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Subject[PATCH tip/core/rcu 31/48] rcu: define __rcu address space modifier for sparse
Date
This patch defines an __rcu annotation that permits sparse to check for
correct use of RCU-protected pointers. If a pointer that is annotated
with __rcu is accessed directly (as opposed to via rcu_dereference(),
rcu_assign_pointer(), or one of their variants), sparse can be made
to complain. To enable such complaints, use the new default-disabled
CONFIG_SPARSE_RCU_POINTER kernel configuration option. Please note that
these sparse complaints are intended to be a debugging aid, -not- a
code-style-enforcement mechanism.

There are special rcu_dereference_protected() and rcu_access_pointer()
accessors for use when RCU read-side protection is not required, for
example, when no other CPU has access to the data structure in question
or while the current CPU hold the update-side lock.

This patch also updates a number of docbook comments that were showing
their age.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Christopher Li <sparse@chrisli.org>
Cc: Josh Triplett <josh@joshtriplett.org>
---
include/linux/compiler.h | 6 +
include/linux/rcupdate.h | 354 ++++++++++++++++++++++++++++------------------
include/linux/srcu.h | 27 +++-
kernel/rcupdate.c | 6 +-
lib/Kconfig.debug | 13 ++
5 files changed, 260 insertions(+), 146 deletions(-)
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index a5a472b..320d6c9 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -16,6 +16,11 @@
# define __release(x) __context__(x,-1)
# define __cond_lock(x,c) ((c) ? ({ __acquire(x); 1; }) : 0)
# define __percpu __attribute__((noderef, address_space(3)))
+#ifdef CONFIG_SPARSE_RCU_POINTER
+# define __rcu __attribute__((noderef, address_space(4)))
+#else
+# define __rcu
+#endif
extern void __chk_user_ptr(const volatile void __user *);
extern void __chk_io_ptr(const volatile void __iomem *);
#else
@@ -34,6 +39,7 @@ extern void __chk_io_ptr(const volatile void __iomem *);
# define __release(x) (void)0
# define __cond_lock(x,c) (c)
# define __percpu
+# define __rcu
#endif

#ifdef __KERNEL__
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 3be0ad7..d922b2e 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -41,6 +41,7 @@
#include <linux/lockdep.h>
#include <linux/completion.h>
#include <linux/debugobjects.h>
+#include <linux/compiler.h>

#ifdef CONFIG_RCU_TORTURE_TEST
extern int rcutorture_runnable; /* for sysctl */
@@ -114,14 +115,15 @@ extern struct lockdep_map rcu_sched_lock_map;
extern int debug_lockdep_rcu_enabled(void);

/**
- * rcu_read_lock_held - might we be in RCU read-side critical section?
+ * rcu_read_lock_held() - might we be in RCU read-side critical section?
*
* If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU
* read-side critical section. In absence of CONFIG_DEBUG_LOCK_ALLOC,
* this assumes we are in an RCU read-side critical section unless it can
- * prove otherwise.
+ * prove otherwise. This is useful for debug checks in functions that
+ * require that they be called within an RCU read-side critical section.
*
- * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
+ * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot
* and while lockdep is disabled.
*/
static inline int rcu_read_lock_held(void)
@@ -138,14 +140,16 @@ static inline int rcu_read_lock_held(void)
extern int rcu_read_lock_bh_held(void);

/**
- * rcu_read_lock_sched_held - might we be in RCU-sched read-side critical section?
+ * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
*
* If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an
* RCU-sched read-side critical section. In absence of
* CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
* critical section unless it can prove otherwise. Note that disabling
* of preemption (including disabling irqs) counts as an RCU-sched
- * read-side critical section.
+ * read-side critical section. This is useful for debug checks in functions
+ * that required that they be called within an RCU-sched read-side
+ * critical section.
*
* Check debug_lockdep_rcu_enabled() to prevent false positives during boot
* and while lockdep is disabled.
@@ -214,41 +218,155 @@ extern int rcu_my_thread_group_empty(void);
} \
} while (0)

