[PATCH] drm/amdgpu: Fix blocking in RCU critical section(v2)

[zhoucm1]

one small nit pick, with that fix, Reviewed-by: Chunming Zhou 
<david1.zhou at amd.com>

On 2017年07月20日 10:46, Alex Xie wrote:
> In RCU read-side critical sections, blocking or sleeping is prohibited.
>
> v2: Unlock RCU for the code path where result==NULL. (David Zhou)
>      Update subject
>
> Tested-by and reported by: Dave Airlie <airlied at redhat.com>
>
> [  141.965723] =============================
> [  141.965724] WARNING: suspicious RCU usage
> [  141.965726] 4.12.0-rc7 #221 Not tainted
> [  141.965727] -----------------------------
> [  141.965728] /home/airlied/devel/kernel/linux-2.6/include/linux/rcupdate.h:531
> Illegal context switch in RCU read-side critical section!
> [  141.965730]
>                 other info that might help us debug this:
>
> [  141.965731]
>                 rcu_scheduler_active = 2, debug_locks = 0
> [  141.965732] 1 lock held by amdgpu_cs:0/1332:
> [  141.965733]  #0:  (rcu_read_lock){......}, at: [<ffffffffa01a0d07>]
> amdgpu_bo_list_get+0x0/0x109 [amdgpu]
> [  141.965774]
>                 stack backtrace:
> [  141.965776] CPU: 6 PID: 1332 Comm: amdgpu_cs:0 Not tainted 4.12.0-rc7 #221
> [  141.965777] Hardware name: To be filled by O.E.M. To be filled by
> O.E.M./M5A97 R2.0, BIOS 2603 06/26/2015
> [  141.965778] Call Trace:
> [  141.965782]  dump_stack+0x68/0x92
> [  141.965785]  lockdep_rcu_suspicious+0xf7/0x100
> [  141.965788]  ___might_sleep+0x56/0x1fc
> [  141.965790]  __might_sleep+0x68/0x6f
> [  141.965793]  __mutex_lock+0x4e/0x7b5
> [  141.965817]  ? amdgpu_bo_list_get+0xa4/0x109 [amdgpu]
> [  141.965820]  ? lock_acquire+0x125/0x1b9
> [  141.965844]  ? amdgpu_bo_list_set+0x464/0x464 [amdgpu]
> [  141.965846]  mutex_lock_nested+0x16/0x18
> [  141.965848]  ? mutex_lock_nested+0x16/0x18
> [  141.965872]  amdgpu_bo_list_get+0xa4/0x109 [amdgpu]
> [  141.965895]  amdgpu_cs_ioctl+0x4a0/0x17dd [amdgpu]
> [  141.965898]  ? radix_tree_node_alloc.constprop.11+0x77/0xab
> [  141.965916]  drm_ioctl+0x264/0x393 [drm]
> [  141.965939]  ? amdgpu_cs_find_mapping+0x83/0x83 [amdgpu]
> [  141.965942]  ? trace_hardirqs_on_caller+0x16a/0x186
>
> Signed-off-by: Alex Xie <AlexBin.Xie at amd.com>
> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c | 12 +++++++++---
>   1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c
> index dc87962..e5d61fe 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c
> @@ -198,12 +198,18 @@ amdgpu_bo_list_get(struct amdgpu_fpriv *fpriv, int id)
>   	result = idr_find(&fpriv->bo_list_handles, id);
>   
>   	if (result) {
> -		if (kref_get_unless_zero(&result->refcount))
> +		if (kref_get_unless_zero(&result->refcount)) {
> +			rcu_read_unlock();
>   			mutex_lock(&result->lock);
> -		else
> +		}
> +		else {
don't need another line, just like } else {
> +			rcu_read_unlock();
>   			result = NULL;
> +		}
> +	}
> +	else {
don't need another line, just like } else {
> +		rcu_read_unlock();
>   	}
> -	rcu_read_unlock();
>   
>   	return result;
>   }