linux-kernel - Re: [RFC] seqlock,lockdep: Add lock primitives to read

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Date:	Tue, 29 Mar 2011 15:14:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC] seqlock,lockdep: Add lock primitives to read_seqbegin().

On Sat, 2011-03-26 at 13:12 +0900, Tetsuo Handa wrote:
> 
> But there is still a problem. It seems to me that lockdep checks for this bug
> only when a new locking pattern (a locking pattern which was not already added
> to lockdep database) is added. 

That's how it works. Why is that a problem?

> This freeze can be triggered by running
> 
>   while :; do newns /sbin/pivot_root /proc/ /proc/sys/; done
> 
> on one terminal and running
> 
>   while :; do /bin/ls -l /proc/*/exe; done
> 
> on another terminal. (The "newns" is a program that unshares the mnt namespace
> before execve() using CLONE_NEWNS.) But even after applying the patch above,
> lockdep does not show the trace above.

Then you're still missing something.

> lockdep shows the trace above only after I run test programs for TOMOYO (which
> causes a locking pattern that was generated by neither
> "/sbin/pivot_root /proc/ /proc/sys/" nor "/bin/ls -l /proc/*/exe" to be added
> to lockdep database).

And tomoyo manages to close the cycle for some reason.

> I think that we want some method for rechecking already added locking pattern.
> Maybe it is run by (e.g.) every 60 seconds. Maybe it is run when stall checking
> mechanisms report the possibility of stall. (The sysrq key didn't work after
> the freeze occurred.) 

I don't get this, why would you need to recheck anything? lockdep does a
full analysis on every addition, rechecking when nothing changed should
not yield another result.

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