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Date: Mon, 10 Oct 2022 12:57:36 -0400
From: Waiman Long <longman@...hat.com>
To: Mukesh Ojha <quic_mojha@...cinc.com>
Cc: linux-kernel@...r.kernel.org, john.p.donnelly@...cle.com,
Hillf Danton <hdanton@...a.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
On 10/10/22 06:24, Mukesh Ojha wrote:
> 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.
>
Good catch!
It is indeed a possible livelock scenario. Will update the patch to
address that.
Thanks,
Longman
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