rcu: Restore barrier() to rcu_read_lock() and rcu_read_unlock()

Commit bb73c52bad36 ("rcu: Don't disable preemption for Tiny and Tree
RCU readers") removed the barrier() calls from rcu_read_lock() and
rcu_write_lock() in CONFIG_PREEMPT=n&&CONFIG_PREEMPT_COUNT=n kernels.
Within RCU, this commit was OK, but it failed to account for things like
get_user() that can pagefault and that can be reordered by the compiler.
Lack of the barrier() calls in rcu_read_lock() and rcu_read_unlock()
can cause these page faults to migrate into RCU read-side critical
sections, which in CONFIG_PREEMPT=n kernels could result in too-short
grace periods and arbitrary misbehavior.  Please see commit 386afc91144b
("spinlocks and preemption points need to be at least compiler barriers")
and Linus's commit 66be4e66a7f4 ("rcu: locking and unlocking need to
always be at least barriers"), this last of which restores the barrier()
call to both rcu_read_lock() and rcu_read_unlock().

This commit removes barrier() calls that are no longer needed given that
the addition of them in Linus's commit noted above.  The combination of
this commit and Linus's commit effectively reverts commit bb73c52bad36
("rcu: Don't disable preemption for Tiny and Tree RCU readers").

Reported-by: Herbert Xu <herbert@gondor.apana.org.au>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
[ paulmck: Fix embarrassing typo located by Alan Stern. ]

1 files changed, 0 insertions, 11 deletions

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index acb225023ed1..3f1b5041de9b 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -288,7 +288,6 @@ void rcu_note_context_switch(bool preempt)
 	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 	struct rcu_node *rnp;
 
-	barrier(); /* Avoid RCU read-side critical sections leaking down. */
 	trace_rcu_utilization(TPS("Start context switch"));
 	lockdep_assert_irqs_disabled();
 	WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0);
@@ -340,7 +339,6 @@ void rcu_note_context_switch(bool preempt)
 	if (rdp->exp_deferred_qs)
 		rcu_report_exp_rdp(rdp);
 	trace_rcu_utilization(TPS("End context switch"));
-	barrier(); /* Avoid RCU read-side critical sections leaking up. */
 }
 EXPORT_SYMBOL_GPL(rcu_note_context_switch);
 
@@ -828,11 +826,6 @@ static void rcu_qs(void)
  * dyntick-idle quiescent state visible to other CPUs, which will in
  * some cases serve for expedited as well as normal grace periods.
  * Either way, register a lightweight quiescent state.
- *
- * The barrier() calls are redundant in the common case when this is
- * called externally, but just in case this is called from within this
- * file.
- *
  */
 void rcu_all_qs(void)
 {
@@ -847,14 +840,12 @@ void rcu_all_qs(void)
 		return;
 	}
 	this_cpu_write(rcu_data.rcu_urgent_qs, false);
-	barrier(); /* Avoid RCU read-side critical sections leaking down. */
 	if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs))) {
 		local_irq_save(flags);
 		rcu_momentary_dyntick_idle();
 		local_irq_restore(flags);
 	}
 	rcu_qs();
-	barrier(); /* Avoid RCU read-side critical sections leaking up. */
 	preempt_enable();
 }
 EXPORT_SYMBOL_GPL(rcu_all_qs);
@@ -864,7 +855,6 @@ EXPORT_SYMBOL_GPL(rcu_all_qs);
  */
 void rcu_note_context_switch(bool preempt)
 {
-	barrier(); /* Avoid RCU read-side critical sections leaking down. */
 	trace_rcu_utilization(TPS("Start context switch"));
 	rcu_qs();
 	/* Load rcu_urgent_qs before other flags. */
@@ -877,7 +867,6 @@ void rcu_note_context_switch(bool preempt)
 		rcu_tasks_qs(current);
 out:
 	trace_rcu_utilization(TPS("End context switch"));
-	barrier(); /* Avoid RCU read-side critical sections leaking up. */
 }
 EXPORT_SYMBOL_GPL(rcu_note_context_switch);