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    Subject[PATCH RFC tip/core/rcu 05/11] rcu: add comment saying why DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT.
    Date
    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>
    ---
    kernel/rcupdate.c | 5 +++++
    1 files changed, 5 insertions(+), 0 deletions(-)

    diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
    index afd21d1..f3240e9 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()) {
    --
    1.7.3.2


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