linux-kernel - Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list is not NULL.

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: <1453411389.30844.38.camel@schen9-desk2.jf.intel.com>
Date:	Thu, 21 Jan 2016 13:23:09 -0800
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Ding Tianhong <dingtianhong@...wei.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Jason Low <jason.low2@...com>, Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list
 is not NULL.

On Thu, 2016-01-21 at 17:29 +0800, Ding Tianhong wrote:

> 
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index 0551c21..596b341 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -256,7 +256,7 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock)
>  	struct task_struct *owner;
>  	int retval = 1;
>  
> -	if (need_resched())
> +	if (need_resched() || atomic_read(&lock->count) == -1)
>  		return 0;
>  

One concern I have is this change will eliminate any optimistic spinning
as long as there is a waiter.  Is there a middle ground that we
can allow only one spinner if there are waiters?  

In other words, we allow spinning when
atomic_read(&lock->count) == -1 but there is no one on the
osq lock that queue up the spinners (i.e. no other process doing
optimistic spinning).

This could allow a bit of spinning without starving out the waiters.

Tim