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

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Message-ID: <2e02321be78d43e9b7d7ead29a10b1d9@AcuMS.aculab.com>
Date:   Thu, 14 Nov 2019 13:22:03 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>
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

From: Peter Zijlstra <peterz@...radead.org>
> 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.

I was probably forgetting that the linux kernel uses (more or less) full 'process'
structures for threads, rather than separate 'thread' data areas.

	David

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