netdev - Re: [PATCH net-next] net: phy: switch to lockdep_assert

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Date:   Mon, 12 Nov 2018 18:44:16 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Heiner Kallweit <hkallweit1@...il.com>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net: phy: switch to lockdep_assert_held in
 phylib

On Sun, Nov 11, 2018 at 10:33:08PM +0100, Heiner Kallweit wrote:
> diff --git a/drivers/net/phy/mdio_bus.c b/drivers/net/phy/mdio_bus.c
> index 2e59a8419..5cb06f021 100644
> --- a/drivers/net/phy/mdio_bus.c
> +++ b/drivers/net/phy/mdio_bus.c
> @@ -541,7 +541,7 @@ int __mdiobus_read(struct mii_bus *bus, int addr, u32 regnum)
>  {
>  	int retval;
>  
> -	WARN_ON_ONCE(!mutex_is_locked(&bus->mdio_lock));
> +	lockdep_assert_held_once(&bus->mdio_lock);

Hi Heiner

I don't think there is a clear right/wrong here. This is not hot path
code. The cost for checking the lock is held is very small compared to
the actual MDIO transaction. So i don't think we really need to
optimise this. I do sometimes build with lockdep on, but not
always. So it is good to know when locking is broken on normal builds.

Florian, what do you think?

	 Andrew