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

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Date:   Thu, 14 Nov 2019 14:09:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Willy Tarreau <w@....eu>, Juergen Gross <jgross@...e.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch V2 11/16] x86/ioperm: Share I/O bitmap if identical

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.