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

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Message-ID: <20110927125020.GB3685@linux.vnet.ibm.com>
Date:	Tue, 27 Sep 2011 18:20:20 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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.

* Peter Zijlstra <peterz@...radead.org> [2011-09-27 14:37:59]:

> 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?

yes, we could do without the lock to.
Will do this in the next patchset.

-- 
Thanks and Regards 
Srikar

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