Commit 22fdcf02 authored Jun 05, 2014 by Trond Myklebust Committed by Ingo Molnar Jun 18, 2014

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: 1ca7d67c: seqcount: Add lockdep functionality to seqcount/seqlock
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Waiman Long <Waiman.Long@hp.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/CAHQdGtRR6SvEhXiqWo24hoUh9AU9cL82Z8Z-d8-7u951F_d+5g@mail.gmail.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 4f3aaf2c

include/linux/seqlock.h

+0 −2

Original line number	Diff line number	Diff line
		@@ -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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