[Intel-gfx] [PATCH] drm: Mark up racy check of drm_gem_object.handle_count

[Daniel Vetter]

On Mon, Mar 09, 2020 at 12:01:51PM +0000, Chris Wilson wrote:
> [ 1715.899800] BUG: KCSAN: data-race in drm_gem_handle_create_tail / drm_gem_object_handle_put_unlocked
> [ 1715.899838]
> [ 1715.899861] write to 0xffff8881830f3604 of 4 bytes by task 7834 on cpu 1:
> [ 1715.899896]  drm_gem_handle_create_tail+0x62/0x250
> [ 1715.899927]  drm_gem_open_ioctl+0xc1/0x160
> [ 1715.899956]  drm_ioctl_kernel+0xe4/0x120
> [ 1715.899981]  drm_ioctl+0x297/0x4c7
> [ 1715.900003]  ksys_ioctl+0x89/0xb0
> [ 1715.900027]  __x64_sys_ioctl+0x42/0x60
> [ 1715.900052]  do_syscall_64+0x6e/0x2c0
> [ 1715.900079]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [ 1715.900100]
> [ 1715.900119] read to 0xffff8881830f3604 of 4 bytes by task 8137 on cpu 0:
> [ 1715.900149]  drm_gem_object_handle_put_unlocked+0x31/0x130
> [ 1715.900180]  drm_gem_object_release_handle+0x93/0xe0
> [ 1715.900208]  drm_gem_handle_delete+0x7b/0xe0
> [ 1715.900235]  drm_gem_close_ioctl+0x61/0x80
> [ 1715.900264]  drm_ioctl_kernel+0xe4/0x120
> [ 1715.900291]  drm_ioctl+0x297/0x4c7
> [ 1715.900316]  ksys_ioctl+0x89/0xb0
> [ 1715.900340]  __x64_sys_ioctl+0x42/0x60
> [ 1715.900363]  do_syscall_64+0x6e/0x2c0
> [ 1715.900388]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

I'm impressed. The dream that we all get replaced by some scripts might be
real :-)

> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

> ---
>  drivers/gpu/drm/drm_gem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index a9e4a610445a..37627d06fb06 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -218,7 +218,7 @@ drm_gem_object_handle_put_unlocked(struct drm_gem_object *obj)
>  	struct drm_device *dev = obj->dev;
>  	bool final = false;
>  
> -	if (WARN_ON(obj->handle_count == 0))
> +	if (WARN_ON(READ_ONCE(obj->handle_count) == 0))
>  		return;
>  
>  	/*
> -- 
> 2.20.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch