Re: Safely giving up a lock before sleeping

[Richard B. Johnson]

On Mon, 23 Jul 2001, Russ Lewis wrote:
[SNIPPED...]

> Of course, since I need the lock to do anything, the first thing I do
> (after removing myself from the wait queue and checking for signals) is
> to relock the lock.
> 
> Is this a common problem?  Is there a more elegant solution to this?
> 

If you need to protect a piece of code, and the variables it
accesses are ONLY accessed by that code, investigate the use
of a simple spin-lock (no disabling interrupts). This can
prevent races such as you describe.

	spin_lock_irqsave(&big_lock, flags);
	do_critical_stuff();    /* Where global variables could change */
	spin_lock(&little_lock);  /* Simple spin for any other entry */
	spin_unlock_irqrestore(&big_lock, flags);  /* Interrupts now on */
	do_less_critical_stuff_including_sleep();
	spin_unlock(&little_lock);

Note that this turns a possible race into a possible CPU time-eating
spin so you need to carefully look at how the code is written. You
could turn that spin-wait into a sleep if you used a semaphore to
protect that section of code "up(), and down()".

Cheers,
Dick Johnson

Penguin : Linux version 2.4.1 on an i686 machine (799.53 BogoMips).

    I was going to compile a list of innovations that could be
    attributed to Microsoft. Once I realized that Ctrl-Alt-Del
    was handled in the BIOS, I found that there aren't any.