linux-kernel - Re: [PATCH] Revert lockdep check in raw_seqcount

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Date:	Fri, 6 Jun 2014 16:07:32 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Trond Myklebust <trond.myklebust@...marydata.com>
Cc:	linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
	Ingo Molnar <mingo@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] Revert lockdep check in raw_seqcount_begin

On Thu, Jun 05, 2014 at 11:31:01AM -0400, Trond Myklebust wrote:
> This commit reverts the addition of lockdep checking to raw_seqcount_begin
> for the following reasons:
> 
> 1) It violates the naming convention that raw_* functions should not
>    do lockdep checks (a convention that is also followed by the other
>    raw_*_seqcount_begin functions).
> 2) raw_seqcount_begin does not spin, so it can only be part of an ABBA
>    deadlock in very special circumstances (for instance if a lock
>    is held across the entire raw_seqcount_begin()+read_seqcount_retry()
>    loop while also being taken inside the write_seqcount protected area).
> 3) It is causing false positives with some existing callers, and there
>    is no non-lockdep alternative for those callers to use.
> 
> None of the three existing callers (__d_lookup_rcu, netdev_get_name, and
> the NFS state code) appear to use the function in a manner that is ABBA
> deadlock prone.
> 
> Fixes: 1ca7d67cf5d5: seqcount: Add lockdep functionality to seqcount/seqlock
> Cc: John Stultz <john.stultz@...aro.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Link: http://lkml.kernel.org/r/CAHQdGtRR6SvEhXiqWo24hoUh9AU9cL82Z8Z-d8-7u951F_d+5g@mail.gmail.com
> Signed-off-by: Trond Myklebust <trond.myklebust@...marydata.com>

Thanks Trond!
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