Messages in this thread | | | Date | Thu, 29 Sep 2022 16:15:26 -0500 | Subject | Re: [PATCH] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath | From | John Donnelly <> |
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On 9/29/22 13:04, 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@redhat.com> > --- > kernel/locking/rwsem.c | 13 ++++++++++--- > 1 file changed, 10 insertions(+), 3 deletions(-) > > diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c > index 65f0262f635e..ad676e99e0b3 100644 > --- a/kernel/locking/rwsem.c > +++ b/kernel/locking/rwsem.c > @@ -628,6 +628,11 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, > new = count; > > if (count & RWSEM_LOCK_MASK) { > + /* > + * A waiter (first or not) can set the handoff bit > + * if it is an RT task or wait in the wait queue > + * for too long. > + */ > if (has_handoff || (!rt_task(waiter->task) && > !time_after(jiffies, waiter->timeout))) > return false; > @@ -643,11 +648,13 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, > } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new)); > > /* > - * We have either acquired the lock with handoff bit cleared or > - * set the handoff bit. > + * We have either acquired the lock with handoff bit cleared or set > + * the handoff bit. Only the first waiter can have its handoff_set > + * set here to enable optimistic spinning in slowpath loop. > */ > if (new & RWSEM_FLAG_HANDOFF) { > - waiter->handoff_set = true; > + if (waiter == first) > + waiter->handoff_set = true; > lockevent_inc(rwsem_wlock_handoff); > return false; > } Hi,.
Are you missing
[PATCH 5.18 87/88] locking/rwsem: Allow slowpath writer to ignore handoff bit if not set by first waiter
Fixes: d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more consistent")
Or is this another regression ?
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