userret: assert td_lk_slocks == 0

[Andriy Gapon]

On 12/08/2019 13:49, Mateusz Guzik wrote:
> On 8/12/19, Andriy Gapon <avg at freebsd.org> wrote:
>>
>> I am trying to debug a leak of a shared vnode lock and I noticed that
>> there is no check for td_lk_slocks in userret.  There are checks for
>> td_rw_rlocks and td_sx_slocks.  I wonder if there is any valid scenario
>> where a thread is allowed to retain a shared lock manager lock across
>> system calls.
>>
> 
> These counters are not for debugging purposes. They are part of poor
> man's starvation prevention for writers.

Yes, I realize that.

> If the target lock is taken for reading and someone wants to take it for
> writing, a bit will be set to denote this fact and prevent more readers
> from showing up. However, this can lead to deadlocks so if someone
> already has a read lock on something, they can bypass the bit and
> grab the extra read lock anyway.
> 
> No locks are allowed to leak back to userspace and witness should
> already handles checking this for readers as well.

Yes.
But since we have those asserts for td_rw_rlocks and td_sx_slocks,
wouldn't it make sense to add one for td_lk_slocks?
If it's considered superfluous for FreeBSD, then at least I'll add it in
the work's fork.


-- 
Andriy Gapon