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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2010-04-09 15:39:10 -0700
committerIngo Molnar <mingo@elte.hu>2010-04-14 12:19:51 +0200
commitb62730baea32f86fe91a7930e4b7ee8d82778b79 (patch)
tree97910bcb8a0790b10ff687dad901e158a2da63f4 /include
parent2ba3abd8186f24c7fb418927025b4e2120e3a362 (diff)
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rcu: Add rcu_access_pointer and rcu_dereference_protected
This patch adds variants of rcu_dereference() that handle situations where the RCU-protected data structure cannot change, perhaps due to our holding the update-side lock, or where the RCU-protected pointer is only to be fetched, not dereferenced. These are needed due to some performance concerns with using rcu_dereference() where it is not required, aside from the need for lockdep/sparse checking. The new rcu_access_pointer() primitive is for the case where the pointer is be fetch and not dereferenced. This primitive may be used without protection, RCU or otherwise, due to the fact that it uses ACCESS_ONCE(). The new rcu_dereference_protected() primitive is for the case where updates are prevented, for example, due to holding the update-side lock. This primitive does neither ACCESS_ONCE() nor smp_read_barrier_depends(), so can only be used when updates are somehow prevented. Suggested-by: David Howells <dhowells@redhat.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com Cc: eric.dumazet@gmail.com LKML-Reference: <1270852752-25278-1-git-send-email-paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'include')
-rw-r--r--include/linux/rcupdate.h32
1 files changed, 32 insertions, 0 deletions
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 872a98e13d6..8fe86609441 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -209,13 +209,45 @@ static inline int rcu_read_lock_sched_held(void)
rcu_dereference_raw(p); \
})
+/**
+ * rcu_dereference_protected - fetch RCU pointer when updates prevented
+ *
+ * Return the value of the specified RCU-protected pointer, but omit
+ * both the smp_read_barrier_depends() and the ACCESS_ONCE(). This
+ * is useful in cases where update-side locks prevent the value of the
+ * pointer from changing. Please note that this primitive does -not-
+ * prevent the compiler from repeating this reference or combining it
+ * with other references, so it should not be used without protection
+ * of appropriate locks.
+ */
+#define rcu_dereference_protected(p, c) \
+ ({ \
+ if (debug_lockdep_rcu_enabled() && !(c)) \
+ lockdep_rcu_dereference(__FILE__, __LINE__); \
+ (p); \
+ })
+
#else /* #ifdef CONFIG_PROVE_RCU */
#define rcu_dereference_check(p, c) rcu_dereference_raw(p)
+#define rcu_dereference_protected(p, c) (p)
#endif /* #else #ifdef CONFIG_PROVE_RCU */
/**
+ * rcu_access_pointer - fetch RCU pointer with no dereferencing
+ *
+ * Return the value of the specified RCU-protected pointer, but omit the
+ * smp_read_barrier_depends() and keep the ACCESS_ONCE(). This is useful
+ * when the value of this pointer is accessed, but the pointer is not
+ * dereferenced, for example, when testing an RCU-protected pointer against
+ * NULL. This may also be used in cases where update-side locks prevent
+ * the value of the pointer from changing, but rcu_dereference_protected()
+ * is a lighter-weight primitive for this use case.
+ */
+#define rcu_access_pointer(p) ACCESS_ONCE(p)
+
+/**
* rcu_read_lock - mark the beginning of an RCU read-side critical section.
*
* When synchronize_rcu() is invoked on one CPU while other CPUs