Messages in this thread | | | Date | Tue, 11 Oct 2022 18:46:20 +0530 | Subject | Re: [PATCH] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath | From | Mukesh Ojha <> |
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Hi @Hilf,
Thanks for looking into this issue.
On 10/11/2022 4:16 PM, Hillf Danton wrote: > On 10/10/22 06:24 Mukesh Ojha <quic_mojha@quicinc.com> >> 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@redhat.com> >>>> --- >>>> 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()
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>> >> 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.
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>> > Hey Mukesh, > > Can you test the diff if it makes sense to you? > > It simply prevents the first waiter from spinning any longer after detecting > it barely makes any progress to spin without lock owner. > > Hillf > > --- mainline/kernel/locking/rwsem.c > +++ b/kernel/locking/rwsem.c > @@ -611,26 +611,15 @@ static inline bool rwsem_try_write_lock( > long count, new; > > lockdep_assert_held(&sem->wait_lock); > + waiter->handoff_set = false; > > count = atomic_long_read(&sem->count); > do { > bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF); > > if (has_handoff) { > - /* > - * Honor handoff bit and yield only when the first > - * waiter is the one that set it. Otherwisee, we > - * still try to acquire the rwsem. > - */ > - if (first->handoff_set && (waiter != first)) > + if (waiter != first) > return false;
you mean, you want to check and change waiter->handoff_set on every run rwsem_try_write_lock().
But does it break optimistic spinning ? @waiman ?
-Mukesh
> - > - /* > - * First waiter can inherit a previously set handoff > - * bit and spin on rwsem if lock acquisition fails. > - */ > - if (waiter == first) > - waiter->handoff_set = true; > } > > new = count;
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