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

[Paul E. McKenney]

On Tue, Feb 25, 2020 at 09:17:11AM -0500, Joel Fernandes wrote:
> 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).

For PREEMPT=n, rcu_read_lock() is preempt_disable(), see the code
in include/linux/rcupdate.h.  ;-)

							Thanx, Paul