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

Andrew Morton
2007-05-31T22:37:56Z

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.

>
> --
>
> diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
> index 7a32df5..ff27587 100644
> --- a/drivers/char/tty_io.c
> +++ b/drivers/char/tty_io.c

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.

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

The IDR API absolutely blows chunks: it should require caller-provided
locking, like radix-tree. But then it'd need gunk like radix_tree_preload
to be reliable. Fact is, storage librares which need to allocate memory at
insert-time are always going to be problematic in-kernel.