Re: [RFC] dma-buf/fence: avoid holding lock while calling cb

[Christian König <deathsimple@xxxxxxxxxxx>]

Am 18.10.2016 um 16:23 schrieb Rob Clark:
> On Tue, Oct 18, 2016 at 6:07 AM, Christian König
> <deathsimple@xxxxxxxxxxx> wrote:
> > Am 16.10.2016 um 18:03 schrieb Rob Clark:
> > > Currently with fence-array, we have a potential deadlock situation.  If we
> > > fence_add_callback() on an array-fence, the array-fence's lock is aquired
> > > first, and in it's ->enable_signaling() callback, it will install cb's on
> > > it's array-member fences, so the array-member's lock is acquired second.
> > >
> > > But in the signal path, the array-member's lock is acquired first, and the
> > > array-fence's lock acquired second.
> > >
> > > One approach to deal with this is avoid holding the fence's lock when
> > > calling the cb.  It is a bit intrusive and I haven't fixed up all the
> > > other drivers that call directly or indirectly fence_signal_locked().
> >
> > In general: Oh! Yes! Please! We have the same issue in the AMD scheduler
> > when we want to register a new callback on the next fence in the list.
> >
> > > I guess the other option would be introduce a work-queue for array-fence?
> >
> > That's what we do in the GPU scheduler and it adds quite a bunch of extra
> > overhead.
> >
> > So my preferences are clearly to fix calling the cb with any locks held
> > before using another work item for the array fences. But I'm not sure if
> > that is possible with all drivers.
> I guess it is probably not 100% possible to ensure driver isn't
> holding other of it's own locks when cb is called.. so I guess wq for
> cb that needs to take other locks is a safe solution.
>
> That said, and maybe I need more coffee, but I think the spinlock for
> iterating cb_list is probably not needed.. I think we could arrange
> things so the test_and_set(SIGNALED) is enough to protect iterating
> the list and calling the cb's.  (Maybe protect the
> test_and_set(SIGNALED) w/ fence->lock just so it doesn't race someone
> who already got past the test_bit(SIGNALED) in _add_callback()?)
>
> Then "all" we have to do is figure out how to kill the
> fence_signal_locked()/fence_is_signaled_locked() paths..
>
> I think I also wouldn't mind pushing the locking down into the
> ->enable_signaling() cb too for drivers that need that.  Maybe we
> don't strictly need that.  From quick look, seems like half the
> drivers just 'return true;' and seems a bit silly to grab a lock for
> that.
>
> Maybe wq in fence-array in the short term at least is the best way
> just to get things working without such an invasive change.  But seems
> like if we could kill the current _locked() paths that there is
> potential to make the fence->lock situation less annoying.

Maybe a heretic question, but do we really need the lock after all ?

I mean the only use case for a double linked list I can see is removing 
the callbacks in the case of a timed out wait and that should be a 
rather rare operation.

So wouldn't a single linked list with atomic swaps do as well?

Christian.

