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.
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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