+#else /* #ifdef CONFIG_PROVE_RCU */
+
+#define __do_rcu_dereference_check(c) do { } while (0)
+
+#endif /* #else #ifdef CONFIG_PROVE_RCU */
+
+/*
+ * Helper functions for rcu_dereference_check(), rcu_dereference_protected()
+ * and rcu_assign_pointer(). Some of these could be folded into their
+ * callers, but they are left separate in order to ease introduction of
+ * multiple flavors of pointers to match the multiple flavors of RCU
+ * (e.g., __rcu_bh, * __rcu_sched, and __srcu), should this make sense in
+ * the future.
+ */
+#define __rcu_access_pointer(p, space) \
+ ({ \
+ typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
+ (void) (((typeof (*p) space *)p) == p); \
+ ((typeof(*p) __force __kernel *)(_________p1)); \
+ })
+#define __rcu_dereference_check(p, c, space) \
+ ({ \
+ typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
+ __do_rcu_dereference_check(c); \
+ (void) (((typeof (*p) space *)p) == p); \
+ smp_read_barrier_depends(); \
+ ((typeof(*p) __force __kernel *)(_________p1)); \
+ })
+#define __rcu_dereference_protected(p, c, space) \
+ ({ \
+ __do_rcu_dereference_check(c); \
+ (void) (((typeof (*p) space *)p) == p); \
+ ((typeof(*p) __force __kernel *)(p)); \
+ })
+
+#define __rcu_dereference_index_check(p, c) \
+ ({ \
+ typeof(p) _________p1 = ACCESS_ONCE(p); \
+ __do_rcu_dereference_check(c); \
+ smp_read_barrier_depends(); \
+ (_________p1); \
+ })
+#define __rcu_assign_pointer(p, v, space) \
+ ({ \
+ if (!__builtin_constant_p(v) || \
+ ((v) != NULL)) \
+ smp_wmb(); \
+ (p) = (typeof(*v) __force space *)(v); \
+ })
+
+
+/**
+ * rcu_access_pointer() - fetch RCU pointer with no dereferencing
+ * @p: The pointer to read
+ *
+ * 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. Although rcu_access_pointer() may also be used in cases where
+ * update-side locks prevent the value of the pointer from changing, you
+ * should instead use rcu_dereference_protected() for this use case.
+ */
+#define rcu_access_pointer(p) __rcu_access_pointer((p), __rcu)
+
/**
- * rcu_dereference_check - rcu_dereference with debug checking
+ * rcu_dereference_check() - rcu_dereference with debug checking
* @p: The pointer to read, prior to dereferencing
* @c: The conditions under which the dereference will take place
*
* Do an rcu_dereference(), but check that the conditions under which the
- * dereference will take place are correct. Typically the conditions indicate
- * the various locking conditions that should be held at that point. The check
- * should return true if the conditions are satisfied.
+ * dereference will take place are correct. Typically the conditions
+ * indicate the various locking conditions that should be held at that
+ * point. The check should return true if the conditions are satisfied.
+ * An implicit check for being in an RCU read-side critical section
+ * (rcu_read_lock()) is included.
*
* For example:
*
- * bar = rcu_dereference_check(foo->bar, rcu_read_lock_held() ||
- * lockdep_is_held(&foo->lock));
+ * bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock));
*
* could be used to indicate to lockdep that foo->bar may only be dereferenced
- * if either the RCU read lock is held, or that the lock required to replace
+ * if either rcu_read_lock() is held, or that the lock required to replace
* the bar struct at foo->bar is held.
*
* Note that the list of conditions may also include indications of when a lock
* need not be held, for example during initialisation or destruction of the
* target struct:
*
- * bar = rcu_dereference_check(foo->bar, rcu_read_lock_held() ||
- * lockdep_is_held(&foo->lock) ||
+ * bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock) ||
* atomic_read(&foo->usage) == 0);
+ *
+ * Inserts memory barriers on architectures that require them
+ * (currently only the Alpha), prevents the compiler from refetching
+ * (and from merging fetches), and, more importantly, documents exactly
+ * which pointers are protected by RCU and checks that the pointer is
+ * annotated as __rcu.
*/
#define rcu_dereference_check(p, c) \
- ({ \
- __do_rcu_dereference_check(c); \
- rcu_dereference_raw(p); \
- })
+ __rcu_dereference_check((p), rcu_read_lock_held() || (c), __rcu)
+
+/**
+ * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking
+ * @p: The pointer to read, prior to dereferencing
+ * @c: The conditions under which the dereference will take place
+ *
+ * This is the RCU-bh counterpart to rcu_dereference_check().
+ */
+#define rcu_dereference_bh_check(p, c) \
+ __rcu_dereference_check((p), rcu_read_lock_bh_held() || (c), __rcu)

