linux-kernel - Re: [PATCH] clk: fix spin_lock/unlock imbalance on bad clk

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Date:   Wed, 20 Dec 2017 15:50:41 -0600
From:   David Lechner <david@...hnology.com>
To:     Michael Turquette <mturquette@...libre.com>,
        linux-clk@...r.kernel.org
Cc:     Stephen Boyd <sboyd@...eaurora.org>, linux-kernel@...r.kernel.org,
        Jerome Brunet <jbrunet@...libre.com>
Subject: Re: [PATCH] clk: fix spin_lock/unlock imbalance on bad clk_enable()
 reentrancy

On 12/20/2017 02:33 PM, David Lechner wrote:
> 
> So, as you can see, we get 4 warnings here. There is no problem with any 
> clock provider or consumer (as far as I can tell). The bug here is that 
> spin_trylock_irqsave() always returns true on non-SMP systems, which 
> messes up the reference counting.
> 
> usb20_phy_clk_enable() currently works because mach-davinci does not use 
> the common clock framework. However, I am trying to move it to the 
> common clock framework, which is how I discovered this bug.

One more thing I mentioned previously, but is worth mentioning again in 
detail is that if you enable CONFIG_DEBUG_SPINLOCK, that changes the 
behavior of spin_trylock_irqsave() on non-SMP systems. It no longer 
always returns true and so everything works as expected in the call 
chain that I described previously.

The difference is that with CONFIG_DEBUG_SPINLOCK=n, we have

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

But if CONFIG_DEBUG_SPINLOCK=y, then we have

static inline int arch_spin_trylock(arch_spinlock_t *lock)
{
	char oldval = lock->slock;

	lock->slock = 0;
	barrier();

	return oldval > 0;
}

This comes from include/linux/spinlock_up.h, which is included from 
include/linux/spinlock.h

#ifdef CONFIG_SMP
# include <asm/spinlock.h>
#else
# include <linux/spinlock_up.h>
#endif


So, the question I have is: what is the actual "correct" behavior of 
spin_trylock_irqsave()? Is it really supposed to always return true when 
CONFIG_DEBUG_SPINLOCK=n and CONFIG_SMP=n or is this a bug?