From: Waiman Long <waiman.long@hpe.com>
To: Jason Low <jason.low2@hpe.com>
Cc: Peter Hurley <peter@hurleysoftware.com>, <jason.low2@hp.com>,
Davidlohr Bueso <dave@stgolabs.net>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, <linux-kernel@vger.kernel.org>,
Dave Chinner <david@fromorbit.com>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Scott J Norton <scott.norton@hpe.com>,
Douglas Hatch <doug.hatch@hpe.com>
Subject: Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE
Date: Tue, 24 May 2016 21:25:46 -0400 [thread overview]
Message-ID: <5744FF1A.8050804@hpe.com> (raw)
In-Reply-To: <1464029181.2479.21.camel@j-VirtualBox>
On 05/23/2016 02:46 PM, Jason Low wrote:
> On Sat, 2016-05-21 at 09:04 -0700, Peter Hurley wrote:
>> On 05/18/2016 12:58 PM, Jason Low wrote:
>>> It should be fine to use the standard READ_ONCE here, even if it's just
>>> for documentation, as it's probably not going to cost anything in
>>> practice. It would be better to avoid adding any special macros for this
>>> which may just add more complexity.
>> See, I don't understand this line of reasoning at all.
>>
>> I read this as "it's ok to be non-optimal here where were spinning CPU
>> time but not ok to be non-optimal generally elsewhere where it's
>> way less important like at init time".
> So I think there is a difference between using it during init time and
> using it here where we're spinning. During init time, initializing the
> owner field locklessly is normal. No other thread should be concurrently
> be writing to the field, since the structure is just getting
> initialized, so there are no surprises there.
>
> Our access of the owner field in this function is special in that we're
> using a bit of "lockless magic" to read and write to a field that gets
> concurrently accessed without any serialization. Since we're not taking
> the wait_lock in a scenario where we'd normally would take a lock, it
> would be good to have this documented.
>
>> And by the way, it's not just "here" but _everywhere_.
>> What about reading ->on_cpu locklessly?
> Sure, we could also use READ_ONCE when reading ->on_cpu :)
>
As on_cpu is just a boolean, load tearing isn't really a problem. You
either see the bit 0 set or not, but not something in between (not a
qbit) :-)
Cheers,
Longman
next prev parent reply other threads:[~2016-05-25 1:26 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-18 1:26 [PATCH v4 0/5] [PATCH v3 0/4] locking/rwsem: Add reader-owned state to the owner field Waiman Long
2016-05-18 1:26 ` [PATCH v4 1/5] " Waiman Long
2016-06-06 17:18 ` Davidlohr Bueso
2016-06-06 20:03 ` Waiman Long
2016-06-06 21:02 ` Peter Zijlstra
2016-06-06 21:49 ` Waiman Long
2016-06-08 14:25 ` [tip:locking/core] " tip-bot for Waiman Long
2016-05-18 1:26 ` [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE() Waiman Long
2016-05-18 14:04 ` Davidlohr Bueso
2016-05-18 17:21 ` [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE Jason Low
2016-05-18 18:29 ` Waiman Long
2016-05-18 19:58 ` Jason Low
2016-05-19 22:21 ` Jason Low
2016-05-20 20:26 ` Waiman Long
2016-05-21 16:04 ` Peter Hurley
2016-05-22 10:42 ` Peter Zijlstra
2016-05-23 18:46 ` Jason Low
2016-05-23 19:44 ` Davidlohr Bueso
2016-05-23 20:15 ` Paul E. McKenney
2016-05-23 21:04 ` Davidlohr Bueso
2016-05-25 1:25 ` Waiman Long [this message]
2016-05-18 17:23 ` [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE() Jason Low
2016-06-08 14:25 ` [tip:locking/core] " tip-bot for Waiman Long
2016-05-18 1:26 ` [PATCH v4 3/5] locking/rwsem: Don't wake up one's own task Waiman Long
2016-05-18 10:30 ` Peter Zijlstra
2016-05-18 16:04 ` Waiman Long
2016-05-18 1:26 ` [PATCH v4 4/5] locking/rwsem: Improve reader wakeup code Waiman Long
2016-06-08 14:25 ` [tip:locking/core] " tip-bot for Waiman Long
2016-05-18 1:26 ` [PATCH v4 5/5] locking/rwsem: Streamline the rwsem_optimistic_spin() code Waiman Long
2016-06-08 14:26 ` [tip:locking/core] " tip-bot for Waiman Long
2016-05-18 10:52 ` [PATCH v4 0/5] [PATCH v3 0/4] locking/rwsem: Add reader-owned state to the owner field Peter Zijlstra
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=5744FF1A.8050804@hpe.com \
--to=waiman.long@hpe.com \
--cc=dave@stgolabs.net \
--cc=david@fromorbit.com \
--cc=doug.hatch@hpe.com \
--cc=jason.low2@hp.com \
--cc=jason.low2@hpe.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=paulmck@linux.vnet.ibm.com \
--cc=peter@hurleysoftware.com \
--cc=peterz@infradead.org \
--cc=scott.norton@hpe.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).