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

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Date:   Thu, 14 Nov 2019 11:02:01 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
CC:     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
> 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?
(I've not looked for a lock, but since kmemdup() can sleep I'd suspect there isn't one.)

Also can another thread call fork() - eg while the kmemdup() is sleeping?

	David

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