linux-kernel - [PATCH 3.14 175/238] lockdep: Revert lockdep check in raw_seqcount

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Message-Id: <20141003212919.272272580@linuxfoundation.org>
Date:	Fri,  3 Oct 2014 14:31:30 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	Peter Zijlstra <peterz@...radead.org>,
	John Stultz <john.stultz@...aro.org>,
	"David S. Miller" <davem@...emloft.net>,
	Al Viro <viro@...iv.linux.org.uk>,
	Waiman Long <Waiman.Long@...com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: [PATCH 3.14 175/238] lockdep: Revert lockdep check in raw_seqcount_begin()

3.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Trond Myklebust <trond.myklebust@...marydata.com>

commit 22fdcf02f6e80d64a927f702dd9d631a927d87d4 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

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

--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -164,8 +164,6 @@ static inline unsigned read_seqcount_beg
 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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