On Thu, Apr 28, 2016 at 10:48:55AM +0200, Maarten Lankhorst wrote:
> Op 27-04-16 om 15:24 schreef Patrik Jakobsson:
> > On Tue, Apr 19, 2016 at 09:52:22AM +0200, Maarten Lankhorst wrote:
> >> Both intel_unpin_work.pending and intel_unpin_work.enable_stall_check
> >> were used to see if work should be enabled. By only using pending
> >> some special cases are gone, and access to unpin_work can be simplified.
> >>
> >> Use this to only access work members untilintel_mark_page_flip_active
> >> is called, or intel_queue_mmio_flip is used. This will prevent
> >> use-after-free, and makes it easier to verify accesses.
> >>
> >> Changes since v1:
> >> - Reword commit message.
> >> - Do not access unpin_work after intel_mark_page_flip_active.
> >> - Add the right memory barriers.
> >>
> >> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> >> ---
> >> drivers/gpu/drm/i915/i915_debugfs.c | 11 +++---
> >> drivers/gpu/drm/i915/intel_display.c | 71 ++++++++++++++----------------------
> >> drivers/gpu/drm/i915/intel_drv.h | 1 -
> >> 3 files changed, 34 insertions(+), 49 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
> >> index 931dc6086f3b..0092aaf47c43 100644
> >> --- a/drivers/gpu/drm/i915/i915_debugfs.c
> >> +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> >> @@ -612,9 +612,14 @@ static int i915_gem_pageflip_info(struct seq_file *m, void *data)
> >> seq_printf(m, "No flip due on pipe %c (plane %c)\n",
> >> pipe, plane);
> >> } else {
> >> + u32 pending;
> >> u32 addr;
> >>
> >> - if (atomic_read(&work->pending) < INTEL_FLIP_COMPLETE) {
> >> + pending = atomic_read(&work->pending);
> >> + if (pending == INTEL_FLIP_INACTIVE) {
> >> + seq_printf(m, "Flip ioctl preparing on pipe %c (plane %c)\n",
> >> + pipe, plane);
> >> + } else if (pending >= INTEL_FLIP_COMPLETE) {
> >> seq_printf(m, "Flip queued on pipe %c (plane %c)\n",
> >> pipe, plane);
> >> } else {
> >> @@ -636,10 +641,6 @@ static int i915_gem_pageflip_info(struct seq_file *m, void *data)
> >> work->flip_queued_vblank,
> >> work->flip_ready_vblank,
> >> drm_crtc_vblank_count(&crtc->base));
> >> - if (work->enable_stall_check)
> >> - seq_puts(m, "Stall check enabled, ");
> >> - else
> >> - seq_puts(m, "Stall check waiting for page flip ioctl, ");
> >> seq_printf(m, "%d prepares\n", atomic_read(&work->pending));
> >>
> >> if (INTEL_INFO(dev)->gen >= 4)
> >> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> >> index 4cb830e2a62e..97a8418f6539 100644
> >> --- a/drivers/gpu/drm/i915/intel_display.c
> >> +++ b/drivers/gpu/drm/i915/intel_display.c
> >> @@ -3896,8 +3896,6 @@ static void page_flip_completed(struct intel_crtc *intel_crtc)
> >> struct drm_i915_private *dev_priv = to_i915(intel_crtc->base.dev);
> >> struct intel_unpin_work *work = intel_crtc->unpin_work;
> >>
> >> - /* ensure that the unpin work is consistent wrt ->pending. */
> >> - smp_rmb();
> >> intel_crtc->unpin_work = NULL;
> >>
> >> if (work->event)
> >> @@ -10980,16 +10978,13 @@ static void do_intel_finish_page_flip(struct drm_device *dev,
> >> spin_lock_irqsave(&dev->event_lock, flags);
> >> work = intel_crtc->unpin_work;
> >>
> >> - /* Ensure we don't miss a work->pending update ... */
> >> - smp_rmb();
> >> + if (work && atomic_read(&work->pending) >= INTEL_FLIP_COMPLETE) {
> >> + /* ensure that the unpin work is consistent wrt ->pending. */
> >> + smp_mb__after_atomic();
> > The docs on smp_mb__after/before_atomic() states that they are used with atomic
> > functions that do not return a value. Why are we using it together with
> > atomic_read() here?
> From Documentation/atomic_ops.txt:
>
> *** WARNING: atomic_read() and atomic_set() DO NOT IMPLY BARRIERS! ***
>
> Plus a whole warning below how the atomic ops may be reordered. The memory
> barriers are definitely required.
Yes, the barriers are required. My point is that _after/before_atomic() should
only be used with set/clear/inc etc atomic operations. For atomic operations
that return a value you should use other macros. At least that is how I
interpret the documentation.
Here's the part from Documentation/atomic_ops.txt:
--
If a caller requires memory barrier semantics around an atomic_t
operation which does not return a value, a set of interfaces are
defined which accomplish this:
void smp_mb__before_atomic(void);
void smp_mb__after_atomic(void);
--
So I interpret this as, there's no guarantee that you'll get a full memory
barrier from these macros.
> >> static void intel_mmio_flip_work_func(struct work_struct *work)
> >> @@ -11529,15 +11517,14 @@ static bool __intel_pageflip_stall_check(struct drm_device *dev,
> >> struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
> >> struct intel_unpin_work *work = intel_crtc->unpin_work;
> >> u32 addr;
> >> + u32 pending;
> >>
> >> - if (atomic_read(&work->pending) >= INTEL_FLIP_COMPLETE)
> >> - return true;
> >> -
> >> - if (atomic_read(&work->pending) < INTEL_FLIP_PENDING)
> >> - return false;
> >> + pending = atomic_read(&work->pending);
> >> + /* ensure that the unpin work is consistent wrt ->pending. */
> >> + smp_mb__after_atomic();
> > Why paired with atomic_read()?
> See above. ^
> >>
> >> - if (!work->enable_stall_check)
> >> - return false;
> >> + if (pending != INTEL_FLIP_PENDING)
> >> + return pending == INTEL_FLIP_COMPLETE;
> > Am I correct in assuming that we can remove the enable_stall_check test here
> > since it's always enabled? If so, that would be useful to explain in the commit
> > message.
> The commit message says stallcheck special handling is removed entirely. I thought it would
> imply that the special case, where a flip may be queued but stallcheck not yet active, is removed entirely.
>
> ~Maarten
The commit message tells what the patch does but not why. This might be obvious
if you're familiar with the code. I stumbled a bit here so I guess I'm not :)
"Both intel_unpin_work.pending and intel_unpin_work.enable_stall_check were used
to see if work should be enabled"
>From this I imply that both checks are needed.
"By only using pending some special cases are gone"
This is what I don't find intuitive. Why can we suddently skip the
enable_stall_check test? It would have been useful to know about the special
case where a flip may be queued but stallcheck not yet active, and that it's no
longer valid (and possibly why).
-Patrik
--
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