/**
- * rcu_dereference_protected - fetch RCU pointer when updates prevented
+ * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking
+ * @p: The pointer to read, prior to dereferencing
+ * @c: The conditions under which the dereference will take place
+ *
+ * This is the RCU-sched counterpart to rcu_dereference_check().
+ */
+#define rcu_dereference_sched_check(p, c) \
+ __rcu_dereference_check((p), rcu_read_lock_sched_held() || (c), \
+ __rcu)
+
+#define rcu_dereference_raw(p) rcu_dereference_check(p, 1) /*@@@ needed? @@@*/
+
+/**
+ * rcu_dereference_index_check() - rcu_dereference for indices with debug checking
+ * @p: The pointer to read, prior to dereferencing
+ * @c: The conditions under which the dereference will take place
+ *
+ * Similar to rcu_dereference_check(), but omits the sparse checking.
+ * This allows rcu_dereference_index_check() to be used on integers,
+ * which can then be used as array indices. Attempting to use
+ * rcu_dereference_check() on an integer will give compiler warnings
+ * because the sparse address-space mechanism relies on dereferencing
+ * the RCU-protected pointer. Dereferencing integers is not something
+ * that even gcc will put up with.
+ *
+ * Note that this function does not implicitly check for RCU read-side
+ * critical sections. If this function gains lots of uses, it might
+ * make sense to provide versions for each flavor of RCU, but it does
+ * not make sense as of early 2010.
+ */
+#define rcu_dereference_index_check(p, c) \
+ __rcu_dereference_index_check((p), (c))
+
+/**
+ * rcu_dereference_protected() - fetch RCU pointer when updates prevented
+ * @p: The pointer to read, prior to dereferencing
+ * @c: The conditions under which the dereference will take place
*
* Return the value of the specified RCU-protected pointer, but omit
* both the smp_read_barrier_depends() and the ACCESS_ONCE(). This
@@ -257,35 +375,61 @@ extern int rcu_my_thread_group_empty(void);
* prevent the compiler from repeating this reference or combining it
* with other references, so it should not be used without protection
* of appropriate locks.
+ *
+ * This function is only for update-side use. Using this function
+ * when protected only by rcu_read_lock() will result in infrequent
+ * but very ugly failures.
*/
#define rcu_dereference_protected(p, c) \
- ({ \
- __do_rcu_dereference_check(c); \
- (p); \
- })
+ __rcu_dereference_protected((p), (c), __rcu)

-#else /* #ifdef CONFIG_PROVE_RCU */
+/**
+ * rcu_dereference_bh_protected() - fetch RCU-bh pointer when updates prevented
+ * @p: The pointer to read, prior to dereferencing
+ * @c: The conditions under which the dereference will take place
+ *
+ * This is the RCU-bh counterpart to rcu_dereference_protected().
+ */
+#define rcu_dereference_bh_protected(p, c) \
+ __rcu_dereference_protected((p), (c), __rcu)

-#define rcu_dereference_check(p, c) rcu_dereference_raw(p)
-#define rcu_dereference_protected(p, c) (p)
+/**
+ * rcu_dereference_sched_protected() - fetch RCU-sched pointer when updates prevented
+ * @p: The pointer to read, prior to dereferencing
+ * @c: The conditions under which the dereference will take place
+ *
+ * This is the RCU-sched counterpart to rcu_dereference_protected().
+ */
+#define rcu_dereference_sched_protected(p, c) \
+ __rcu_dereference_protected((p), (c), __rcu)