> BR,
> -R
>
> > Regards,
> > Christian.
> >
> >
> > > Or??
> > >
> > > lockdep splat:
> > >
> > >    ======================================================
> > > [ INFO: possible circular locking dependency detected ]
> > > 4.7.0-rc7+ #489 Not tainted
> > > -------------------------------------------------------
> > > surfaceflinger/2034 is trying to acquire lock:
> > >    (&(&array->lock)->rlock){......}, at: [<ffff00000858cddc>]
> > > fence_signal+0x5c/0xf8
> > >
> > > but task is already holding lock:
> > >    (&(&obj->child_list_lock)->rlock){......}, at: [<ffff0000085902f8>]
> > > sw_sync_ioctl+0x228/0x3b0
> > >
> > > which lock already depends on the new lock.
> > >
> > > the existing dependency chain (in reverse order) is:
> > >
> > > -> #1 (&(&obj->child_list_lock)->rlock){......}:
> > >          [<ffff000008108924>] __lock_acquire+0x173c/0x18d8
> > >          [<ffff000008108e0c>] lock_acquire+0x4c/0x68
> > >          [<ffff000008ac6a6c>] _raw_spin_lock_irqsave+0x54/0x70
> > >          [<ffff00000858d05c>] fence_add_callback+0x3c/0x100
> > >          [<ffff00000858f100>] fence_array_enable_signaling+0x80/0x170
> > >          [<ffff00000858d0d8>] fence_add_callback+0xb8/0x100
> > >          [<ffff00000858f504>] sync_file_poll+0xd4/0xf0
> > >          [<ffff0000081fd3a0>] do_sys_poll+0x220/0x438
> > >          [<ffff0000081fd8d0>] SyS_ppoll+0x1b0/0x1d8
> > >          [<ffff000008084f30>] el0_svc_naked+0x24/0x28
> > >
> > > -> #0 (&(&array->lock)->rlock){......}:
> > >          [<ffff000008104768>] print_circular_bug+0x80/0x2e0
> > >          [<ffff0000081089ac>] __lock_acquire+0x17c4/0x18d8
> > >          [<ffff000008108e0c>] lock_acquire+0x4c/0x68
> > >          [<ffff000008ac6a6c>] _raw_spin_lock_irqsave+0x54/0x70
> > >          [<ffff00000858cddc>] fence_signal+0x5c/0xf8
> > >          [<ffff00000858f268>] fence_array_cb_func+0x78/0x88
> > >          [<ffff00000858cb28>] fence_signal_locked+0x80/0xe0
> > >          [<ffff0000085903c8>] sw_sync_ioctl+0x2f8/0x3b0
> > >          [<ffff0000081faf6c>] do_vfs_ioctl+0xa4/0x790
> > >          [<ffff0000081fb6e4>] SyS_ioctl+0x8c/0xa0
> > >          [<ffff000008084f30>] el0_svc_naked+0x24/0x28
> > >
> > > other info that might help us debug this:
> > >
> > >    Possible unsafe locking scenario:
> > >
> > >          CPU0                    CPU1
> > >          ----                    ----
> > >     lock(&(&obj->child_list_lock)->rlock);
> > >                                  lock(&(&array->lock)->rlock);
> > >                                  lock(&(&obj->child_list_lock)->rlock);
> > >     lock(&(&array->lock)->rlock);
> > >
> > >    *** DEADLOCK ***
> > >
> > > 1 lock held by surfaceflinger/2034:
> > >    #0:  (&(&obj->child_list_lock)->rlock){......}, at: [<ffff0000085902f8>]
> > > sw_sync_ioctl+0x228/0x3b0
> > > ---
> > >    drivers/dma-buf/fence.c      | 22 ++++++++++++++--------
> > >    drivers/dma-buf/sw_sync.c    |  2 +-
> > >    drivers/dma-buf/sync_debug.c | 16 +++++++++-------
> > >    include/linux/fence.h        |  6 +++---
> > >    4 files changed, 27 insertions(+), 19 deletions(-)
> > >
> > > diff --git a/drivers/dma-buf/fence.c b/drivers/dma-buf/fence.c
> > > index cc05ddd..917f905 100644
> > > --- a/drivers/dma-buf/fence.c
> > > +++ b/drivers/dma-buf/fence.c
> > > @@ -63,9 +63,9 @@ EXPORT_SYMBOL(fence_context_alloc);
> > >     *
> > >     * Unlike fence_signal, this function must be called with fence->lock
> > > held.
> > >     */
> > > -int fence_signal_locked(struct fence *fence)
> > > +int fence_signal_locked(struct fence *fence, unsigned long flags)
> > >    {
> > > -       struct fence_cb *cur, *tmp;
> > > +       struct fence_cb *cur;
> > >          int ret = 0;
> > >          lockdep_assert_held(fence->lock);
> > > @@ -88,9 +88,12 @@ int fence_signal_locked(struct fence *fence)
