Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) != current) with fsfreeze (4.19.25-rt16)

From: Peter Zijlstra
Date: Fri May 03 2019 - 11:47:31 EST

On Fri, May 03, 2019 at 05:37:48PM +0200, Oleg Nesterov wrote:
> (And if we change this code to use wait_event(xchg(readers_block) == 0) we
> can remove rw_sem altogether).

That patch you just saw and didn't look at did just that.

> The main problem is that this is sub-optimal. We can have a lot of readers
> sleeping in __down_read() when percpu_down_write() succeeds, then after
> percpu_down_write_non_owner() does up_write() they all will be woken just
> to hang in readers_block(). Plus the new readers will need to pass the
> lock-check-unlock-schedule path.

Yes, that's gone. Still, write side locking on percpu-rwsem _should_ be
relatively rare and is certainly not a fast path.

> Peter, just in case... I see another patch from you but I need to run away
> till Monday.

n/p, enjoy the weekend!