linux-kernel - Re: [PATCH v2] clk: Evict unregistered clks from parent caches

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Message-Id: <20190917172424.AAF5C214AF@mail.kernel.org>
Date:   Tue, 17 Sep 2019 10:24:23 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
Subject: Re: [PATCH v2] clk: Evict unregistered clks from parent caches

Quoting Stephen Boyd (2019-08-28 11:19:59)
> 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()")
> Reviewed-by: Bjorn Andersson <bjorn.andersson@...aro.org>
> Tested-by: Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
> Signed-off-by: Stephen Boyd <sboyd@...nel.org>
> ---

Applied to clk-next