[PATCH 2/4] drm/shmem: Use mutex_trylock in drm_gem_shmem_purge

[Steven Price]

On 19/08/2019 17:12, Rob Herring wrote:
> Lockdep reports a circular locking dependency with pages_lock taken in
> the shrinker callback. The deadlock can't actually happen with current
> users at least as a BO will never be purgeable when pages_lock is held.
> To be safe, let's use mutex_trylock() instead and bail if a BO is locked
> already.
> 
> WARNING: possible circular locking dependency detected
> 5.3.0-rc1+ #100 Tainted: G             L
> ------------------------------------------------------
> kswapd0/171 is trying to acquire lock:
> 000000009b9823fd (&shmem->pages_lock){+.+.}, at: drm_gem_shmem_purge+0x20/0x40
> 
> but task is already holding lock:
> 00000000f82369b6 (fs_reclaim){+.+.}, at: __fs_reclaim_acquire+0x0/0x40
> 
> which lock already depends on the new lock.
> 
> the existing dependency chain (in reverse order) is:
> 
> -> #1 (fs_reclaim){+.+.}:
>        fs_reclaim_acquire.part.18+0x34/0x40
>        fs_reclaim_acquire+0x20/0x28
>        __kmalloc_node+0x6c/0x4c0
>        kvmalloc_node+0x38/0xa8
>        drm_gem_get_pages+0x80/0x1d0
>        drm_gem_shmem_get_pages+0x58/0xa0
>        drm_gem_shmem_get_pages_sgt+0x48/0xd0
>        panfrost_mmu_map+0x38/0xf8 [panfrost]
>        panfrost_gem_open+0xc0/0xe8 [panfrost]
>        drm_gem_handle_create_tail+0xe8/0x198
>        drm_gem_handle_create+0x3c/0x50
>        panfrost_gem_create_with_handle+0x70/0xa0 [panfrost]
>        panfrost_ioctl_create_bo+0x48/0x80 [panfrost]
>        drm_ioctl_kernel+0xb8/0x110
>        drm_ioctl+0x244/0x3f0
>        do_vfs_ioctl+0xbc/0x910
>        ksys_ioctl+0x78/0xa8
>        __arm64_sys_ioctl+0x1c/0x28
>        el0_svc_common.constprop.0+0x90/0x168
>        el0_svc_handler+0x28/0x78
>        el0_svc+0x8/0xc
> 
> -> #0 (&shmem->pages_lock){+.+.}:
>        __lock_acquire+0xa2c/0x1d70
>        lock_acquire+0xdc/0x228
>        __mutex_lock+0x8c/0x800
>        mutex_lock_nested+0x1c/0x28
>        drm_gem_shmem_purge+0x20/0x40
>        panfrost_gem_shrinker_scan+0xc0/0x180 [panfrost]
>        do_shrink_slab+0x208/0x500
>        shrink_slab+0x10c/0x2c0
>        shrink_node+0x28c/0x4d8
>        balance_pgdat+0x2c8/0x570
>        kswapd+0x22c/0x638
>        kthread+0x128/0x130
>        ret_from_fork+0x10/0x18
> 
> other info that might help us debug this:
> 
>  Possible unsafe locking scenario:
> 
>        CPU0                    CPU1
>        ----                    ----
>   lock(fs_reclaim);
>                                lock(&shmem->pages_lock);
>                                lock(fs_reclaim);
>   lock(&shmem->pages_lock);
> 
>  *** DEADLOCK ***
> 
> 3 locks held by kswapd0/171:
>  #0: 00000000f82369b6 (fs_reclaim){+.+.}, at: __fs_reclaim_acquire+0x0/0x40
>  #1: 00000000ceb37808 (shrinker_rwsem){++++}, at: shrink_slab+0xbc/0x2c0
>  #2: 00000000f31efa81 (&pfdev->shrinker_lock){+.+.}, at: panfrost_gem_shrinker_scan+0x34/0x180 [panfrost]
> 
> Fixes: 17acb9f35ed7 ("drm/shmem: Add madvise state and purge helpers")
> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Cc: Maxime Ripard <maxime.ripard at bootlin.com>
> Cc: Sean Paul <sean at poorly.run>
> Cc: David Airlie <airlied at linux.ie>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Signed-off-by: Rob Herring <robh at kernel.org>

Seems reasonable, like you state I don't think this can actually happen,
but keeping lockdep happy is a good idea.

Reviewed-by: Steven Price <steven.price at arm.com>

Steve

> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 7 +++++--
>  include/drm/drm_gem_shmem_helper.h     | 2 +-
>  2 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 5423ec56b535..f5918707672f 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -415,13 +415,16 @@ void drm_gem_shmem_purge_locked(struct drm_gem_object *obj)
>  }
>  EXPORT_SYMBOL(drm_gem_shmem_purge_locked);
>  
> -void drm_gem_shmem_purge(struct drm_gem_object *obj)
> +bool drm_gem_shmem_purge(struct drm_gem_object *obj)
>  {
>  	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
>  
> -	mutex_lock(&shmem->pages_lock);
> +	if (!mutex_trylock(&shmem->pages_lock))
> +		return false;
>  	drm_gem_shmem_purge_locked(obj);
>  	mutex_unlock(&shmem->pages_lock);
> +
> +	return true;
>  }
>  EXPORT_SYMBOL(drm_gem_shmem_purge);
>  
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index ce1600fdfc3e..01f514521687 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -134,7 +134,7 @@ static inline bool drm_gem_shmem_is_purgeable(struct drm_gem_shmem_object *shmem
>  }
>  
>  void drm_gem_shmem_purge_locked(struct drm_gem_object *obj);
> -void drm_gem_shmem_purge(struct drm_gem_object *obj);
> +bool drm_gem_shmem_purge(struct drm_gem_object *obj);
>  
>  struct drm_gem_shmem_object *
>  drm_gem_shmem_create_with_handle(struct drm_file *file_priv,
>