The patch titled
Immunize rcu_dereference() against crazy compiler writers
has been removed from the -mm tree. Its filename was
immunize-rcu_dereference-against-crazy-compiler-writers.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
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>
Acked-by: Josh Triplett <josh@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/rcupdate.h | 14 +++++++++++++-
1 file 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
@@ -231,6 +231,18 @@ extern struct lockdep_map rcu_lock_map;
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
@@ -242,7 +254,7 @@ extern struct lockdep_map rcu_lock_map;
*/
#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
origin.patch
make-access-to-tasks-nsproxy-lighter.patch
make-access-to-tasks-nsproxy-lighterpatch-breaks-unshare.patch
make-access-to-tasks-nsproxy-lighter-update-get_net_ns_by_pid.patch
-
To unsubscribe from this list: send the line "unsubscribe mm-commits" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
