linux-kernel - Inconsistent description in memory-barrier.txt

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Date:	Tue, 22 Dec 2015 14:21:31 +0800
From:	Jianyu Zhan <nasa4836@...il.com>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Inconsistent description in memory-barrier.txt

Hi, Paul,


I noticed that in the control dependency section in
memory-barrier.txt,  you mistakenly made an inconsistent
description:


On the description part:

 641 It is tempting to try to enforce ordering on identical stores on both
 642 branches of the "if" statement as follows:
 643
 644         q = READ_ONCE(a);
 645         if (q) {
 646                 barrier();
 647                 WRITE_ONCE(b, p);
 648                 do_something();
 649         } else {
 650                 barrier();
 651                 WRITE_ONCE(b, p);
 652                 do_something_else();
 653         }
 654
 655 Unfortunately, current compilers will transform this as follows at high
 656 optimization levels:
 657
 658         q = READ_ONCE(a);
 659         barrier();
 660         WRITE_ONCE(b, p);  /* BUG: No ordering vs. load from a!!! */
 661         if (q) {
 662                 /* WRITE_ONCE(b, p); -- moved up, BUG!!! */
 663                 do_something();
 664         } else {
 665                 /* WRITE_ONCE(b, p); -- moved up, BUG!!! */
 666                 do_something_else();
 667         }
 668

This part is incorporated in commit 2456d2a617de ("memory-barriers: Fix
 description of 2-legged-if-based control dependencies") on 2014-08-13.

However,  on the summary part:

 803   (*) If both legs of the "if" statement begin with identical stores
 804       to the same variable, a barrier() statement is required at the
 805       beginning of each leg of the "if" statement.


This part is incorporated in commit 9b2b3bf53124
("Documentation/memory-barriers.txt:
Need barriers() for some control dependencies"), on 2014-02-12.



I think you missed fixing the summary part?




Thanks,
Jianyu Zhan
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