Re: [PATCH] clk: fix spin_lock/unlock imbalance on bad clk_enable() reentrancy

[David Lechner]

On 12/12/2017 05:43 PM, David Lechner wrote:
> If clk_enable() is called in reentrant way and spin_trylock_irqsave() is
> not working as expected, it is possible to get a negative enable_refcnt
> which results in a missed call to spin_unlock_irqrestore().
>
> It works like this:
>
> 1. clk_enable() is called.
> 2. clk_enable_unlock() calls spin_trylock_irqsave() and sets
>     enable_refcnt = 1.
> 3. Another clk_enable() is called before the first has returned
>     (reentrant), but somehow spin_trylock_irqsave() is returning true.
>     (I'm not sure how/why this is happening yet, but it is happening to me
>     with arch/arm/mach-davinci clocks that I am working on).

I think I have figured out that since CONFIG_SMP=n and 
CONFIG_DEBUG_SPINLOCK=n on my kernel that

#define arch_spin_trylock(lock)({ barrier(); (void)(lock); 1; })

in include/linux/spinlock_up.h is causing the problem.

So, basically, reentrancy of clk_enable() is broken for non-SMP systems, 
but I'm not sure I know how to fix it.


> 4. Because spin_trylock_irqsave() returned true, enable_lock has been
>     locked twice without being unlocked and enable_refcnt = 1 is called
>     instead of enable_refcnt++.
> 5. After the inner clock is enabled clk_enable_unlock() is called which
>     decrements enable_refnct to 0 and calls spin_unlock_irqrestore()
> 6. The inner clk_enable() function returns.
> 7. clk_enable_unlock() is called again for the outer clock. enable_refcnt
>     is decremented to -1 and spin_unlock_irqrestore() is *not* called.
> 8. The outer clk_enable() function returns.
> 9. Unrelated code called later issues a BUG warning about sleeping in an
>     atomic context because of the unbalanced calls for the spin lock.
>
> This patch fixes the problem of unbalanced calls by calling
> spin_unlock_irqrestore() if enable_refnct <= 0 instead of just checking if
> it is == 0.
>
> The BUG warning about sleeping in an atomic context in the unrelated code
> is eliminated with this patch, but there are still warnings printed from
> clk_enable_unlock() and clk_enable_unlock() because of the reference
> counting problems.
>
> Signed-off-by: David Lechner <david@xxxxxxxxxxxxxx>
> ---
>   drivers/clk/clk.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index 647d056..bb1b1f9 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -162,7 +162,7 @@ static void clk_enable_unlock(unsigned long flags)
>   	WARN_ON_ONCE(enable_owner != current);
>   	WARN_ON_ONCE(enable_refcnt == 0);
>   
> -	if (--enable_refcnt) {
> +	if (--enable_refcnt > 0) {
>   		__release(enable_lock);
>   		return;
>   	}