> > >          } else
> > >                  trace_fence_signaled(fence);
> > >    -     list_for_each_entry_safe(cur, tmp, &fence->cb_list, node) {
> > > +       while (!list_empty(&fence->cb_list)) {
> > > +               cur = list_first_entry(&fence->cb_list, struct fence_cb,
> > > node);
> > >                  list_del_init(&cur->node);
> > > +               spin_unlock_irqrestore(fence->lock, flags);
> > >                  cur->func(fence, cur);
> > > +               spin_lock_irqsave(fence->lock, flags);
> > >          }
> > >          return ret;
> > >    }
> > > @@ -124,12 +127,15 @@ int fence_signal(struct fence *fence)
> > >          trace_fence_signaled(fence);
> > >          if (test_bit(FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags)) {
> > > -               struct fence_cb *cur, *tmp;
> > > +               struct fence_cb *cur;
> > >                  spin_lock_irqsave(fence->lock, flags);
> > > -               list_for_each_entry_safe(cur, tmp, &fence->cb_list, node)
> > > {
> > > +               while (!list_empty(&fence->cb_list)) {
> > > +                       cur = list_first_entry(&fence->cb_list, struct
> > > fence_cb, node);
> > >                          list_del_init(&cur->node);
> > > +                       spin_unlock_irqrestore(fence->lock, flags);
> > >                          cur->func(fence, cur);
> > > +                       spin_lock_irqsave(fence->lock, flags);
> > >                  }
> > >                  spin_unlock_irqrestore(fence->lock, flags);
> > >          }
> > > @@ -211,7 +217,7 @@ void fence_enable_sw_signaling(struct fence *fence)
> > >                  spin_lock_irqsave(fence->lock, flags);
> > >                  if (!fence->ops->enable_signaling(fence))
> > > -                       fence_signal_locked(fence);
> > > +                       fence_signal_locked(fence, flags);
> > >                  spin_unlock_irqrestore(fence->lock, flags);
> > >          }
> > > @@ -266,7 +272,7 @@ int fence_add_callback(struct fence *fence, struct
> > > fence_cb *cb,
> > >                  trace_fence_enable_signal(fence);
> > >                  if (!fence->ops->enable_signaling(fence)) {
> > > -                       fence_signal_locked(fence);
> > > +                       fence_signal_locked(fence, flags);
> > >                          ret = -ENOENT;
> > >                  }
> > >          }
> > > @@ -366,7 +372,7 @@ fence_default_wait(struct fence *fence, bool intr,
> > > signed long timeout)
> > >                  trace_fence_enable_signal(fence);
> > >                  if (!fence->ops->enable_signaling(fence)) {
> > > -                       fence_signal_locked(fence);
> > > +                       fence_signal_locked(fence, flags);
> > >                          goto out;
> > >                  }
> > >          }
> > > diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c
> > > index 62e8e6d..2271f7f 100644
> > > --- a/drivers/dma-buf/sw_sync.c
> > > +++ b/drivers/dma-buf/sw_sync.c
> > > @@ -146,7 +146,7 @@ static void sync_timeline_signal(struct sync_timeline
> > > *obj, unsigned int inc)
> > >          list_for_each_entry_safe(pt, next, &obj->active_list_head,
> > >                                   active_list) {
> > > -               if (fence_is_signaled_locked(&pt->base))
> > > +               if (fence_is_signaled_locked(&pt->base, flags))
> > >                          list_del_init(&pt->active_list);
> > >          }
> > >    diff --git a/drivers/dma-buf/sync_debug.c b/drivers/dma-buf/sync_debug.c
> > > index 2dd4c3d..a7556d3 100644
> > > --- a/drivers/dma-buf/sync_debug.c
> > > +++ b/drivers/dma-buf/sync_debug.c
> > > @@ -71,12 +71,13 @@ static const char *sync_status_str(int status)
> > >          return "error";
> > >    }
