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rcu: add comment saying why DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT. 

The build will break if you change the Kconfig to allow
DEBUG_OBJECTS_RCU_HEAD and !PREEMPT, so document the reasoning
near where the breakage would occur.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2011-01-30 23:56:46 -08:00
parent fe8e64071a
commit e611eecd6f
1 changed files with 5 additions and 0 deletions
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5
kernel/rcupdate.c
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@ -214,6 +214,11 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state)
* Ensure that queued callbacks are all executed.
* If we detect that we are nested in a RCU read-side critical
* section, we should simply fail, otherwise we would deadlock.
* Note that the machinery to reliably determine whether
* or not we are in an RCU read-side critical section
* exists only in the preemptible RCU implementations
* (TINY_PREEMPT_RCU and TREE_PREEMPT_RCU), which is why
* DEBUG_OBJECTS_RCU_HEAD is disallowed if !PREEMPT.
*/
if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
irqs_disabled()) {