Messages in this thread | | | Date | Thu, 31 May 2007 15:37:12 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] use mutex instead of semaphore in tty_io.c |
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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.
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