linux-kernel - [PATCH tip/core/rcu 06/14] documentation: Subsequent writes ordered by rcu

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Date:	Tue, 23 Feb 2016 21:00:39 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 06/14] documentation: Subsequent writes ordered by rcu_dereference()

The current memory-barriers.txt does not address the possibility of
a write to a dereferenced pointer.  This should be rare, but when it
happens, we need that write -not- to be clobbered by the initialization.
This commit therefore adds an example showing a data dependency ordering
a later data-dependent write.

Reported-by: Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 Documentation/memory-barriers.txt | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 6bee0a2c43ab..e9ebeb3b1077 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -555,6 +555,30 @@ between the address load and the data load:
 This enforces the occurrence of one of the two implications, and prevents the
 third possibility from arising.
 
+A data-dependency barrier must also order against dependent writes:
+
+	CPU 1		      CPU 2
+	===============	      ===============
+	{ A == 1, B == 2, C = 3, P == &A, Q == &C }
+	B = 4;
+	<write barrier>
+	WRITE_ONCE(P, &B);
+			      Q = READ_ONCE(P);
+			      <data dependency barrier>
+			      *Q = 5;
+
+The data-dependency barrier must order the read into Q with the store
+into *Q.  This prohibits this outcome:
+
+	(Q == B) && (B == 4)
+
+Please note that this pattern should be rare.  After all, the whole point
+of dependency ordering is to -prevent- writes to the data structure, along
+with the expensive cache misses associated with those writes.  This pattern
+can be used to record rare error conditions and the like, and the ordering
+prevents such records from being lost.
+
+
 [!] Note that this extremely counterintuitive situation arises most easily on
 machines with split caches, so that, for example, one cache bank processes
 even-numbered cache lines and the other bank processes odd-numbered cache
-- 
2.5.2