Re: [Linux-kernel-mentees] [PATCH RESEND] lockdep: Pass lockdep expression to RCU lists

[Joel Fernandes <joel@joelfernandes.org>]

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

Could it use hlist_for_each_entry_rcu_notrace() if that's better for this
code? That one does not need the additional condition passed. Though I find
rcu_dereference_raw_nocheck() in that macro a bit odd since it does sparse
checking, where as the rcu_dereference_raw() in hlist_for_each_entry() does
nothing.

And perf can do the same thing if it iss too annoying, like the tracing code
does.

This came up mainly because list_for_each_entry_rcu() does some checking of
it is in a reader section, but it is helpless in its checking when a lock is
held.

thanks,

 - Joel

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