Re: [patch V2 11/16] x86/ioperm: Share I/O bitmap if identical

[Peter Zijlstra]

On Thu, Nov 14, 2019 at 11:02:01AM +0000, David Laight wrote:
> From: Peter Zijlstra
> > Sent: 12 November 2019 09:15
> ...
> > > +	/*
> > > +	 * If the bitmap is not shared, then nothing can take a refcount as
> > > +	 * current can obviously not fork at the same time. If it's shared
> > > +	 * duplicate it and drop the refcount on the original one.
> > > +	 */
> > > +	if (refcount_read(&iobm->refcnt) > 1) {
> > > +		iobm = kmemdup(iobm, sizeof(*iobm), GFP_KERNEL);
> > > +		if (!iobm)
> > > +			return -ENOMEM;
> > > +		io_bitmap_exit();
> > 		refcount_set(&iobm->refcnd, 1);
> > >  	}
> 
> What happens if two threads of the same process enter the above
> at the same time?

Suppose there's just the two threads, and both will change it. Then both
do copy-on-write and the original gets freed.