linux-kernel - Re: [PATCH 2/2] lockdep: check that no locks held at freeze time

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Message-ID: <20130505011334.GB25454@amd.pavel.ucw.cz>
Date:	Sun, 5 May 2013 03:13:34 +0200
From:	Pavel Machek <pavel@....cz>
To:	Colin Cross <ccross@...roid.com>
Cc:	Andrew Morton <akpm@...l.org>, lkml <linux-kernel@...r.kernel.org>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Len Brown <len.brown@...el.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	"David S. Miller" <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mandeep Singh Baines <msb@...omium.org>,
	Paul Walmsley <paul@...an.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Oleg Nesterov <oleg@...hat.com>, linux-nfs@...r.kernel.org,
	Linux PM list <linux-pm@...r.kernel.org>,
	netdev <netdev@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Ben Chan <benchan@...omium.org>
Subject: Re: [PATCH 2/2] lockdep: check that no locks held at freeze time

On Sat 2013-05-04 17:23:01, Colin Cross wrote:
> On Sat, May 4, 2013 at 5:05 PM, Pavel Machek <pavel@....cz> wrote:
> > 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.
> 
> That translates to 3 instructions, with no memory accesses:
> c0008350:       e1a0300d        mov     r3, sp
> c0008354:       e3c32d7f        bic     r2, r3, #8128   ; 0x1fc0
> c0008358:       e3c2203f        bic     r2, r2, #63     ; 0x3f

On ARM, you are right. It seems to have memory access on s390 and
x86-64. Ok, it is probably going to be in cache, but...

									Pavel
-- 
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