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Date:   Mon, 17 Feb 2020 10:28:31 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Amol Grover <frextrite@...il.com>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Madhuparna Bhowmik <madhuparnabhowmik04@...il.com>
Subject: Re: [PATCH RESEND] lockdep: Pass lockdep expression to RCU lists

On Mon, Feb 17, 2020 at 04:12:46PM +0100, Peter Zijlstra wrote:
> On Sun, Feb 16, 2020 at 01:16:36PM +0530, Amol Grover wrote:
> > Data is traversed using hlist_for_each_entry_rcu outside an
> > RCU read-side critical section but under the protection
> > of either lockdep_lock or with irqs disabled.
> > 
> > Hence, add corresponding lockdep expression to silence false-positive
> > lockdep warnings, and harden RCU lists. Also add macro for
> > corresponding lockdep expression.
> > 
> > Two things to note:
> > - RCU traversals protected under both, irqs disabled and
> > graph lock, have both the checks in the lockdep expression.
> > - RCU traversals under the protection of just disabled irqs
> > don't have a corresponding lockdep expression as it is implicitly
> > checked for.
> > 
> > Signed-off-by: Amol Grover <frextrite@...il.com>
> > ---
> >  kernel/locking/lockdep.c | 21 +++++++++++++--------
> >  1 file changed, 13 insertions(+), 8 deletions(-)
> > 
> > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > index 32282e7112d3..696ad5d4daed 100644
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -85,6 +85,8 @@ module_param(lock_stat, int, 0644);
> >   * code to recurse back into the lockdep code...
> >   */
> >  static arch_spinlock_t lockdep_lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED;
> > +#define graph_lock_held() \
> > +	arch_spin_is_locked(&lockdep_lock)
> >  static struct task_struct *lockdep_selftest_task_struct;
> >  
> >  static int graph_lock(void)
> > @@ -1009,7 +1011,7 @@ static bool __check_data_structures(void)
> >  	/* Check the chain_key of all lock chains. */
> >  	for (i = 0; i < ARRAY_SIZE(chainhash_table); i++) {
> >  		head = chainhash_table + i;
> > -		hlist_for_each_entry_rcu(chain, head, entry) {
> > +		hlist_for_each_entry_rcu(chain, head, entry, graph_lock_held()) {
> >  			if (!check_lock_chain_key(chain))
> >  				return false;
> >  		}
> 
> URGH.. this patch combines two horribles to create a horrific :/
> 
>  - spin_is_locked() is an abomination

Agreed, I would prefer use of lockdep assertions myself.  And yes, I
did try to get rid of spin_is_locked() some time back, but there were
a few use cases that proved stubborn.  :-(

>  - this RCU list stuff is just plain annoying
> 
> I'm tempted to do something like:
> 
> #define STFU (true)
> 
> 	hlist_for_each_entry_rcu(chain, head, entry, STFU) {

Now that is just plain silly.  It is easier to type "true" than "STFU",
satisfying though the latter might feel to you right now.

> Paul, are we going a little over-board with this stuff? Do we really
> have to annotate all of this?

Like rcu_dereference_raw()?  

My goal is to provide infrastructure that allows people to gain the
benefit of automated code review if they so choose.  And a number have
so chosen.  In this case, it is pretty easy to disable the checking by
adding "true" as the last argument, so I am not seeing a real problem.

Just don't come crying to me if doing so ends up hiding a bug.  ;-)

							Thanx, Paul

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