[PATCH] dma-buf/sw_sync: fix lockdep anger

[Chris Wilson]

On Thu, Oct 13, 2016 at 04:04:05PM -0400, Rob Clark wrote:
> We were holding the wrong lock to be using fence_is_signaled_locked().
> And holding the child_list_lock over something that could end up calling
> fence cb's angers lockdep:
> 
> ======================================================
> [ 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
> 
> Signed-off-by: Rob Clark <robdclark at gmail.com>
> ---
> The fence_get()/_put() might be overkill.. wasn't sure if there was
> any path where the ref could get dropped while child_list_lock was
> released..
> 
>  drivers/dma-buf/sw_sync.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c
> index 62e8e6d..3bf8f5c 100644
> --- a/drivers/dma-buf/sw_sync.c
> +++ b/drivers/dma-buf/sw_sync.c
> @@ -146,8 +146,17 @@ 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))
> +		struct fence *fence = fence_get(&pt->base);
> +		bool signaled;
> +
> +		spin_unlock_irqrestore(&obj->child_list_lock, flags);

Hmm. The obj->child_list_lock and fence->lock are one and the same (
fence->lock = &obj->child_list_lock). The problem lockdep is complaining
about appears to nesting of identical lockclasses. (The current fence_cb
design allows for unbounded recursion from the signal callbacks.)
Aiui, this patch shouldn't be fixing anything as the fence_signal is
still fired from under the very same obj->child_list_lock.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre