linux-kernel - Re: [PATCH 1/2] [RFC v2] seqcount: Add lockdep functionality to seqcount/seqlock structures

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Date:	Tue, 17 Sep 2013 10:28:05 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Li Zefan <lizefan@...wei.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] [RFC v2] seqcount: Add lockdep functionality to
 seqcount/seqlock structures


* John Stultz <john.stultz@...aro.org> wrote:

> On 09/13/2013 05:19 PM, John Stultz wrote:
> > Currently seqlocks and seqcounts don't support lockdep.
> >
> > After running across a seqcount related deadlock in the timekeeping
> > code, I used a less-refined and more focused varient of this patch
> > to narrow down the cause of the issue.
> >
> > This is a first-pass attempt to properly enable lockdep functionality
> > on seqlocks and seqcounts.
> >
> > Since seqcounts are used in the vdso gettimeofday code, I've provided
> > lockdep accessors.
> >
> > I've also handled one cases where there were nested seqlock writers
> > and there may be more edge cases.
> 
> Oof.
> 
> So I just noticed there's a bunch of places in the network code that use
> fairly deeply embedded seqcounter: u64_stats_sync. There's almost never
> an explicit initialization, as they assume they're zeroed when
> allocated, but this causes trouble with the lockdep key initialization.
> 
> I'll have to go through each of these (about 25 cases) and make them
> call seqcount_init(), but since I'm heading to plumbers tomorrow I might
> not get to it until next week.
> 
> Anyway, let me know if you have any other thoughts on the patches.

Explicit initialization is generally a bonus for readability, 
debuggability and ease of development, we enforce that for spinlocks as 
well.

Thanks,

	Ingo
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