From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: "Ingo Molnar" <mingo@redhat.com>, "Will Deacon" <will@kernel.org>,
"Boqun Feng" <boqun.feng@gmail.com>,
linux-kernel@vger.kernel.org, john.p.donnelly@oracle.com,
"Hillf Danton" <hdanton@sina.com>,
"Mukesh Ojha" <quic_mojha@quicinc.com>,
"Ting11 Wang 王婷" <wangting11@xiaomi.com>
Subject: Re: [PATCH v3 2/5] locking/rwsem: Limit # of null owner retries for handoff writer
Date: Tue, 25 Oct 2022 13:22:22 +0200 [thread overview]
Message-ID: <Y1fG7nQxiLyKIhQ6@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <980d882c-01b8-2ce1-663f-41a8a337f350@redhat.com>
On Mon, Oct 24, 2022 at 11:55:53AM -0400, Waiman Long wrote:
> 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.
>
> ----------------------
>
> To fix that we either has to disable preemption in down_read() and reenable
> it in rwsem_down_read_slowpath after decrementing the RWSEM_READER_BIAS or
> to limit the number of trylock-spinning attempt like this patch. The latter
> approach seems a bit less messy and I am going to take it back out anyway in
> patch 4. I will put a summary of that special case in the patch description.
Funny, I find the former approach much saner. Disabling preemption
around the whole thing fixes the fundamental problem while spin-limiting
is a band-aid.
Note how rwsem_write_trylock() already does preempt_disable(), having
the read-side do something similar only makes sense.
next prev parent reply other threads:[~2022-10-25 11:22 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-17 21:13 [PATCH v3 0/5] lockinig/rwsem: Fix rwsem bugs & enable true lock handoff Waiman Long
2022-10-17 21:13 ` [PATCH v3 1/5] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath Waiman Long
2022-10-24 13:17 ` Peter Zijlstra
2022-10-24 13:50 ` Waiman Long
2022-10-17 21:13 ` [PATCH v3 2/5] locking/rwsem: Limit # of null owner retries for handoff writer Waiman Long
2022-10-24 13:31 ` Peter Zijlstra
2022-10-24 15:55 ` Waiman Long
2022-10-25 11:22 ` Peter Zijlstra [this message]
2022-10-25 11:48 ` Peter Zijlstra
2022-10-25 19:55 ` Waiman Long
2022-10-25 20:14 ` Peter Zijlstra
2022-10-26 1:44 ` Waiman Long
[not found] ` <20221025145843.2953-1-hdanton@sina.com>
2022-10-25 19:00 ` Waiman Long
2022-10-17 21:13 ` [PATCH v3 3/5] locking/rwsem: Change waiter->hanodff_set to a handoff_state enum Waiman Long
2022-10-17 21:13 ` [PATCH v3 4/5] locking/rwsem: Enable direct rwsem lock handoff Waiman Long
2022-10-17 21:13 ` [PATCH v3 5/5] locking/rwsem: Update handoff lock events tracking Waiman Long
[not found] ` <20221018111424.1007-1-hdanton@sina.com>
2022-10-18 14:13 ` [PATCH v3 4/5] locking/rwsem: Enable direct rwsem lock handoff Mukesh Ojha
2022-10-18 17:37 ` Waiman Long
[not found] ` <20221018235138.1088-1-hdanton@sina.com>
2022-10-19 0:39 ` Waiman Long
[not found] ` <20221019022934.1166-1-hdanton@sina.com>
2022-10-19 2:49 ` Waiman Long
[not found] ` <20221019070559.1220-1-hdanton@sina.com>
2022-10-19 15:02 ` Waiman Long
2022-10-24 16:18 ` Waiman Long
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