rcu: Make Tiny call_rcu() safe to call from any context

Give Tiny call_rcu() the same treatment as Tree RCU.  When interrupts are
disabled and the scheduler is up, stage the callback on a lockless list
that an irq_work re-issues later.  One global list and irq_work suffice
since Tiny RCU is uniprocessor, and there is no CPU-offline drain.

The re-issue runs with interrupts disabled and can be re-entered by
instrumentation, so a draining flag drops a deferring call_rcu() seen
mid-drain (unless from an NMI), as in Tree RCU.  Gated by CONFIG_RCU_DEFER,
though the deferral state is unconditional.

Interrupts stay off for the whole batch, but the re-issue is a tail append
with no locks.  TINY_RCU implies !SMP, where arch_irq_work_has_interrupt()
is false, so the drain always waits for the tick and a batch is whatever
one tick's worth of interrupts-disabled call_rcu()s staged.  As in Tree
RCU the drain clears ->next before re-issuing, which bounds a node
self-linked by a double call_rcu(): rcu_do_enqueue()'s duplicate path
returns without clearing it.  A longer cycle is not bounded; a double
call_rcu() stays undefined.

The idle-task reschedule moves out of the enqueue helper so that a drain
does it once for the batch rather than once per callback, which would
otherwise take the runqueue lock N times with interrupts disabled.

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

0 files changed, 0 insertions, 0 deletions