linux-kernel - Re: [PATCH] make idr_remove_all() do removal -before- free

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Date:	Sun, 8 Mar 2009 16:33:36 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, manfred@...orfullife.com,
	Nadia.Derbey@...l.net, miltonm@...tin.ibm.com,
	akpm@...ux-foundation.org, peterz@...radead.org,
	lnxninja@...ux.vnet.ibm.com, efault@....de, riel@...hat.com
Subject: Re: [PATCH] make idr_remove_all() do removal -before- free_layer()


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> The following patch fixes a problem in the IDR system, where 
> an idr_remove_all() hands a data element to call_rcu() (via 
> free_layer()) before making that data element inaccessible to 
> new readers.  This is very bad, and results in readers still 
> having a reference to this data element at the end of the 
> grace period.  Tests on large machines that concurrently map 
> and unmap user-space memory within the same multithreaded 
> process result in crashes within about five minutes.  Applying 
> this patch increases the kernel's longevity to the 
> three-to-eight-hour range.
> 
> There appear to be other similar problems in 
> idr_get_empty_slot() and sub_remove(), but I fixed the easy 
> one in idr_remove_all() first.  It is therefore no surprise 
> that failures still occur.
> 
> (Yes, and I did look at the relevant patch last year without 
> spotting this one.  Goes to show the value of testing as well 
> as code review, I guess...)
> 
> Nadia, Manfred, any thoughts?
> 
> Located-by: Milton Miller II <miltonm@...tin.ibm.com>
> Tested-by: Milton Miller II <miltonm@...tin.ibm.com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Hm, looks like something we really want to see fixed in 
2.6.29-final, right?

	Ingo
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