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Date: Wed, 28 Nov 2001 16:26:16 -0800
From: "David C. Hansen" <haveblue@us.ibm.com>
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To: Russell King <rmk@arm.linux.org.uk>
CC: Alan Cox <alan@lxorguk.ukuu.org.uk>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] remove BKL from drivers' release functions
In-Reply-To: <E169EFX-0006TA-00@the-village.bc.nu> <3C057410.3090201@us.ibm.com> <20011128234505.C2561@flint.arm.linux.org.uk>
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Russell King wrote:

>On Wed, Nov 28, 2001 at 03:32:32PM -0800, David C. Hansen wrote:
>
>>Nothing, because the BKL is not held for all opens anymore.  In most of 
>>the cases that we addressed, the BKL was in release _only_, not in open 
>>at all.  There were quite a few drivers where we added a spinlock, or 
>>used atomic operations to keep open from racing with release.  
>>
>
>All char and block devs are opened with the BKL held - see chrdev_open in
>fs/devices.c and do_open in fs/block_dev.c
>
I wrote a quick and dirty char device driver to see if this happened. 
 If I run two tasks doing a bunch of opens and closes, the -EBUSY 
condition in the open function does happen.  Is my driver doing 
something wrong?

Here is the meat of the driver:

static int Device_Open = 0;

int testdev_open(struct inode *inode,  struct file *file)
{
  if ( test_and_set_bit(0,&Device_Open) )    {
      printk( "attempt to open testdev more than once\n" );
      return -EBUSY;
    }
  MOD_INC_USE_COUNT;
  return SUCCESS;
}

int testdev_release(struct inode *inode,  struct file *file)
{
  clear_bit(0,&Device_Open);
  MOD_DEC_USE_COUNT;
  return 0;
}

static int Major;
struct file_operations Fops = {
     open: testdev_open,
  release: testdev_release,
};

/* Initialize the module - Register the character device */
int init_module(void)
{
  Major = register_chrdev(0,
                          DEVICE_NAME,
                          &Fops);

  /* Negative values signify an error */
  if (Major < 0) {
    printk ("%s: %s device failed with %d\n",MODULE_NAME,
            "Sorry, registering the character",
            Major);
    return Major;
  }
  printk( "%s: loaded successfully on Major:%d\n",MODULE_NAME,Major);
  return 0;
}

void cleanup_module(void)
{
  int ret;
  ret = unregister_chrdev(Major, DEVICE_NAME);
  if (ret < 0)
    printk("%s: Error in unregister_MODULE_NAME: %d\n",
       MODULE_NAME, ret);
}




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