Re: [PATCH v2 1/2] watchdog: dw_wdt: No need for a spinlock

Dmitry Torokhov
2015-05-08T05:23:30Z

On Thu, May 07, 2015 at 09:27:44PM -0700, Doug Anderson wrote:
> Right now the dw_wdt uses a spinlock to protect dw_wdt_open(). The
> problem is that while holding the spinlock we call:
> -> dw_wdt_set_top()
> -> dw_wdt_top_in_seconds()
> -> clk_get_rate()
> -> clk_prepare_lock()
> -> mutex_lock()
>
> Locking a mutex while holding a spinlock is not allowed and leads to
> warnings like "BUG: spinlock wrong CPU on CPU#1", among other
> problems.
>
> There's no reason to use a spinlock. Only dw_wdt_open() was protected
> and the test_and_set_bit() at the start of that function protects us
> anyway.
>
> Signed-off-by: Doug Anderson <dianders@chromium.org>
> ---
> Changes in v2:
> - Don't switch to mutex; just don't use spinlock at all as per Dmitry

Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

>
> drivers/watchdog/dw_wdt.c | 7 -------
> 1 file changed, 7 deletions(-)
>
> diff --git a/drivers/watchdog/dw_wdt.c b/drivers/watchdog/dw_wdt.c
> index d0bb949..a284abd 100644
> --- a/drivers/watchdog/dw_wdt.c
> +++ b/drivers/watchdog/dw_wdt.c
> @@ -35,7 +35,6 @@
> #include <linux/pm.h>
> #include <linux/platform_device.h>
> #include <linux/reboot.h>
> -#include <linux/spinlock.h>
> #include <linux/timer.h>
> #include <linux/uaccess.h>
> #include <linux/watchdog.h>
> @@ -61,7 +60,6 @@ MODULE_PARM_DESC(nowayout, "Watchdog cannot be stopped once started "
> #define WDT_TIMEOUT (HZ / 2)
>
> static struct {
> - spinlock_t lock;
> void __iomem *regs;
> struct clk *clk;
> unsigned long in_use;
> @@ -177,7 +175,6 @@ static int dw_wdt_open(struct inode *inode, struct file *filp)
> /* Make sure we don't get unloaded. */
> __module_get(THIS_MODULE);
>
> - spin_lock(&dw_wdt.lock);
> if (!dw_wdt_is_enabled()) {
> /*
> * The watchdog is not currently enabled. Set the timeout to
> @@ -190,8 +187,6 @@ static int dw_wdt_open(struct inode *inode, struct file *filp)
>
> dw_wdt_set_next_heartbeat();
>
> - spin_unlock(&dw_wdt.lock);
> -
> return nonseekable_open(inode, filp);
> }
>
> @@ -348,8 +343,6 @@ static int dw_wdt_drv_probe(struct platform_device *pdev)
> if (ret)
> return ret;
>
> - spin_lock_init(&dw_wdt.lock);
> -
> ret = misc_register(&dw_wdt_miscdev);
> if (ret)
> goto out_disable_clk;
> --
> 2.2.0.rc0.207.ga3a616c
>

--
Dmitry