Re: [PATCH] use mutex instead of semaphore in tty_io.c

Matthias Kaehlcke
2007-06-01T07:46:52Z

El Thu, May 31, 2007 at 03:37:12PM -0700 Andrew Morton ha dit:

> On Thu, 31 May 2007 15:42:26 +0200
> Matthias Kaehlcke <matthias.kaehlcke@gmail.com> wrote:
>
> > drivers/char/tty_io.c: Use spinlock instead of a (binary) semaphore
> >
>
> hm.
>
> >
>
> We end up with this:
>
> /* find a device that is not in use. */
> if (!idr_pre_get(&allocated_ptys, GFP_KERNEL))
> return -ENOMEM;
>
> spin_lock(&allocated_ptys_lock);
>
> idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
> if (idr_ret < 0) {
> spin_unlock(&allocated_ptys_lock);
> if (idr_ret == -EAGAIN)
> return -ENOMEM;
> return -EIO;
> }
> if (index >= pty_limit) {
> idr_remove(&allocated_ptys, index);
> spin_unlock(&allocated_ptys_lock);
> return -EIO;
> }
> spin_unlock(&allocated_ptys_lock);
>
> this leaves a small window in which another thread can come in and steal
> away the idr tree's reserves, causing the idr_get_new() to fail. It's
> highly improbable, but it's real.

i agree, thanks for pointing it out

> Hence I think a straight semaphore->mutex conversion would be better.

that leads us back to the initial patch. christoph: is that ok for
you or do you have another proposal?

--
Matthias Kaehlcke
Linux Application Developer
Barcelona

The assumption that what currently exists must necessarily
exist is the acid that corrodes all visionary thinking
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