Re: [Question] srcu: is it making sense to recursively invoke srcu_read_lock?

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Apr 21, 2022 at 06:34:14AM -0700, Paul E. McKenney wrote:
> On Thu, Apr 21, 2022 at 12:22:11PM +0800, Hillf Danton wrote:
> > Given rcu_lock_acquire() in srcu_read_lock(),
> > 
> > 	iA = srcu_read_lock(foo);
> > 	iB = srcu_read_lock(foo); // not bar
> > 	...
> > 	srcu_read_unlock(foo, iB);
> > 	srcu_read_unlock(foo, iA);
> > 
> > can the call sequence above trigger warning with CONFIG_DEBUG_LOCK_ALLOC enabled?
> 
> I hope not!  After all, nesting SRCU read-side critical sections is
> perfectly legal.  But why not just try it and see?
> 
> > Does it make sense to add srcu_lock_acquire() in line with rwsem_acquire_read() if
> > warning is expected but not triggered?
> 
> Please understand that while SRCU can often be used where an rwsem
> might otherwise be used, SRCU is not an rwsem.  For one thing, rwsem
> readers can deadlock in ways that SRCU reader cannot.
> 
> Now, I don't yet know of a non-destructive use case for partially
> overlapping SRCU read-side critical sections, for example, if you
> switched the two srcu_read_unlock() calls above.  But at the same
> time, I cannot prove that there is no valid use case, not yet,
> anyway.

But I do see one now.

Imagine someone passing a callback to another function, with an SRCU
read-side critical section starting before this other function is invoked
and ending within the callback.  Suppose that this other function also
uses SRCU to protect the invocation of the callback.  The two partially
overlapping SRCU read-side critical sections are independent, so they
are OK.

When the critical sections are not independent, trouble ensues:
https://paulmck.livejournal.com/40593.html

						Thanx, Paul
> 
> > Thanks
> > Hillf
> > 
> > static inline void rcu_lock_acquire(struct lockdep_map *map)
> > {
> > 	lock_acquire(map, 0, 0, 2, 0, NULL, _THIS_IP_);
> > }
> > 
> > static inline void srcu_lock_acquire(struct lockdep_map *map)
> > {
> > 	lock_acquire(map, 0, 0, 1, 0, NULL, _THIS_IP_);
> > }