linux-kernel - Re: [PATCH v5 3.1.0-rc4-tip 12/26] Uprobes: Handle breakpoint and Singlestep

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Date:	Wed, 5 Oct 2011 19:48:42 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	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>,
	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 12/26]   Uprobes: Handle breakpoint
	and Singlestep

On 09/26, Peter Zijlstra wrote:
>
> On Mon, 2011-09-26 at 21:31 +0530, Srikar Dronamraju wrote:
> > * Peter Zijlstra <peterz@...radead.org> [2011-09-26 15:59:13]:
> >
> > > On Tue, 2011-09-20 at 17:32 +0530, Srikar Dronamraju wrote:
> > > > 						Hence provide some extra
> > > > + * time (by way of synchronize_sched() for breakpoint hit threads to acquire
> > > > + * the uprobes_treelock before the uprobe is removed from the rbtree.
> > >
> > > 'Some extra time' doesn't make me all warm an fuzzy inside, but instead
> > > screams we fudge around a race condition.
> >
> > The extra time provided is sufficient to avoid the race. So will modify
> > it to mean "sufficient" instead of "some".
> >
> > Would that suffice?
>
> Much better, for extra point, explain why its sufficient as well ;-)

+1 ;)

I can't understand why synchronize_sched helps. In fact it is very
possible I simply misunderstood the problem, I'll appreciate if you
can explain.

Just for example. Suppose that uprobe_notify_resume() sleeps in
down_read(mmap_sem). In this case synchronize_sched() can return
even before it takes this sem, how this can help the subsequent
find_uprobe() ? Or that task  can be simply preempted before.

Or I missed the point completely?

Oleg.

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