Re: [PATCH] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath

[Mukesh Ojha]

Hi Waiman,


On 9/29/2022 11:36 PM, Waiman Long wrote:
> On 9/29/22 14:04, Waiman Long wrote:
> > A non-first waiter can potentially spin in the for loop of
> > rwsem_down_write_slowpath() without sleeping but fail to acquire the
> > lock even if the rwsem is free if the following sequence happens:
> >
> >    Non-first waiter       First waiter      Lock holder
> >    ----------------       ------------      -----------
> >    Acquire wait_lock
> >    rwsem_try_write_lock():
> >      Set handoff bit if RT or
> >        wait too long
> >      Set waiter->handoff_set
> >    Release wait_lock
> >                           Acquire wait_lock
> >                           Inherit waiter->handoff_set
> >                           Release wait_lock
> >                        Clear owner
> >                                             Release lock
> >    if (waiter.handoff_set) {
> >      rwsem_spin_on_owner(();
> >      if (OWNER_NULL)
> >        goto trylock_again;
> >    }
> >    trylock_again:
> >    Acquire wait_lock
> >    rwsem_try_write_lock():
> >       if (first->handoff_set && (waiter != first))
> >           return false;
> >    Release wait_lock
> >
> > It is especially problematic if the non-first waiter is an RT task and
> > it is running on the same CPU as the first waiter as this can lead to
> > live lock.
> >
> > Fixes: d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more 
> > consistent")
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   kernel/locking/rwsem.c | 13 ++++++++++---
> >   1 file changed, 10 insertions(+), 3 deletions(-)
>
> Mukesh, can you test if this patch can fix the RT task lockup problem?

Looks like, There is still a window for a race.

There is a chance when a reader who came first added it's BIAS and goes 
to slowpath and before it gets added to wait list it got preempted by RT 
task which  goes to slowpath as well and being the first waiter gets its 
hand-off bit set and not able to get the lock due to following condition 
in rwsem_try_write_lock()

 630                 if (count & RWSEM_LOCK_MASK) {  ==> reader has 
sets its bias
..
...

 634
 635                         new |= RWSEM_FLAG_HANDOFF;
 636                 } else {
 637                         new |= RWSEM_WRITER_LOCKED;


---------------------->----------------------->-------------------------

First reader (1)    	  writer(2) RT task             Lock holder(3)

It sets
RWSEM_READER_BIAS.
while it is going to
slowpath(as the lock
was held by (3)) and
before it got added
to the waiters list
it got preempted
by (2).
                        RT task also takes
                        the slowpath and add              release the 

                        itself into waiting list          rwsem lock 

			and since it is the first         clear the
                        it is the next one to get         owner.
                        the lock but it can not
                        get the lock as (count &
                        RWSEM_LOCK_MASK) is set
                        as (1) has added it but
                        not able to remove its
			adjustment.


-Mukesh


> Thanks,
> Longman