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Subject[PATCH 3.16 173/233] rcu: Move preemption disabling out of __srcu_read_lock()
3.16.48-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

commit 49f5903b473c5f63f3b57856d1bd4593db0a2eef upstream.

Currently, __srcu_read_lock() cannot be invoked from restricted
environments because it contains calls to preempt_disable() and
preempt_enable(), both of which can invoke lockdep, which is a bad
idea in some restricted execution modes. This commit therefore moves
the preempt_disable() and preempt_enable() from __srcu_read_lock()
to srcu_read_lock(). It also inserts the preempt_disable() and
preempt_enable() around the call to __srcu_read_lock() in do_exit().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
[bwh: Backported to 3.16:
- Drop changes in do_exit()
- Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -217,8 +217,11 @@ static inline int srcu_read_lock_held(st
*/
static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
{
- int retval = __srcu_read_lock(sp);
+ int retval;

+ preempt_disable();
+ retval = __srcu_read_lock(sp);
+ preempt_enable();
rcu_lock_acquire(&(sp)->dep_map);
return retval;
}
--- a/kernel/rcu/srcu.c
+++ b/kernel/rcu/srcu.c
@@ -297,11 +297,9 @@ int __srcu_read_lock(struct srcu_struct
int idx;

idx = ACCESS_ONCE(sp->completed) & 0x1;
- preempt_disable();
ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
smp_mb(); /* B */ /* Avoid leaking the critical section. */
ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;
- preempt_enable();
return idx;
}
EXPORT_SYMBOL_GPL(__srcu_read_lock);
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