Re: [PATCH] Revert lockdep check in raw_seqcount_begin

From: Peter Zijlstra
Date: Fri Jun 06 2014 - 10:07:41 EST

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@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Link:
> Signed-off-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>

Thanks Trond!
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