-#endif /* #else #ifdef CONFIG_PROVE_RCU */

/**
- * rcu_access_pointer - fetch RCU pointer with no dereferencing
+ * rcu_dereference() - fetch RCU-protected pointer for dereferencing
+ * @p: The pointer to read, prior to 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.
+ * This is a simple wrapper around rcu_dereference_check().
*/
-#define rcu_access_pointer(p) ACCESS_ONCE(p)
+#define rcu_dereference(p) rcu_dereference_check(p, 0)

/**
- * rcu_read_lock - mark the beginning of an RCU read-side critical section.
+ * rcu_dereference_bh() - fetch an RCU-bh-protected pointer for dereferencing
+ * @p: The pointer to read, prior to dereferencing
+ *
+ * Makes rcu_dereference_check() do the dirty work.
+ */
+#define rcu_dereference_bh(p) rcu_dereference_bh_check(p, 0)
+
+/**
+ * rcu_dereference_sched() - fetch RCU-sched-protected pointer for dereferencing
+ * @p: The pointer to read, prior to dereferencing
+ *
+ * Makes rcu_dereference_check() do the dirty work.
+ */
+#define rcu_dereference_sched(p) rcu_dereference_sched_check(p, 0)
+
+/**
+ * rcu_read_lock() - mark the beginning of an RCU read-side critical section
*
* When synchronize_rcu() is invoked on one CPU while other CPUs
* are within RCU read-side critical sections, then the
@@ -331,7 +475,7 @@ static inline void rcu_read_lock(void)
*/

/**
- * rcu_read_unlock - marks the end of an RCU read-side critical section.
+ * rcu_read_unlock() - marks the end of an RCU read-side critical section.
*
* See rcu_read_lock() for more information.
*/
@@ -343,15 +487,16 @@ static inline void rcu_read_unlock(void)
}

/**
- * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical section
+ * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
*
* This is equivalent of rcu_read_lock(), but to be used when updates
- * are being done using call_rcu_bh(). Since call_rcu_bh() callbacks
- * consider completion of a softirq handler to be a quiescent state,
- * a process in RCU read-side critical section must be protected by
- * disabling softirqs. Read-side critical sections in interrupt context
- * can use just rcu_read_lock().
- *
+ * are being done using call_rcu_bh() or synchronize_rcu_bh(). Since
+ * both call_rcu_bh() and synchronize_rcu_bh() consider completion of a
+ * softirq handler to be a quiescent state, a process in RCU read-side
+ * critical section must be protected by disabling softirqs. Read-side
+ * critical sections in interrupt context can use just rcu_read_lock(),
+ * though this should at least be commented to avoid confusing people
+ * reading the code.
*/
static inline void rcu_read_lock_bh(void)
{
@@ -373,13 +518,12 @@ static inline void rcu_read_unlock_bh(void)
}

/**
- * rcu_read_lock_sched - mark the beginning of a RCU-classic critical section
+ * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
*
- * Should be used with either
- * - synchronize_sched()
- * or
- * - call_rcu_sched() and rcu_barrier_sched()
- * on the write-side to insure proper synchronization.
+ * This is equivalent of rcu_read_lock(), but to be used when updates
+ * are being done using call_rcu_sched() or synchronize_rcu_sched().
+ * Read-side critical sections can also be introduced by anything that
+ * disables preemption, including local_irq_disable() and friends.
*/
static inline void rcu_read_lock_sched(void)
{
@@ -414,54 +558,14 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
preempt_enable_notrace();
}

