Re: srcu hung task panic

[Paul E. McKenney]

On Fri, Oct 26, 2018 at 04:00:53AM +0000, Krein, Dennis wrote:
> I have a patch attached that fixes the problem for us.  I also tried a
> version with an smb_mb() call added at end of rcu_segcblist_enqueue()
> - but that turned out not to be needed.  I think the key part of
> this is locking srcu_data in srcu_gp_start().  I also put in the
> preempt_disable/enable in __call_srcu() so that it couldn't get scheduled
> out and possibly moved to another CPU.  I had one hung task panic where
> the callback that would complete the wait was properly set up but for some
> reason the delayed work never happened.  Only thing I could determine to
> cause that was if __call_srcu() got switched out after dropping spin lock.

Good show!!!

You are quite right, the srcu_data structure's ->lock
must be held across the calls to rcu_segcblist_advance() and
rcu_segcblist_accelerate().  Color me blind, given that I repeatedly
looked at the "lockdep_assert_held(&ACCESS_PRIVATE(sp, lock));" and
repeatedly misread it as "lockdep_assert_held(&ACCESS_PRIVATE(sdp,
lock));".

A few questions and comments:

o	Are you OK with my adding your Signed-off-by as shown in the
	updated patch below?

o	I removed the #ifdefs because this is needed everywhere.
	However, I do agree that it can be quite helpful to use these
	while experimenting with different potential solutions.

o	Preemption is already disabled across all of srcu_gp_start()
	because the sp->lock is an interrupt-disabling lock.  This means
	that disabling preemption would have no effect.  I therefore
	removed the preempt_disable() and preempt_enable().

o	What sequence of events would lead to the work item never being
	executed?  Last I knew, workqueues were supposed to be robust
	against preemption.

I have added Christoph and Bart on CC (along with their Reported-by tags)
because they were recently seeing an intermittent failure that might
have been caused gby tyhis same bug.  Could you please check to see if
the below patch fixes your problem, give or take the workqueue issue?

							Thanx, Paul

------------------------------------------------------------------------

commit 1c1d315dfb7049d0233b89948a3fbcb61ea15d26
Author: Dennis Krein <Dennis.Krein@xxxxxxxxxx>
Date:   Fri Oct 26 07:38:24 2018 -0700

    srcu: Lock srcu_data structure in srcu_gp_start()
    
    The srcu_gp_start() function is called with the srcu_struct structure's
    ->lock held, but not with the srcu_data structure's ->lock.  This is
    problematic because this function accesses and updates the srcu_data
    structure's ->srcu_cblist, which is protected by that lock.  Failing to
    hold this lock can result in corruption of the SRCU callback lists,
    which in turn can result in arbitrarily bad results.
    
    This commit therefore makes srcu_gp_start() acquire the srcu_data
    structure's ->lock across the calls to rcu_segcblist_advance() and
    rcu_segcblist_accelerate(), thus preventing this corruption.
    
    Reported-by: Bart Van Assche <bvanassche@xxxxxxx>
    Reported-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
    Signed-off-by: Dennis Krein <Dennis.Krein@xxxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 60f3236beaf7..697a2d7e8e8a 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -451,10 +451,12 @@ static void srcu_gp_start(struct srcu_struct *sp)
 
 	lockdep_assert_held(&ACCESS_PRIVATE(sp, lock));
 	WARN_ON_ONCE(ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed));
+	spin_lock_rcu_node(sdp);  /* Interrupts already disabled. */
 	rcu_segcblist_advance(&sdp->srcu_cblist,
 			      rcu_seq_current(&sp->srcu_gp_seq));
 	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
 				       rcu_seq_snap(&sp->srcu_gp_seq));
+	spin_unlock_rcu_node(sdp);  /* Interrupts remain disabled. */
 	smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
 	rcu_seq_start(&sp->srcu_gp_seq);
 	state = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));