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

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Date:	Thu,  5 Jun 2014 11:31:01 -0400
From:	Trond Myklebust <trond.myklebust@...marydata.com>
To:	Peter Zijlstra <peterz@...radead.org>
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: [PATCH] Revert lockdep check in raw_seqcount_begin

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>
---
 include/linux/seqlock.h | 2 --
 1 file changed, 2 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 535f158977b9..8cf350325dc6 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -164,8 +164,6 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
 static inline unsigned raw_seqcount_begin(const seqcount_t *s)
 {
 	unsigned ret = ACCESS_ONCE(s->sequence);
-
-	seqcount_lockdep_reader_access(s);
 	smp_rmb();
 	return ret & ~1;
 }
-- 
1.9.3

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