On Tue, Oct 18, 2016 at 11:27 AM, Christian König
<deathsimple@xxxxxxxxxxx> wrote:
> 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?
Possibly, although I guess keeping the lock as long as it isn't held
over callbacks would be sufficient. Either way, I guess this approach
starts by untangling the
fence_signal_locked()/fence_is_signaled_locked() paths.. that is the
part I'm less sure how to do yet.
BR,
-R
> 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;
>>>> }
>>>>
>>>
>>>
>
_______________________________________________
dri-devel mailing list
dri-devel@xxxxxxxxxxxxxxxxxxxxx
https://lists.freedesktop.org/mailman/listinfo/dri-devel
