linux-kernel - Re: [PATCH v5 3.1.0-rc4-tip 18/26] uprobes: slot allocation.

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Message-ID: <1317127079.15383.52.camel@twins>
Date:	Tue, 27 Sep 2011 14:37:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 3.1.0-rc4-tip 18/26]   uprobes: slot allocation.

On Tue, 2011-09-20 at 17:33 +0530, Srikar Dronamraju wrote:
> +static unsigned long xol_take_insn_slot(struct uprobes_xol_area *area)
> +{
> +       unsigned long slot_addr, flags;
> +       int slot_nr;
> +
> +       do {
> +               spin_lock_irqsave(&area->slot_lock, flags);
> +               slot_nr = find_first_zero_bit(area->bitmap, UINSNS_PER_PAGE);
> +               if (slot_nr < UINSNS_PER_PAGE) {
> +                       __set_bit(slot_nr, area->bitmap);
> +                       slot_addr = area->vaddr +
> +                                       (slot_nr * UPROBES_XOL_SLOT_BYTES);
> +                       atomic_inc(&area->slot_count);
> +               }
> +               spin_unlock_irqrestore(&area->slot_lock, flags);
> +               if (slot_nr >= UINSNS_PER_PAGE)
> +                       xol_wait_event(area);
> +
> +       } while (slot_nr >= UINSNS_PER_PAGE);
> +
> +       return slot_addr;
> +} 

Why isn't a find_first_bit() + set_and_test_bit() not sufficient? That
is, what do you need that lock for?
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