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

Andrew Morton
2007-04-26T02:20:27Z

On Wed, 25 Apr 2007 20:13:59 +0100 Christoph Hellwig <hch@infradead.org> wrote:

> On Wed, Apr 25, 2007 at 05:49:34PM +0200, Matthias Kaehlcke wrote:
> > drivers/char/tty_io.c uses a semaphore as mutex. use the mutex API
> > instead of the (binary) semaphore
>
> This looks like it should be a spinlock:
>
> > - down(&allocated_ptys_lock);
> > + mutex_lock(&allocated_ptys_lock);
> > idr_remove(&allocated_ptys, idx);
> > - up(&allocated_ptys_lock);
> > + mutex_unlock(&allocated_ptys_lock);
>
> idr_remove is a quick operation that doesn't sleep.
>
> > @@ -2639,24 +2639,24 @@ static int ptmx_open(struct inode * inode, struct file * filp)
> > nonseekable_open(inode, filp);
> >
> > /* find a device that is not in use. */
> > - down(&allocated_ptys_lock);
> > + mutex_lock(&allocated_ptys_lock);
> > if (!idr_pre_get(&allocated_ptys, GFP_KERNEL)) {
> > - up(&allocated_ptys_lock);
>
> The idr_pre_get should be moved out of the lock, that's the whole
> point for it's existance..
>

I think having it inside the lock makes sense:

mutex_lock()
idr_pre_get()
idr_get_new()
mutex_unlock()

here, if idr_pre_get() succeeded, we know that idr_get_new() will succeed.

otoh:

try_again:
idr_pre_get()
mutex_lock()
if (idr_get_new() == failed) {
mutex_unlock()
goto try_again;
}
mutex_unlock()

is not nice.

the IDR api is awful. A little project is to rip out all its internal
locking and to implement caller-provided locking.

Unfortunately the fact that the library allocates memory means that we
might need to do awkward things like radix_tree_preload() to make it
reliable for callers who use spinlocking.