wait_event_interruptible() seems non-atomic

[Jan Engelhardt]

Hi list,


upon reviewing some of my code for a device driver, I've come across:


static ssize_t uif_read(struct file *filp, char __user *buf,
  size_t count, loff_t *ppos)
{
    // Nothing read, nothing done
    if(count == 0) { return 0; }

    // Must sleep as long as there is no data
    if(down_interruptible(&Buffer_lock)) { return -ERESTARTSYS; }
    while(BufRP == BufWP) {
        up(&Buffer_lock);
        if(filp->f_flags & O_NONBLOCK) { return -EAGAIN; }

        // hm, the condition is not atomic or locked...
        if(wait_event_interruptible(Pull_queue, (BufRP != BufWP))) {
            return -ERESTARTSYS;
        }
        if(down_interruptible(&Buffer_lock)) { return -ERESTARTSYS; }
    }

    // Data is available, so give it to the user
    ...
}

As you can see, I lock Buffer_lock and then check BufRP == BufWP. I do that
because in an SMP environment, either of BufRP or BufWP might have been
updated. (I.e. one CPU currently does "mov BufRP to eax; mov BufWP to ebx; cmp
eax, ebx" while another does "inc BufWP")

I would like to also lock Buffer_lock around BufRP != BufWP, but don't see a
way on how to accomplish this.

Does anybody know a way how this could be achieved?



Jan Engelhardt
-- 
Gesellschaft fÃr Wissenschaftliche Datenverarbeitung
Am Fassberg, 37077 GÃttingen, www.gwdg.de
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