linux-kernel - Re: [PATCH] cfg80211: clear cfg80211_inform

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Message-ID: <43e72e890909041421m243bbea6i95c26ab21dc8732d@mail.gmail.com>
Date:	Fri, 4 Sep 2009 14:21:40 -0700
From:	"Luis R. Rodriguez" <lrodriguez@...eros.com>
To:	Catalin Marinas <catalin.marinas@....com>,
	"John W. Linville" <linville@...driver.com>
Cc:	Johannes Berg <johannes@...solutions.net>,
	Luis Rodriguez <Luis.Rodriguez@...eros.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>
Subject: Re: [PATCH] cfg80211: clear cfg80211_inform_bss() from kmemleak 
	reports

On Fri, Sep 4, 2009 at 1:25 AM, Catalin Marinas<catalin.marinas@....com> wrote:
> On Fri, 2009-09-04 at 07:04 +0200, Johannes Berg wrote:
>> On Thu, 2009-09-03 at 13:43 -0700, Luis R. Rodriguez wrote:
>> > On Thu, Sep 03, 2009 at 11:17:17AM -0700, Johannes Berg wrote:
>> > > On Thu, 2009-09-03 at 11:13 -0700, Luis R. Rodriguez wrote:
>> > >
>> > > > What I meant is it gobbles it up and spits another thing out. When it
>> > > > gobbles it up the routine then uses kref_put().
>> > > >
>> > > > > Why can it not track this?
>> > > >
>> > > > It probably can, just not sure if it follows kref_put(), I was under
>> > > > the impression here it doesn't and because of it we were getting false
>> > > > positives. Catalin, can you confirm?
>> > >
>> > > Ah I'd think that if it can't track it then that's because we use a
>> > > pointer to the middle of the struct to keep track of it much of the
>> > > time.
>> >
>> > So you agree with the patch but not the commit log entry?
>>
>> I'm not sure -- I think kmemleak should be able to figure it out, and if
>> you were using IBSS then we actually have a leak that we need to plug,
>> but otherwise I'd prefer to get some more input from Catalin first.
>
> First of all, kmemleak_ignore() is not the right function to mark a
> false positive as it completely ignores an object even though it may
> have pointers to others. The kmemleak_not_leak() function should be
> used. However, there are only two places in the kernel where this was
> actually needed (one of them is a real leak but we ignore it as it makes
> the code more complicated).
>
> So, I think we should try to figure out why kmemleak reports it. There
> are a few common cases:
>     1. transient false positive - this should disappear after a few
>        scans
>     2. a pointer leading to the reported object is stored in an area of
>        memory not scanned by kmemleak - most commonly pages allocated
>        explicitly (alloc_pages etc.) as kmemleak doesn't track these.
>        The preferred solution is to inform kmemleak about such page
>        (kmemleak_alloc/kmemleak_free) rather than marking the false
>        positive
>     3. a pointer leading to the reported object isn't actually pointing
>        to anywhere inside the structure (i.e. using the physical
>        address). Here we would use kmemleak_not_leak()

John please revert this merged patch
(b563f91105758c35d7cd4589992198b9da52d579) on wireless-testing as we'd
like to investigate further why we get this.

BTW I should not I got this kmemleak report after using the clear
command by painting objects black. I'll test it now with your
suggested changes.

  Luis
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