Commit 511a0868 authored by Lai Jiangshan's avatar Lai Jiangshan Committed by Paul E. McKenney
Browse files

srcu: Remove checks preventing idle CPUs from calling srcu_read_lock() 



SRCU has its own statemachine and no longer relies on normal RCU.
Its read-side critical section can now be used by an offline CPU, so this
commit removes the check and the comments, reverting the SRCU portion
of ff195cb6 (rcu: Warn when srcu_read_lock() is used in an extended
quiescent state).

It also makes the codes match the comments in whatisRCU.txt:

g.	Do you need read-side critical sections that are respected
	even though they are in the middle of the idle loop, during
	user-mode execution, or on an offlined CPU?  If so, SRCU is the
	only choice that will work for you.

[ paulmck: There is at least one remaining issue, namely use of lockdep
	   with tracing enabled. ]

Signed-off-by: default avatarLai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 3bc97a78

include/linux/srcu.h

+3 −18
Original line number Diff line number Diff line
@@ -151,25 +151,14 @@ void srcu_barrier(struct srcu_struct *sp); 
 * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot

 * and while lockdep is disabled.

 *

 * Note that if the CPU is in the idle loop from an RCU point of view

 * (ie: that we are in the section between rcu_idle_enter() and

 * rcu_idle_exit()) then srcu_read_lock_held() returns false even if

 * the CPU did an srcu_read_lock().  The reason for this is that RCU

 * ignores CPUs that are in such a section, considering these as in

 * extended quiescent state, so such a CPU is effectively never in an

 * RCU read-side critical section regardless of what RCU primitives it

 * invokes.  This state of affairs is required --- we need to keep an

 * RCU-free window in idle where the CPU may possibly enter into low

 * power mode. This way we can notice an extended quiescent state to

 * other CPUs that started a grace period. Otherwise we would delay any

 * grace period as long as we run in the idle task.

 * Note that SRCU is based on its own statemachine and it doesn't

 * relies on normal RCU, it can be called from the CPU which

 * is in the idle loop from an RCU point of view or offline.

 */

static inline int srcu_read_lock_held(struct srcu_struct *sp)

{

	if (!debug_lockdep_rcu_enabled())

		return 1;

	if (rcu_is_cpu_idle())

		return 0;

	return lock_is_held(&sp->dep_map);

}
@@ -231,8 +220,6 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp) 
	int retval = __srcu_read_lock(sp);



	rcu_lock_acquire(&(sp)->dep_map);

	rcu_lockdep_assert(!rcu_is_cpu_idle(),

			   "srcu_read_lock() used illegally while idle");

	return retval;

}
@@ -246,8 +233,6 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp) 
static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)

	__releases(sp)

{

	rcu_lockdep_assert(!rcu_is_cpu_idle(),

			   "srcu_read_unlock() used illegally while idle");

	rcu_lock_release(&(sp)->dep_map);

	__srcu_read_unlock(sp, idx);

}