srcu: Create an srcu_read_lock_nmisafe() and srcu_read_unlock_nmisafe()

On strict load-store architectures, the use of this_cpu_inc() by
srcu_read_lock() and srcu_read_unlock() is not NMI-safe in TREE SRCU.
To see this suppose that an NMI arrives in the middle of srcu_read_lock(),
just after it has read ->srcu_lock_count, but before it has written
the incremented value back to memory.  If that NMI handler also does
srcu_read_lock() and srcu_read_lock() on that same srcu_struct structure,
then upon return from that NMI handler, the interrupted srcu_read_lock()
will overwrite the NMI handler's update to ->srcu_lock_count, but
leave unchanged the NMI handler's update by srcu_read_unlock() to
->srcu_unlock_count.

This can result in a too-short SRCU grace period, which can in turn
result in arbitrary memory corruption.

If the NMI handler instead interrupts the srcu_read_unlock(), this
can result in eternal SRCU grace periods, which is not much better.

This commit therefore creates a pair of new srcu_read_lock_nmisafe()
and srcu_read_unlock_nmisafe() functions, which allow SRCU readers in
both NMI handlers and in process and IRQ context.  It is bad practice
to mix the existing and the new _nmisafe() primitives on the same
srcu_struct structure.  Use one set or the other, not both.

Just to underline that "bad practice" point, using srcu_read_lock() at
process level and srcu_read_lock_nmisafe() in your NMI handler will not,
repeat NOT, work.  If you do not immediately understand why this is the
case, please review the earlier paragraphs in this commit log.

[ paulmck: Apply kernel test robot feedback. ]
[ paulmck: Apply feedback from Randy Dunlap. ]
[ paulmck: Apply feedback from John Ogness. ]
[ paulmck: Apply feedback from Frederic Weisbecker. ]

Link: https://lore.kernel.org/all/20220910221947.171557773@linutronix.de/

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: John Ogness <john.ogness@linutronix.de>
Cc: Petr Mladek <pmladek@suse.com>

1 files changed, 51 insertions, 0 deletions

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 01226e4d960a..4fac088072c3 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -64,6 +64,20 @@ unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
 unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
 bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
 
+#ifdef CONFIG_NEED_SRCU_NMI_SAFE
+int __srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp);
+void __srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx) __releases(ssp);
+#else
+static inline int __srcu_read_lock_nmisafe(struct srcu_struct *ssp)
+{
+	return __srcu_read_lock(ssp);
+}
+static inline void __srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx)
+{
+	__srcu_read_unlock(ssp, idx);
+}
+#endif /* CONFIG_NEED_SRCU_NMI_SAFE */
+
 #ifdef CONFIG_SRCU
 void srcu_init(void);
 #else /* #ifdef CONFIG_SRCU */
@@ -166,6 +180,25 @@ static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp)
 	return retval;
 }
 
+/**
+ * srcu_read_lock_nmisafe - register a new reader for an SRCU-protected structure.
+ * @ssp: srcu_struct in which to register the new reader.
+ *
+ * Enter an SRCU read-side critical section, but in an NMI-safe manner.
+ * See srcu_read_lock() for more information.
+ */
+static inline int srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp)
+{
+	int retval;
+
+	if (IS_ENABLED(CONFIG_NEED_SRCU_NMI_SAFE))
+		retval = __srcu_read_lock_nmisafe(ssp);
+	else
+		retval = __srcu_read_lock(ssp);
+	rcu_lock_acquire(&(ssp)->dep_map);
+	return retval;
+}
+
 /* Used by tracing, cannot be traced and cannot invoke lockdep. */
 static inline notrace int
 srcu_read_lock_notrace(struct srcu_struct *ssp) __acquires(ssp)
@@ -191,6 +224,24 @@ static inline void srcu_read_unlock(struct srcu_struct *ssp, int idx)
 	__srcu_read_unlock(ssp, idx);
 }
 
+/**
+ * srcu_read_unlock_nmisafe - unregister a old reader from an SRCU-protected structure.
+ * @ssp: srcu_struct in which to unregister the old reader.
+ * @idx: return value from corresponding srcu_read_lock().
+ *
+ * Exit an SRCU read-side critical section, but in an NMI-safe manner.
+ */
+static inline void srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx)
+	__releases(ssp)
+{
+	WARN_ON_ONCE(idx & ~0x1);
+	rcu_lock_release(&(ssp)->dep_map);
+	if (IS_ENABLED(CONFIG_NEED_SRCU_NMI_SAFE))
+		__srcu_read_unlock_nmisafe(ssp, idx);
+	else
+		__srcu_read_unlock(ssp, idx);
+}
+
 /* Used by tracing, cannot be traced and cannot call lockdep. */
 static inline notrace void
 srcu_read_unlock_notrace(struct srcu_struct *ssp, int idx) __releases(ssp)