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, 39 insertions, 4 deletions

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 25e9458da6a2..32a94b254d29 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -654,6 +654,41 @@ void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
 }
 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
 
+#ifdef CONFIG_NEED_SRCU_NMI_SAFE
+
+/*
+ * Counts the new reader in the appropriate per-CPU element of the
+ * srcu_struct, but in an NMI-safe manner using RMW atomics.
+ * Returns an index that must be passed to the matching srcu_read_unlock().
+ */
+int __srcu_read_lock_nmisafe(struct srcu_struct *ssp)
+{
+	int idx;
+	struct srcu_data *sdp = raw_cpu_ptr(ssp->sda);
+
+	idx = READ_ONCE(ssp->srcu_idx) & 0x1;
+	atomic_long_inc(&sdp->srcu_lock_count[idx]);
+	smp_mb__after_atomic(); /* B */  /* Avoid leaking the critical section. */
+	return idx;
+}
+EXPORT_SYMBOL_GPL(__srcu_read_lock_nmisafe);
+
+/*
+ * Removes the count for the old reader from the appropriate per-CPU
+ * element of the srcu_struct.  Note that this may well be a different
+ * CPU than that which was incremented by the corresponding srcu_read_lock().
+ */
+void __srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx)
+{
+	struct srcu_data *sdp = raw_cpu_ptr(ssp->sda);
+
+	smp_mb__before_atomic(); /* C */  /* Avoid leaking the critical section. */
+	atomic_long_inc(&sdp->srcu_unlock_count[idx]);
+}
+EXPORT_SYMBOL_GPL(__srcu_read_unlock_nmisafe);
+
+#endif // CONFIG_NEED_SRCU_NMI_SAFE
+
 /*
  * Start an SRCU grace period.
  */
@@ -1090,7 +1125,7 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
 	int ss_state;
 
 	check_init_srcu_struct(ssp);
-	idx = srcu_read_lock(ssp);
+	idx = __srcu_read_lock_nmisafe(ssp);
 	ss_state = smp_load_acquire(&ssp->srcu_size_state);
 	if (ss_state < SRCU_SIZE_WAIT_CALL)
 		sdp = per_cpu_ptr(ssp->sda, 0);
@@ -1123,7 +1158,7 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
 		srcu_funnel_gp_start(ssp, sdp, s, do_norm);
 	else if (needexp)
 		srcu_funnel_exp_start(ssp, sdp_mynode, s);
-	srcu_read_unlock(ssp, idx);
+	__srcu_read_unlock_nmisafe(ssp, idx);
 	return s;
 }
 
@@ -1427,13 +1462,13 @@ void srcu_barrier(struct srcu_struct *ssp)
 	/* Initial count prevents reaching zero until all CBs are posted. */
 	atomic_set(&ssp->srcu_barrier_cpu_cnt, 1);
 
-	idx = srcu_read_lock(ssp);
+	idx = __srcu_read_lock_nmisafe(ssp);
 	if (smp_load_acquire(&ssp->srcu_size_state) < SRCU_SIZE_WAIT_BARRIER)
 		srcu_barrier_one_cpu(ssp, per_cpu_ptr(ssp->sda, 0));
 	else
 		for_each_possible_cpu(cpu)
 			srcu_barrier_one_cpu(ssp, per_cpu_ptr(ssp->sda, cpu));
-	srcu_read_unlock(ssp, idx);
+	__srcu_read_unlock_nmisafe(ssp, idx);
 
 	/* Remove the initial count, at which point reaching zero can happen. */
 	if (atomic_dec_and_test(&ssp->srcu_barrier_cpu_cnt))