Re: [PATCH] drivers: char: applicom.c: Add lock for protecting DeviceErrorCount

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Aug 03, 2020 at 04:20:49PM +0530, madhuparnabhowmik10@gmail.com wrote:
> From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> 
> The variable DeviceErrorCount is used to keep track of the number of
> errors in read, write and interrupt routines, however it was not
> protected by proper locking.
> Therefore, this patch adds a spinlock: error_lock to protect the
> variable.
> 
> Found by Linux Driver Verification project (linuxtesting.org).
> 
> Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> ---
>  drivers/char/applicom.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/char/applicom.c b/drivers/char/applicom.c
> index 14b2d8034c51..6df7450b8b99 100644
> --- a/drivers/char/applicom.c
> +++ b/drivers/char/applicom.c
> @@ -106,6 +106,7 @@ static DECLARE_WAIT_QUEUE_HEAD(FlagSleepRec);
>  static unsigned int WriteErrorCount;	/* number of write error      */
>  static unsigned int ReadErrorCount;	/* number of read error       */
>  static unsigned int DeviceErrorCount;	/* number of device error     */
> +DEFINE_SPINLOCK(error_lock);		/* lock to protect error count variables */

That's a horrible global name, shouldn't it be static?

>  
>  static ssize_t ac_read (struct file *, char __user *, size_t, loff_t *);
>  static ssize_t ac_write (struct file *, const char __user *, size_t, loff_t *);
> @@ -428,7 +429,9 @@ static ssize_t ac_write(struct file *file, const char __user *buf, size_t count,
>  		spin_unlock_irqrestore(&apbs[IndexCard].mutex, flags);
>  		printk(KERN_WARNING "APPLICOM driver write error board %d, DataFromPcReady = %d\n",
>  		       IndexCard,(int)readb(apbs[IndexCard].RamIO + DATA_FROM_PC_READY));
> +		spin_lock_irqsave(&error_lock, flags);

Why all of these irqsave?

>  		DeviceErrorCount++;

Does this really matter?  Who cares if we drop one of these, or any
other of these debugging-only values?

thanks,

greg k-h