From: Paul E. McKenney Date: Mon, 31 Jan 2011 07:56:46 +0000 (-0800) Subject: rcu: add comment saying why DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT. X-Git-Tag: v2.6.39-rc1~493^2~1 X-Git-Url: https://asedeno.scripts.mit.edu/gitweb/?a=commitdiff_plain;h=e611eecd6f9f91d74beda8cfbb3b5e02abdeb5a1;p=linux.git 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 --- diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c index afd21d17c081..f3240e987928 100644 --- a/kernel/rcupdate.c +++ b/kernel/rcupdate.c @@ -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()) {