The patch titled
Immunize rcu_dereference() against crazy compiler writers
has been added to the -mm tree. Its filename is
immunize-rcu_dereference-against-crazy-compiler-writers.patch
*** Remember to use Documentation/SubmitChecklist when testing your code ***
See http://www.zip.com.au/~akpm/linux/patches/stuff/added-to-mm.txt to find
out what to do about this
------------------------------------------------------
Subject: Immunize rcu_dereference() against crazy compiler writers
From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Turns out that compiler writers are a bit more aggressive about optimizing
than one might expect. This patch prevents a number of such optimizations
from messing up rcu_deference(). This is not merely a theoretical problem, as
evidenced by the rmb() in mce_log().
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/rcupdate.h | 14 +++++++++++++-
1 files changed, 13 insertions(+), 1 deletion(-)
diff -puN include/linux/rcupdate.h~immunize-rcu_dereference-against-crazy-compiler-writers include/linux/rcupdate.h
--- a/include/linux/rcupdate.h~immunize-rcu_dereference-against-crazy-compiler-writers
+++ a/include/linux/rcupdate.h
@@ -217,6 +217,18 @@ extern int rcu_needs_cpu(int cpu);
local_bh_enable(); \
} while(0)
+/*
+ * Prevent the compiler from merging or refetching accesses. The compiler
+ * is also forbidden from reordering successive instances of ACCESS_ONCE(),
+ * but only when the compiler is aware of some particular ordering. One way
+ * to make the compiler aware of ordering is to put the two invocations of
+ * ACCESS_ONCE() in different C statements.
+ *
+ * This macro does absolutely -nothing- to prevent the CPU from reordering,
+ * merging, or refetching absolutely anything at any time.
+ */
+#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+
/**
* rcu_dereference - fetch an RCU-protected pointer in an
* RCU read-side critical section. This pointer may later
@@ -228,7 +240,7 @@ extern int rcu_needs_cpu(int cpu);
*/
#define rcu_dereference(p) ({ \
- typeof(p) _________p1 = p; \
+ typeof(p) _________p1 = ACCESS_ONCE(p); \
smp_read_barrier_depends(); \
(_________p1); \
})
_
Patches currently in -mm which might be from paulmck@xxxxxxxxxxxxxxxxxx are
document-the-fact-that-rcu-callbacks-can-run-in-parallel.patch
immunize-rcu_dereference-against-crazy-compiler-writers.patch
remove-workaround-for-unimmunized-rcu_dereference-from-mce_log.patch
-
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