linux-kernel - Re: [Question] srcu: is it making sense to recursively invoke srcu_read

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Message-ID: <20220421133414.GP4285@paulmck-ThinkPad-P17-Gen-1>
Date:   Thu, 21 Apr 2022 06:34:14 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [Question] srcu: is it making sense to recursively invoke
 srcu_read_lock?

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.

						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_);
> }