-
-/**
- * rcu_dereference_raw - fetch an RCU-protected pointer
- *
- * The caller must be within some flavor of RCU read-side critical
- * section, or must be otherwise preventing the pointer from changing,
- * for example, by holding an appropriate lock. This pointer may later
- * be safely dereferenced. It is the caller's responsibility to have
- * done the right thing, as this primitive does no checking of any kind.
- *
- * Inserts memory barriers on architectures that require them
- * (currently only the Alpha), and, more importantly, documents
- * exactly which pointers are protected by RCU.
- */
-#define rcu_dereference_raw(p) ({ \
- typeof(p) _________p1 = ACCESS_ONCE(p); \
- smp_read_barrier_depends(); \
- (_________p1); \
- })
-
-/**
- * rcu_dereference - fetch an RCU-protected pointer, checking for RCU
- *
- * Makes rcu_dereference_check() do the dirty work.
- */
-#define rcu_dereference(p) \
- rcu_dereference_check(p, rcu_read_lock_held())
-
-/**
- * rcu_dereference_bh - fetch an RCU-protected pointer, checking for RCU-bh
- *
- * Makes rcu_dereference_check() do the dirty work.
- */
-#define rcu_dereference_bh(p) \
- rcu_dereference_check(p, rcu_read_lock_bh_held())
-
-/**
- * rcu_dereference_sched - fetch RCU-protected pointer, checking for RCU-sched
- *
- * Makes rcu_dereference_check() do the dirty work.
- */
-#define rcu_dereference_sched(p) \
- rcu_dereference_check(p, rcu_read_lock_sched_held())
-
/**
- * rcu_assign_pointer - assign (publicize) a pointer to a newly
- * initialized structure that will be dereferenced by RCU read-side
- * critical sections. Returns the value assigned.
+ * rcu_assign_pointer() - assign to RCU-protected pointer
+ * @p: pointer to assign to
+ * @v: value to assign (publish)
+ *
+ * Assigns the specified value to the specified RCU-protected
+ * pointer, ensuring that any concurrent RCU readers will see
+ * any prior initialization. Returns the value assigned.
*
* Inserts memory barriers on architectures that require them
* (pretty much all of them other than x86), and also prevents
@@ -470,14 +574,17 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
* call documents which pointers will be dereferenced by RCU read-side
* code.
*/
-
#define rcu_assign_pointer(p, v) \
- ({ \
- if (!__builtin_constant_p(v) || \
- ((v) != NULL)) \
- smp_wmb(); \
- (p) = (v); \
- })
+ __rcu_assign_pointer((p), (v), __rcu)
+
+/**
+ * RCU_INIT_POINTER() - initialize an RCU protected pointer
+ *
+ * Initialize an RCU-protected pointer in such a way to avoid RCU-lockdep
+ * splats.
+ */
+#define RCU_INIT_POINTER(p, v) \
+ p = (typeof(*v) __force __rcu *)(v)

/* Infrastructure to implement the synchronize_() primitives. */

@@ -489,7 +596,7 @@ struct rcu_synchronize {
extern void wakeme_after_rcu(struct rcu_head *head);

/**
- * call_rcu - Queue an RCU callback for invocation after a grace period.
+ * call_rcu() - Queue an RCU callback for invocation after a grace period.
* @head: structure to be used for queueing the RCU updates.
* @func: actual update function to be invoked after the grace period
*
@@ -503,7 +610,7 @@ extern void call_rcu(struct rcu_head *head,
void (*func)(struct rcu_head *head));

/**
- * call_rcu_bh - Queue an RCU for invocation after a quicker grace period.
+ * call_rcu_bh() - Queue an RCU for invocation after a quicker grace period.
* @head: structure to be used for queueing the RCU updates.
* @func: actual update function to be invoked after the grace period
*
@@ -560,37 +667,4 @@ static inline void debug_rcu_head_unqueue(struct rcu_head *head)
}
#endif /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */

