linux-kernel - Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <5620103B.2050002@hpe.com>
Date:	Thu, 15 Oct 2015 16:44:43 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt

On 10/13/2015 04:44 PM, Peter Zijlstra wrote:
> On Tue, Oct 13, 2015 at 04:41:41PM -0400, Waiman Long wrote:
>> On 10/13/2015 02:23 PM, Peter Zijlstra wrote:
>>> On Tue, Sep 22, 2015 at 04:50:43PM -0400, Waiman Long wrote:
>>>>   	for (;; waitcnt++) {
>>>> +		loop = SPIN_THRESHOLD;
>>>> +		while (loop) {
>>>> +			/*
>>>> +			 * Spin until the lock is free
>>>> +			 */
>>>> +			for (; loop&&   READ_ONCE(l->locked); loop--)
>>>> +				cpu_relax();
>>>> +			/*
>>>> +			 * Seeing the lock is free, this queue head vCPU is
>>>> +			 * the rightful next owner of the lock. However, the
>>>> +			 * lock may have just been stolen by another task which
>>>> +			 * has entered the slowpath. So we need to use atomic
>>>> +			 * operation to make sure that we really get the lock.
>>>> +			 * Otherwise, we have to wait again.
>>>> +			 */
>>>> +			if (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0)
>>>> +				goto gotlock;
>>>>   		}
>>> 		for (loop = SPIN_THRESHOLD; loop; --loop) {
>>> 			if (!READ_ONCE(l->locked)&&
>>> 			cmpxchg(&l->locked, 0, _Q_LOCKED_VA) == 0)
>>> 				goto gotlock;
>>>
>>> 			cpu_relax();
>>> 		}
>>>
>> This was the code that I used in my original patch, but it seems to confuse
>> you about doing too many lock stealing. So I separated it out to make my
>> intention more explicit. I will change it back to the old code.
> Code should be compact; its the purpose of Changelogs and comments to
> explain it if its subtle.
>
> Here you made weird code and the comments still don't explain how its
> starvation proof and the Changelog is almost empty of useful.

You are right. The current patch can't guarantee that there will be no 
lock starvation. I will make some code changes to make sure that lock 
starvation won't happen. I will also try to clean up the code and the 
comments at the same time.

Cheers,
Longman
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/