lkml.org 
[lkml]   [2013]   [Sep]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] [RFC] seqcount: Add lockdep functionality to seqcount/seqlock structures
On Mon, Sep 09, 2013 at 09:42:46PM -0700, 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.
>
> Due to seqlocks/seqcounts having slightly different possible semantics
> then standard locks (ie: reader->reader and reader->writer recursion is
> fine, but writer->reader is not), this implementation is probably not
> as exact as I'd like (currently using a hack by only spot checking
> readers), and may be overly strict in some cases.
>
> I've handled one cases where there were nested seqlock writers, and
> there may be more edge cases, as while I've gotten it to run cleanly,
> depending on config its reporting issues that I'm not sure if they are
> flaws in the implementation or actual bugs. But I wanted to send this
> out for some initial thoughts as until today I hadn't looked at much
> of the lockdep infrastructure. So I'm sure there are improvements
> that could be made.
>
> Comments and feedback would be appreciated!

> --- a/arch/x86/vdso/vclock_gettime.c
> +++ b/arch/x86/vdso/vclock_gettime.c
> @@ -178,13 +178,15 @@ notrace static int __always_inline do_realtime(struct timespec *ts)
>
> ts->tv_nsec = 0;
> do {
> - seq = read_seqcount_begin(&gtod->seq);
> + seq = __read_seqcount_begin(&gtod->seq);
> + smp_rmb();
> mode = gtod->clock.vclock_mode;
> ts->tv_sec = gtod->wall_time_sec;
> ns = gtod->wall_time_snsec;
> ns += vgetsns(&mode);
> ns >>= gtod->clock.shift;
> - } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
> + smp_rmb();
> + } while (unlikely(__read_seqcount_retry(&gtod->seq, seq)));
>
> timespec_add_ns(ts, ns);
> return mode;


You didn't mention the VDSO 'fun' in you Changelog!


\
 
 \ /
  Last update: 2013-09-10 11:21    [W:0.064 / U:0.172 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site