Re: [PATCH v2 5/6] maple_tree: Update check_forking() and bench_forking()

[Matthew Wilcox]

On Thu, Sep 07, 2023 at 02:03:01PM -0400, Liam R. Howlett wrote:
> > >  WARNING: possible recursive locking detected
> > >  6.5.0-rc4-00632-g2730245bd6b1 #1 Tainted: G                TN
> > >  --------------------------------------------
> > >  swapper/1 is trying to acquire lock:
> > > ffffffff86485058 (&mt->ma_lock){+.+.}-{2:2}, at: check_forking (include/linux/spinlock.h:? lib/test_maple_tree.c:1854)
> > > 
> > >  but task is already holding lock:
> > >  ffff888110847a30 (&mt->ma_lock){+.+.}-{2:2}, at: check_forking (include/linux/spinlock.h:351 lib/test_maple_tree.c:1854)
> > Thanks for the test. I checked that these are two different locks, why
> > is this warning reported? Did I miss something?
> 
> I don't think you can nest spinlocks like this.  In my previous test I
> avoided nesting, but in your case we cannot avoid having both locks at
> the same time.
> 
> You can get around this by using an rwsemaphore, set the two trees as
> external and use down_write_nested(&lock2, SINGLE_DEPTH_NESTING) like
> the real fork.  Basically, switch the locking to exactly what fork does.

spin_lock_nested() exists.

You should probably both read through
Documentation/locking/lockdep-design.rst It's not the best user
documentation in the world, but it's what we have.