Re: [PATCH 1/3] rcu: Use static initializer for krc.lock

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Apr 16, 2020 at 01:25:30PM -0700, Paul E. McKenney wrote:
> On Thu, Apr 16, 2020 at 10:05:57PM +0200, Uladzislau Rezki wrote:
> > On Thu, Apr 16, 2020 at 12:53:27PM -0700, Paul E. McKenney wrote:
> > > On Thu, Apr 16, 2020 at 03:26:23PM -0400, Steven Rostedt wrote:
> > > > On Thu, 16 Apr 2020 14:59:34 -0400
> > > > Joel Fernandes <joel@joelfernandes.org> wrote:
> > > > 
> > > > > But, then will it be safe for kfree_rcu() callers from hard IRQ context to
> > > > > call this in PREEMPT_RT? That could would just break then as you cannot sleep
> > > > > in hard IRQ context even on PREEMPT_RT.
> > > > 
> > > > But where in PREEMPT_RT would it be called in hard IRQ context?
> > > 
> > > I believe that call_rcu() is invoked with raw spinlocks held, so we should
> > > allow kfree_rcu() to be invoked from similar contexts.  It obviously
> > > cannot allocate memory in such contexts, so perhaps the rule is that
> > > single-argument kfree_rcu() cannot be invoked within hard IRQ contexts
> > > or with raw spinlocks held.  In those contexts, you would instead need
> > > to invoke two-argument kfree_rcu(), which never needs to allocate memory.
> > > 
> > > Seem reasonable?
> > > 
> > Paul, just to make it more clear, even invoking two arguments fkree_rcu()
> > currently does an allocation. We maintain an array that contains pointers
> > for "bulk logic". 
> 
> True, but that is an optimization rather than an absolute necessity.
> In addition, most two-argument kfree_rcu() callers will take another slot
> from the array that has already been allocated.  So one alternative is
> to do the allocation only if both interrupts and preemption are enabled.
> As long as most kfree_rcu() invocations can allocate, you get good
> performance and things work nicely in -rt.
> 
Agree it is not absolutely necessary :) The easiest way is something like that:

#ifdef CONFIG_PREEMPT_RT
    if (is_there_any_space_in_array())
        put_the_pointer_into_arry()
	finish.
    else
        queue_it_using_rcu_head_helper()
#endif

as for kfree_rcu() with one argument, we are not allowed to call it from atomic
context.

Thanks.

--
Vlad Rezki