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Date: Tue, 11 Oct 2022 18:46:20 +0530
From: Mukesh Ojha <quic_mojha@...cinc.com>
To: Hillf Danton <hdanton@...a.com>
CC: <linux-kernel@...r.kernel.org>, <john.p.donnelly@...cle.com>,
<linux-mm@...ck.org>, Waiman Long <longman@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Will Deacon <will@...nel.org>,
Boqun Feng <boqun.feng@...il.com>,
Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] locking/rwsem: Prevent non-first waiter from spinning in
down_write() slowpath
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@...cinc.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@...hat.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()
[]
>>
>> 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.
[]
>>
> 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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