Re: [PATCH 1/3, RFC] misc char dev BKL pushdown

From: Jonathan Corbet
Date: Tue May 20 2008 - 19:25:48 EST

Mike Frysinger <vapier.adi@xxxxxxxxx> wrote:

> please drop the coreb.c changes from your patch

At a minimum, I would hope such a request would say something like "I've
looked at the driver's locking and am convinced that the BKL is not
needed." Have you done that? There is a certain leap of faith involved
in removing that protection from a driver.

I decided to take a quick look...

- You use spin_lock_irq(&coreb_lock) in a number of places, but you do
not take the lock in the interrupt handler. You also do not take the
lock in coreb_write() or coreb_read(), so those can race with the
interrupt handler, with ioctl(), and with each other.

- coreb_write() and coreb_read() do interruptible waits, but do not
check to see whether they were interrupted. They will, in fact,
continue in their I/O loops after a signal.

- In both functions you have:

unsigned long p = *ppos;

if (p + count > coreb_size)
return -EFAULT;

that calculation can overflow.

- You also do this:

static ssize_t coreb_write(struct file *file, const char *buf, size_t count,
loff_t * ppos)
/* ... */
set_dma_start_addr(CH_MEM_STREAM2_SRC, (unsigned long)buf);

In other words, the DMA is done directly to/from a user-space
address. Maybe that's safe on Blackfin, I don't know...

- I have no idea why some of your functions are using d_inode->i_mutex.

- In coreb_ioctl():

if (coreb_status & COREB_IS_RUNNING) {
retval = -EBUSY;

this will exit the function with the spinlock still held and
interrupts disabled.

printk(KERN_INFO "Resetting Core B\n");
bfin_write_SICB_SYSCR(bfin_read_SICB_SYSCR() | 0x0080);

You do not acquire the lock here, so this can race against other
ioctl() calls. And ioctl() can race against read() and write().

Registration and such seem reasonable, so I can't come up with a
scenario where loss of BKL protection will create trouble. Given the
other problems there, though, I'll confess to being a bit nervous about

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at