> > >    -static void sync_print_fence(struct seq_file *s, struct fence *fence,
> > > bool show)
> > > +static void sync_print_fence(struct seq_file *s, struct fence *fence,
> > > +               bool show, unsigned long flags)
> > >    {
> > >          int status = 1;
> > >          struct sync_timeline *parent = fence_parent(fence);
> > >    -     if (fence_is_signaled_locked(fence))
> > > +       if (fence_is_signaled_locked(fence, flags))
> > >                  status = fence->status;
> > >          seq_printf(s, "  %s%sfence %s",
> > > @@ -124,13 +125,14 @@ static void sync_print_obj(struct seq_file *s,
> > > struct sync_timeline *obj)
> > >          list_for_each(pos, &obj->child_list_head) {
> > >                  struct sync_pt *pt =
> > >                          container_of(pos, struct sync_pt, child_list);
> > > -               sync_print_fence(s, &pt->base, false);
> > > +               sync_print_fence(s, &pt->base, false, flags);
> > >          }
> > >          spin_unlock_irqrestore(&obj->child_list_lock, flags);
> > >    }
> > >      static void sync_print_sync_file(struct seq_file *s,
> > > -                                 struct sync_file *sync_file)
> > > +                                 struct sync_file *sync_file,
> > > +                                 unsigned long flags)
> > >    {
> > >          int i;
> > >    @@ -141,9 +143,9 @@ static void sync_print_sync_file(struct seq_file *s,
> > >                  struct fence_array *array =
> > > to_fence_array(sync_file->fence);
> > >                  for (i = 0; i < array->num_fences; ++i)
> > > -                       sync_print_fence(s, array->fences[i], true);
> > > +                       sync_print_fence(s, array->fences[i], true,
> > > flags);
> > >          } else {
> > > -               sync_print_fence(s, sync_file->fence, true);
> > > +               sync_print_fence(s, sync_file->fence, true, flags);
> > >          }
> > >    }
> > >    @@ -172,7 +174,7 @@ static int sync_debugfs_show(struct seq_file *s,
> > > void *unused)
> > >                  struct sync_file *sync_file =
> > >                          container_of(pos, struct sync_file,
> > > sync_file_list);
> > >    -             sync_print_sync_file(s, sync_file);
> > > +               sync_print_sync_file(s, sync_file, flags);
> > >                  seq_puts(s, "\n");
> > >          }
> > >          spin_unlock_irqrestore(&sync_file_list_lock, flags);
> > > diff --git a/include/linux/fence.h b/include/linux/fence.h
> > > index 0d76305..073f380 100644
> > > --- a/include/linux/fence.h
> > > +++ b/include/linux/fence.h
> > > @@ -220,7 +220,7 @@ static inline void fence_put(struct fence *fence)
> > >    }
> > >      int fence_signal(struct fence *fence);
> > > -int fence_signal_locked(struct fence *fence);
> > > +int fence_signal_locked(struct fence *fence, unsigned long flags);
> > >    signed long fence_default_wait(struct fence *fence, bool intr, signed
> > > long timeout);
> > >    int fence_add_callback(struct fence *fence, struct fence_cb *cb,
> > >                         fence_func_t func);
> > > @@ -239,13 +239,13 @@ void fence_enable_sw_signaling(struct fence *fence);
> > >     * This function requires fence->lock to be held.
> > >     */
> > >    static inline bool
> > > -fence_is_signaled_locked(struct fence *fence)
> > > +fence_is_signaled_locked(struct fence *fence, unsigned long flags)
> > >    {
> > >          if (test_bit(FENCE_FLAG_SIGNALED_BIT, &fence->flags))
> > >                  return true;
> > >          if (fence->ops->signaled && fence->ops->signaled(fence)) {
> > > -               fence_signal_locked(fence);
> > > +               fence_signal_locked(fence, flags);
> > >                  return true;
> > >          }

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