1+
---
   drivers/gpu/drm/i915/gt/intel_breadcrumbs.c | 11 ++----
   drivers/gpu/drm/i915/gt/intel_timeline.c    | 10 +++---
   drivers/gpu/drm/i915/i915_request.h         | 37 ++++++++++++++++++---
   3 files changed, 39 insertions(+), 19 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
index 3c62fd6daa76..f96cd7d9b419 100644
--- 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)
   {
@@ -245,7 +240,7 @@ 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,
@@ -380,7 +375,7 @@ 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)) {
               irq_signal_request(rq, b);
               return;
       }
@@ -468,7 +463,7 @@ void i915_request_cancel_breadcrumb(struct i915_request *rq)
       if (release)
               intel_context_put(ce);
- if (__request_completed(rq))
+     if (__i915_request_is_complete(rq))
               irq_signal_request(rq, b);
i915_request_put(rq);
diff --git a/drivers/gpu/drm/i915/gt/intel_timeline.c b/drivers/gpu/drm/i915/gt/intel_timeline.c
index 512afacd2bdc..a005d0165bf4 100644
--- 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 92e4320c50c4..7c4453e60323 100644
--- a/drivers/gpu/drm/i915/i915_request.h
+++ b/drivers/gpu/drm/i915/i915_request.h
@@ -440,7 +440,7 @@ 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);
   }
/**
@@ -471,11 +471,19 @@ 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;
   }
/**
@@ -488,10 +496,16 @@ 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;
   }
/**
@@ -515,12 +529,25 @@ 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)


Should rq->hwsp_seqno be marked as rcu pointer?

50:50; we're careful enough with the helper, but we don't always call it
from under a rcu read lock. Hmm, that used to be true but now the
interrupt handler is under rcu context. That might not be a huge task to
annotate now, but still a few places that know the request->hwsp is valid
since they are before request submit.

We reset the fence signaled status before re-assigning the timeline. So
if we were to query a request in the process of being allocated, do we
need to do something extra to make sure !signaled status is not seen
before the rq->hwsp_seqno is replaced? Like should the order of re-init
be inverted?

You're thinking of request reuse. The question is where do we look at
i915_request_competed outside of an explicit ref. Off the top of my head
that is only gem_busy. (Similar thought exercise for plain
i915_request_is_signaled, just less pointer dancing.) gem_busy is
protected by the seqlock, that is if the dma-resv is changed and the
fence freed, we restart the inspection.

So... I don't think we have a use where the order matters across
recycling, and so nothing to define the rules.

Okay, sounds believable.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

Regards,

Tvrtko




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