Re: [PATCH v2 1/2] media: exynos-gsc: fix lockdep warning

[Hans Verkuil <hverkuil@xs4all.nl>]

On 10/16/2017 05:16 PM, Shuah Khan wrote:
> The driver mmap functions shouldn't take lock when calling vb2_mmap().
> Fix it to not take the lock.
> 
> Reference: commit log for f035eb4e976ef5a059e30bc91cfd310ff030a7d3
> and e752577ed7bf55c81e10343fced8b378cda2b63b
> 
> The following lockdep warning is fixed with this change.
> 
> [ 1990.972058] ======================================================
> [ 1990.978172] WARNING: possible circular locking dependency detected
> [ 1990.984327] 4.14.0-rc2-00002-gfab205f-dirty #4 Not tainted
> [ 1990.989783] ------------------------------------------------------
> [ 1990.995937] qtdemux0:sink/2765 is trying to acquire lock:
> [ 1991.001309]  (&gsc->lock){+.+.}, at: [<bf1729f0>] gsc_m2m_mmap+0x24/0x5c [exynos_gsc]
> [ 1991.009108]
>                but task is already holding lock:
> [ 1991.014913]  (&mm->mmap_sem){++++}, at: [<c01df2e4>] vm_mmap_pgoff+0x44/0xb8
> [ 1991.021932]
>                which lock already depends on the new lock.
> 
> [ 1991.030078]
>                the existing dependency chain (in reverse order) is:
> [ 1991.037530]
>                -> #1 (&mm->mmap_sem){++++}:
> [ 1991.042913]        __might_fault+0x80/0xb0
> [ 1991.047096]        video_usercopy+0x1cc/0x510 [videodev]
> [ 1991.052297]        v4l2_ioctl+0xa4/0xdc [videodev]
> [ 1991.057036]        do_vfs_ioctl+0xa0/0xa18
> [ 1991.061102]        SyS_ioctl+0x34/0x5c
> [ 1991.064834]        ret_fast_syscall+0x0/0x28
> [ 1991.069072]
>                -> #0 (&gsc->lock){+.+.}:
> [ 1991.074193]        lock_acquire+0x6c/0x88
> [ 1991.078179]        __mutex_lock+0x68/0xa34
> [ 1991.082247]        mutex_lock_interruptible_nested+0x1c/0x24
> [ 1991.087888]        gsc_m2m_mmap+0x24/0x5c [exynos_gsc]
> [ 1991.093029]        v4l2_mmap+0x54/0x88 [videodev]
> [ 1991.097673]        mmap_region+0x3a8/0x638
> [ 1991.101743]        do_mmap+0x330/0x3a4
> [ 1991.105470]        vm_mmap_pgoff+0x90/0xb8
> [ 1991.109542]        SyS_mmap_pgoff+0x90/0xc0
> [ 1991.113702]        ret_fast_syscall+0x0/0x28
> [ 1991.117945]
>                other info that might help us debug this:
> 
> [ 1991.125918]  Possible unsafe locking scenario:
> 
> [ 1991.131810]        CPU0                    CPU1
> [ 1991.136315]        ----                    ----
> [ 1991.140821]   lock(&mm->mmap_sem);
> [ 1991.144201]                                lock(&gsc->lock);
> [ 1991.149833]                                lock(&mm->mmap_sem);
> [ 1991.155725]   lock(&gsc->lock);
> [ 1991.158845]
>                 *** DEADLOCK ***
> 
> [ 1991.164740] 1 lock held by qtdemux0:sink/2765:
> [ 1991.169157]  #0:  (&mm->mmap_sem){++++}, at: [<c01df2e4>] vm_mmap_pgoff+0x44/0xb8
> [ 1991.176609]
>                stack backtrace:
> [ 1991.180946] CPU: 2 PID: 2765 Comm: qtdemux0:sink Not tainted 4.14.0-rc2-00002-gfab205f-dirty #4
> [ 1991.189608] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
> [ 1991.195686] [<c01102c8>] (unwind_backtrace) from [<c010cabc>] (show_stack+0x10/0x14)
> [ 1991.203393] [<c010cabc>] (show_stack) from [<c08543a4>] (dump_stack+0x98/0xc4)
> [ 1991.210586] [<c08543a4>] (dump_stack) from [<c016b2fc>] (print_circular_bug+0x254/0x410)
> [ 1991.218644] [<c016b2fc>] (print_circular_bug) from [<c016c580>] (check_prev_add+0x468/0x938)
> [ 1991.227049] [<c016c580>] (check_prev_add) from [<c016f4dc>] (__lock_acquire+0x1314/0x14fc)
> [ 1991.235281] [<c016f4dc>] (__lock_acquire) from [<c016fefc>] (lock_acquire+0x6c/0x88)
> [ 1991.242993] [<c016fefc>] (lock_acquire) from [<c0869fb4>] (__mutex_lock+0x68/0xa34)
> [ 1991.250620] [<c0869fb4>] (__mutex_lock) from [<c086aa08>] (mutex_lock_interruptible_nested+0x1c/0x24)
> [ 1991.259812] [<c086aa08>] (mutex_lock_interruptible_nested) from [<bf1729f0>] (gsc_m2m_mmap+0x24/0x5c [exynos_gsc])
> [ 1991.270159] [<bf1729f0>] (gsc_m2m_mmap [exynos_gsc]) from [<bf037120>] (v4l2_mmap+0x54/0x88 [videodev])
> [ 1991.279510] [<bf037120>] (v4l2_mmap [videodev]) from [<c01f4798>] (mmap_region+0x3a8/0x638)
> [ 1991.287792] [<c01f4798>] (mmap_region) from [<c01f4d58>] (do_mmap+0x330/0x3a4)
> [ 1991.294986] [<c01f4d58>] (do_mmap) from [<c01df330>] (vm_mmap_pgoff+0x90/0xb8)
> [ 1991.302178] [<c01df330>] (vm_mmap_pgoff) from [<c01f28cc>] (SyS_mmap_pgoff+0x90/0xc0)
> [ 1991.309977] [<c01f28cc>] (SyS_mmap_pgoff) from [<c0108820>] (ret_fast_syscall+0x0/0x28)
> 
> Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
> Suggested-by: Hans Verkuil <hansverk@cisco.com>
> Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>

Acked-by: Hans Verkuil <hansverk@cisco.com>

Regards,

	Hans

> ---
>  drivers/media/platform/exynos-gsc/gsc-m2m.c | 5 -----
>  1 file changed, 5 deletions(-)
> 
> diff --git a/drivers/media/platform/exynos-gsc/gsc-m2m.c b/drivers/media/platform/exynos-gsc/gsc-m2m.c
> index 2a2994e..722d7c4 100644
> --- a/drivers/media/platform/exynos-gsc/gsc-m2m.c
> +++ b/drivers/media/platform/exynos-gsc/gsc-m2m.c
> @@ -726,14 +726,9 @@ static unsigned int gsc_m2m_poll(struct file *file,
>  static int gsc_m2m_mmap(struct file *file, struct vm_area_struct *vma)
>  {
>  	struct gsc_ctx *ctx = fh_to_ctx(file->private_data);
> -	struct gsc_dev *gsc = ctx->gsc_dev;
>  	int ret;
>  
> -	if (mutex_lock_interruptible(&gsc->lock))
> -		return -ERESTARTSYS;
> -
>  	ret = v4l2_m2m_mmap(file, ctx->m2m_ctx, vma);
> -	mutex_unlock(&gsc->lock);
>  
>  	return ret;
>  }
>