-#ifndef CONFIG_PROVE_RCU
-#define __do_rcu_dereference_check(c) do { } while (0)
-#endif /* #ifdef CONFIG_PROVE_RCU */
-
-#define __rcu_dereference_index_check(p, c) \
- ({ \
- typeof(p) _________p1 = ACCESS_ONCE(p); \
- __do_rcu_dereference_check(c); \
- smp_read_barrier_depends(); \
- (_________p1); \
- })
-
-/**
- * rcu_dereference_index_check() - rcu_dereference for indices with debug checking
- * @p: The pointer to read, prior to dereferencing
- * @c: The conditions under which the dereference will take place
- *
- * Similar to rcu_dereference_check(), but omits the sparse checking.
- * This allows rcu_dereference_index_check() to be used on integers,
- * which can then be used as array indices. Attempting to use
- * rcu_dereference_check() on an integer will give compiler warnings
- * because the sparse address-space mechanism relies on dereferencing
- * the RCU-protected pointer. Dereferencing integers is not something
- * that even gcc will put up with.
- *
- * Note that this function does not implicitly check for RCU read-side
- * critical sections. If this function gains lots of uses, it might
- * make sense to provide versions for each flavor of RCU, but it does
- * not make sense as of early 2010.
- */
-#define rcu_dereference_index_check(p, c) \
- __rcu_dereference_index_check((p), (c))
-
#endif /* __LINUX_RCUPDATE_H */
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 4d5d2f5..6f456a7 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -108,12 +108,31 @@ static inline int srcu_read_lock_held(struct srcu_struct *sp)
#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */

/**
- * srcu_dereference - fetch SRCU-protected pointer with checking
+ * srcu_dereference_check - fetch SRCU-protected pointer for later dereferencing
+ * @p: the pointer to fetch and protect for later dereferencing
+ * @sp: pointer to the srcu_struct, which is used to check that we
+ * really are in an SRCU read-side critical section.
+ * @c: condition to check for update-side use
*
- * Makes rcu_dereference_check() do the dirty work.
+ * If PROVE_RCU is enabled, invoking this outside of an RCU read-side
+ * critical section will result in an RCU-lockdep splat, unless @c evaluates
+ * to 1. The @c argument will normally be a logical expression containing
+ * lockdep_is_held() calls.
*/
-#define srcu_dereference(p, sp) \
- rcu_dereference_check(p, srcu_read_lock_held(sp))
+#define srcu_dereference_check(p, sp, c) \
+ __rcu_dereference_check((p), srcu_read_lock_held(sp) || (c), __rcu)
+
+/**
+ * srcu_dereference - fetch SRCU-protected pointer for later dereferencing
+ * @p: the pointer to fetch and protect for later dereferencing
+ * @sp: pointer to the srcu_struct, which is used to check that we
+ * really are in an SRCU read-side critical section.
+ *
+ * Makes rcu_dereference_check() do the dirty work. If PROVE_RCU
+ * is enabled, invoking this outside of an RCU read-side critical
+ * section will result in an RCU-lockdep splat.
+ */
+#define srcu_dereference(p, sp) srcu_dereference_check((p), (sp), 0)

/**
* srcu_read_lock - register a new reader for an SRCU-protected structure.
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index be12514..10d1dfa 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -73,12 +73,14 @@ int debug_lockdep_rcu_enabled(void)
EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);

/**
- * rcu_read_lock_bh_held - might we be in RCU-bh read-side critical section?
+ * rcu_read_lock_bh_held() - might we be in RCU-bh read-side critical section?
*
* Check for bottom half being disabled, which covers both the
* CONFIG_PROVE_RCU and not cases. Note that if someone uses
* rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled)
- * will show the situation.
+ * will show the situation. This is useful for debug checks in functions
+ * that require that they be called within an RCU read-side critical
+ * section.
*
* Check debug_lockdep_rcu_enabled() to prevent false positives during boot.
*/
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 8689646..7698752 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -528,6 +528,19 @@ config PROVE_RCU_REPEATEDLY
disabling, allowing multiple RCU-lockdep warnings to be printed
on a single reboot.

+config SPARSE_RCU_POINTER
+ bool "RCU debugging: sparse-based checks for pointer usage"
+ default n
+ help
+ This feature enables the __rcu sparse annotation for
+ RCU-protected pointers. This annotation will cause sparse
+ to flag any non-RCU used of annotated pointers. This can be
+ helpful when debugging RCU usage. Please note that this feature
+ is not intended to enforce code cleanliness; it is instead merely
+ a debugging aid.
+
+ Say Y to make sparse flag questionable use of RCU-protected pointers
+
Say N if you are unsure.

config LOCKDEP
--
1.7.0


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