Re: [PATCH] clk: Evict unregistered clks from parent caches

[Bjorn Andersson <bjorn.andersson@linaro.org>]

On Mon 26 Aug 16:43 PDT 2019, Stephen Boyd wrote:

> We leave a dangling pointer in each clk_core::parents array that has an
> unregistered clk as a potential parent when that clk_core pointer is
> freed by clk{_hw}_unregister(). It is impossible for the true parent of
> a clk to be set with clk_set_parent() once the dangling pointer is left
> in the cache because we compare parent pointers in
> clk_fetch_parent_index() instead of checking for a matching clk name or
> clk_hw pointer.
> 
> Before commit ede77858473a ("clk: Remove global clk traversal on fetch
> parent index"), we would check clk_hw pointers, which has a higher
> chance of being the same between registration and unregistration, but it
> can still be allocated and freed by the clk provider. In fact, this has
> been a long standing problem since commit da0f0b2c3ad2 ("clk: Correct
> lookup logic in clk_fetch_parent_index()") where we stopped trying to
> compare clk names and skipped over entries in the cache that weren't
> NULL.
> 
> There are good (performance) reasons to not do the global tree lookup in
> cases where the cache holds dangling pointers to parents that have been
> unregistered. Let's take the performance hit on the uncommon
> registration path instead. Loop through all the clk_core::parents arrays
> when a clk is unregistered and set the entry to NULL when the parent
> cache entry and clk being unregistered are the same pointer. This will
> fix this problem and avoid the overhead for the "normal" case.
> 
> Based on a patch by Bjorn Andersson.
> 
> Fixes: da0f0b2c3ad2 ("clk: Correct lookup logic in clk_fetch_parent_index()")
> Cc: Bjorn Andersson <bjorn.andersson@linaro.org>

Thanks for writing this up.

Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>

> Cc: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
> Signed-off-by: Stephen Boyd <sboyd@kernel.org>
> ---
>  drivers/clk/clk.c | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
> 
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index c0990703ce54..f3982bfa39d6 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -3737,6 +3737,34 @@ static const struct clk_ops clk_nodrv_ops = {
>  	.set_parent	= clk_nodrv_set_parent,
>  };
>  
> +static void clk_core_evict_parent_cache_subtree(struct clk_core *root,
> +						struct clk_core *target)
> +{
> +	int i;
> +	struct clk_core *child;
> +
> +	for (i = 0; i < root->num_parents; i++)
> +		if (root->parents[i].core == target)
> +			root->parents[i].core = NULL;
> +
> +	hlist_for_each_entry(child, &root->children, child_node)
> +		clk_core_evict_parent_cache_subtree(child, target);
> +}
> +
> +/* Remove this clk from all parent caches */
> +static void clk_core_evict_parent_cache(struct clk_core *core)
> +{
> +	struct hlist_head **lists;
> +	struct clk_core *root;
> +
> +	lockdep_assert_held(&prepare_lock);
> +
> +	for (lists = all_lists; *lists; lists++)
> +		hlist_for_each_entry(root, *lists, child_node)
> +			clk_core_evict_parent_cache_subtree(root, core);
> +
> +}
> +
>  /**
>   * clk_unregister - unregister a currently registered clock
>   * @clk: clock to unregister
> @@ -3775,6 +3803,8 @@ void clk_unregister(struct clk *clk)
>  			clk_core_set_parent_nolock(child, NULL);
>  	}
>  
> +	clk_core_evict_parent_cache(clk->core);
> +
>  	hlist_del_init(&clk->core->child_node);
>  
>  	if (clk->core->prepare_count)
> -- 
> Sent by a computer through tubes
>