linux-kernel - Re: [PATCH RFC 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation

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Message-ID: <51FAA5F8.2070209@linux.vnet.ibm.com>
Date:	Thu, 01 Aug 2013 23:46:24 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Waiman Long <waiman.long@...com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Richard Weinberger <richard@....at>,
	Catalin Marinas <catalin.marinas@....com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Matt Fleming <matt.fleming@...el.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Akinobu Mita <akinobu.mita@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	George Spelvin <linux@...izon.com>,
	Harvey Harrison <harvey.harrison@...il.com>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH RFC 1/2] qspinlock: Introducing a 4-byte queue spinlock
 implementation

On 08/01/2013 11:28 PM, Waiman Long wrote:
> On 08/01/2013 05:40 AM, Peter Zijlstra wrote:
>> On Wed, Jul 31, 2013 at 10:37:10PM -0400, Waiman Long wrote:
>>
[...]
>>
>>> +         */
>>> +        for (qn_idx = 1; qn_idx<  MAX_QNODES; qn_idx++) {
>>> +            if (!node[qn_idx].used)
>>> +                break;
>>         }
>>
>>> +        if (unlikely(qn_idx == MAX_QNODES)) {
>>> +            /*
>>> +             * This shouldn't happen, print a warning message
>>> +             *&  busy spinning on the lock.
>>> +             */
>>> +            pr_warn("qspinlock: queue node table exhausted at "
>>> +                "cpu %d!\n", cpu_nr);
>> This could make your machine die hard.. not all contexts can printk().
>
> Do you have any suggestion? I could skip the warning and silently do the
> busy spinning. I just want some way to notify the user of this rare event.

We have used debugfs in pv-spinlock to avoid that since printk uses
spinlock again. may be it will help to profile many other parts of
code too.

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