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Commit 97b43032 authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Linus Torvalds
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Immunize rcu_dereference() against crazy compiler writers 



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: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: default avatarJosh Triplett <josh@kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent f6b450d4

include/linux/rcupdate.h

+13 −1
Original line number Diff line number Diff line
@@ -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); \

				})