Re: [PATCH v4 3/3] rcu: Use _full() API to debug synchronize_rcu()

[Uladzislau Rezki <urezki@gmail.com>]

On Fri, Feb 28, 2025 at 10:21:57AM -0800, Paul E. McKenney wrote:
> On Fri, Feb 28, 2025 at 05:36:47PM +0100, Uladzislau Rezki wrote:
> > On Fri, Feb 28, 2025 at 07:41:40AM -0800, Paul E. McKenney wrote:
> > > On Thu, Feb 27, 2025 at 06:44:15PM +0100, Uladzislau Rezki wrote:
> > > > On Thu, Feb 27, 2025 at 09:26:40AM -0800, Paul E. McKenney wrote:
> > > > > On Thu, Feb 27, 2025 at 09:12:39AM -0800, Boqun Feng wrote:
> > > > > > Hi Ulad,
> > > > > > 
> > > > > > I put these three patches into next (and misc.2025.02.27a) for some
> > > > > > testing, hopefully it all goes well and they can make it v6.15.
> > > > > > 
> > > > > > A few tag changed below:
> > > > > > 
> > > > > > On Thu, Feb 27, 2025 at 02:16:13PM +0100, Uladzislau Rezki (Sony) wrote:
> > > > > > > Switch for using of get_state_synchronize_rcu_full() and
> > > > > > > poll_state_synchronize_rcu_full() pair to debug a normal
> > > > > > > synchronize_rcu() call.
> > > > > > > 
> > > > > > > Just using "not" full APIs to identify if a grace period is
> > > > > > > passed or not might lead to a false-positive kernel splat.
> > > > > > > 
> > > > > > > It can happen, because get_state_synchronize_rcu() compresses
> > > > > > > both normal and expedited states into one single unsigned long
> > > > > > > value, so a poll_state_synchronize_rcu() can miss GP-completion
> > > > > > > when synchronize_rcu()/synchronize_rcu_expedited() concurrently
> > > > > > > run.
> > > > > > > 
> > > > > > > To address this, switch to poll_state_synchronize_rcu_full() and
> > > > > > > get_state_synchronize_rcu_full() APIs, which use separate variables
> > > > > > > for expedited and normal states.
> > > > > > > 
> > > > > > > Link: https://lore.kernel.org/lkml/Z5ikQeVmVdsWQrdD@pc636/T/
> > > > > > 
> > > > > > I switch this into "Closes:" per checkpatch.
> > > > > > 
> > > > > > > Fixes: 988f569ae041 ("rcu: Reduce synchronize_rcu() latency")
> > > > > > > Reported-by: cheung wall <zzqq0103.hey@gmail.com>
> > > > > > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > > > > > 
> > > > > > You seem to forget add Paul's Reviewed-by, so I add it in rcu/next.
> > > > > > Would you or Paul double-check the Reviewed-by should be here?
> > > > > 
> > > > > I am good with keeping my Reviewed-by tags.
> > > > > 
> > > > Thanks Paul!
> > > 
> > > Except that I got this from overnight testing of rcu/dev on the shared
> > > RCU tree:
> > > 
> > > WARNING: CPU: 5 PID: 14 at kernel/rcu/tree.c:1636 rcu_sr_normal_complete+0x5c/0x80
> > > 
> > > I see this only on TREE05.  Which should not be too surprising, given
> > > that this is the scenario that tests it.  It happened within five minutes
> > > on all 14 of the TREE05 runs.
> > > 
> > Hm.. This is not fun. I tested this on my system and i did not manage to
> > trigger this whereas you do. Something is wrong.
> 
> If you have a debug patch, I would be happy to give it a go.
> 
I can trigger it. But.

Some background. I tested those patches during many hours on the stable
kernel which is 6.13. On that kernel i was not able to trigger it. Running
the rcutorture on the our shared "dev" tree, which i did now, triggers this
right away.

--
Uladzislau Rezki