linux-kernel - Re: [PATCH] lockdep: Show address of "struct lockdep_map" at print

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Message-ID: <20180326132057.GJ5652@dhcp22.suse.cz>
Date:   Mon, 26 Mar 2018 15:20:57 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Borislav Petkov <bp@...e.de>,
        David Rientjes <rientjes@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] lockdep: Show address of "struct lockdep_map" at
 print_lock().

On Mon 26-03-18 15:19:11, Michal Hocko wrote:
> On Mon 26-03-18 19:18:33, Tetsuo Handa wrote:
> > Currently, print_lock() is printing hlock->acquire_ip field in both
> > "[<%px>]" and "%pS" format. But "[<%px>]" is little useful nowadays, for
> > we use scripts/faddr2line which receives "%pS" for finding the location
> > in the source code.
> > 
> > Since "struct lockdep_map" is embedded into lock objects, we can know
> > which instance of a lock object is acquired using hlock->instance field.
> > This will help finding which threads are causing a lock contention when
> > e.g. the OOM reaper failed to acquire an OOM victim's mmap_sem for read.
> 
> How? All I can see is that we can match which instances are the same.
> This would be an interesting thing to know AFAICS because you can tell
> different instances of lock apart. So the patch makes some sense to me,
> I am just not sure about changelog.

Also, are you sure that %px is appropriate? Can this be abused to leak
the kernel pointer and infere other useful data from it? %p should be
sufficient to tell different lock instances even with the hashed
addresses.
-- 
Michal Hocko
SUSE Labs