Re: [PATCH RFC] ext4: fix potential race between online resizing and write operations

[Joel Fernandes]

On Mon, Feb 24, 2020 at 10:55 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
[...]
> > > As for "task_struct's rcu_read_lock_nesting". Will it be enough just
> > > have a look at preempt_count of current process? If we have for example
> > > nested rcu_read_locks:
> > >
> > > <snip>
> > > rcu_read_lock()
> > >     rcu_read_lock()
> > >         rcu_read_lock()
> > > <snip>
> > >
> > > the counter would be 3.
> >
> > No, because preempt_count is not incremented during rcu_read_lock(). RCU
> > reader sections can be preempted, they just cannot goto sleep in a reader
> > section (unless the kernel is RT).
>
> You are both right.
>
> Vlad is correct for CONFIG_PREEMPT=n and CONFIG_DEBUG_ATOMIC_SLEEP=y
> and Joel is correct otherwise, give or take the possibility of other
> late-breaking corner cases.  ;-)

Oh yes, but even for PREEMPT=n, rcu_read_lock() is just a NOOP for
that configuration and doesn't really mess around with preempt_count
if I recall :-D. (doesn't need to mess with preempt_count because
being in kernel mode is non-preemptible for PREEMPT=n anyway).

thanks,

- Joel