linux-kernel - [tip:locking/core] lockdep: Revert lockdep check in raw_seqcount

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Message-ID: <tip-22fdcf02f6e80d64a927f702dd9d631a927d87d4@git.kernel.org>
Date:	Thu, 19 Jun 2014 05:36:52 -0700
From:	tip-bot for Trond Myklebust <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
	torvalds@...ux-foundation.org, trond.myklebust@...marydata.com,
	peterz@...radead.org, sboyd@...eaurora.org,
	viro@...iv.linux.org.uk, davem@...emloft.net,
	john.stultz@...aro.org, Waiman.Long@...com, tglx@...utronix.de
Subject: [tip:locking/core] lockdep:
  Revert lockdep check in raw_seqcount_begin()

Commit-ID:  22fdcf02f6e80d64a927f702dd9d631a927d87d4
Gitweb:     http://git.kernel.org/tip/22fdcf02f6e80d64a927f702dd9d631a927d87d4
Author:     Trond Myklebust <trond.myklebust@...marydata.com>
AuthorDate: Thu, 5 Jun 2014 11:31:01 -0400
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 18 Jun 2014 18:41:23 +0200

lockdep: 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
Signed-off-by: Trond Myklebust <trond.myklebust@...marydata.com>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Cc: John Stultz <john.stultz@...aro.org>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Waiman Long <Waiman.Long@...com>
Cc: Stephen Boyd <sboyd@...eaurora.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Link: http://lkml.kernel.org/r/CAHQdGtRR6SvEhXiqWo24hoUh9AU9cL82Z8Z-d8-7u951F_d+5g@mail.gmail.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 include/linux/seqlock.h | 2 --
 1 file changed, 2 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 535f158..8cf3503 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;
 }
--
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