Date: Sun, 5 May 2013 02:05:28 +0200
From: Pavel Machek <pavel@ucw.cz>
To: Colin Cross <ccross@android.com>, Andrew Morton <akpm@osdl.org>
Cc: lkml <linux-kernel@vger.kernel.org>,
        Trond Myklebust <Trond.Myklebust@netapp.com>,
        Len Brown <len.brown@intel.com>, "Rafael J. Wysocki" <rjw@sisk.pl>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        "David S. Miller" <davem@davemloft.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mandeep Singh Baines <msb@chromium.org>,
        Paul Walmsley <paul@pwsan.com>, Al Viro <viro@zeniv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Oleg Nesterov <oleg@redhat.com>, linux-nfs@vger.kernel.org,
        Linux PM list <linux-pm@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>, Ben Chan <benchan@chromium.org>
Subject: Re: [PATCH 2/2] lockdep: check that no locks held at freeze time
Message-ID: <20130505000528.GA25454@amd.pavel.ucw.cz>
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Sender: linux-nfs-owner@vger.kernel.org

Hi!

> >> >> --- a/kernel/exit.c
> >> >> +++ b/kernel/exit.c
> >> >> @@ -835,7 +835,7 @@ void do_exit(long code)
> >> >>       /*
> >> >>        * Make sure we are holding no locks:
> >> >>        */
> >> >> -     debug_check_no_locks_held(tsk);
> >> >> +     debug_check_no_locks_held();
> >> >
> >> > Is task guaranteed == current?
> >>
> >> Yes, the first line of do_exit is:
> >>         struct task_struct *tsk = current;
> >
> > Aha, I understand it now.
> >
> > Accessing current is slower than local variable. So your "new" code
> > will work but will be slower. Please revert this part.
> 
> Using current instead of passing in tsk was done at Andrew Morton's
> suggestion, and makes no difference from the freezer's perspective
> since it would have to use current to get the task to pass in, so I'm
> going to leave it as is.

Well, current is:

static inline struct thread_info *current_thread_info(void)
{
        register unsigned long sp asm ("sp");
        return (struct thread_info *)(sp & ~(THREAD_SIZE - 1));
}

#define get_current() (current_thread_info()->task)

#define current get_current()

Instead of passing computed value to debug_check_no_locks_held(), you
force it to be computed again. do_exit() performance matters for
configure scripts, etc.

I'd say it makes sense to keep the optimalization. akpm can correct
me.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html