[tip:locking/core] Documentation/atomic_t.txt: Clarify pure non-rmw usage

tip-bot for Peter Zijlstra
2019-06-03T13:47:09Z

Commit-ID: fff9b6c7d26943a8eb32b58364b7ec6b9369746a
Gitweb: https://git.kernel.org/tip/fff9b6c7d26943a8eb32b58364b7ec6b9369746a
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Fri, 24 May 2019 13:52:31 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 3 Jun 2019 12:32:57 +0200

Documentation/atomic_t.txt: Clarify pure non-rmw usage

Clarify that pure non-RMW usage of atomic_t is pointless, there is
nothing 'magical' about atomic_set() / atomic_read().

This is something that seems to confuse people, because I happen upon it
semi-regularly.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190524115231.GN2623@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
Documentation/atomic_t.txt | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
index dca3fb0554db..89eae7f6b360 100644
--- a/Documentation/atomic_t.txt
+++ b/Documentation/atomic_t.txt
@@ -81,9 +81,11 @@ Non-RMW ops:

The non-RMW ops are (typically) regular LOADs and STOREs and are canonically
implemented using READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and
-smp_store_release() respectively.
+smp_store_release() respectively. Therefore, if you find yourself only using
+the Non-RMW operations of atomic_t, you do not in fact need atomic_t at all
+and are doing it wrong.

-The one detail to this is that atomic_set{}() should be observable to the RMW
+A subtle detail of atomic_set{}() is that it should be observable to the RMW
ops. That is:

C atomic-set