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

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Message-ID: <20191112091521.GX4131@hirez.programming.kicks-ass.net>
Date:   Tue, 12 Nov 2019 10:15:21 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.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 Mon, Nov 11, 2019 at 11:03:25PM +0100, Thomas Gleixner wrote:
> @@ -59,8 +71,26 @@ long ksys_ioperm(unsigned long from, uns
>  			return -ENOMEM;
>  
>  		memset(iobm->bits, 0xff, sizeof(iobm->bits));
> +		refcount_set(&iobm->refcnt, 1);
> +	}
> +
> +	/*
> +	 * 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);
>  	}
>  
> +	/* Set the tasks io_bitmap pointer (might be the same) */
> +	t->io_bitmap = iobm;
> +	/* Mark it active for context switching and exit to user mode */
> +	set_thread_flag(TIF_IO_BITMAP);
> +