Message-ID: <54DCB7E6.8040802@linux.vnet.ibm.com>
Date: Thu, 12 Feb 2015 19:55:42 +0530
From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Organization: IBM
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To: Oleg Nesterov <oleg@redhat.com>
CC: tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, peterz@infradead.org,
        torvalds@linux-foundation.org, konrad.wilk@oracle.com,
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Subject: Re: [PATCH V3] x86 spinlock: Fix memory corruption on completing
 completions
References: <1423741647-2156-1-git-send-email-raghavendra.kt@linux.vnet.ibm.com> <20150212140248.GA11050@redhat.com>
In-Reply-To: <20150212140248.GA11050@redhat.com>
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On 02/12/2015 07:32 PM, Oleg Nesterov wrote:
> Damn, sorry for noise, forgot to mention...
>
> On 02/12, Raghavendra K T wrote:
>>
>> +static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock,
>> +							__ticket_t head)
>> +{
>> +	if (head & TICKET_SLOWPATH_FLAG) {
>> +		arch_spinlock_t old, new;
>> +
>> +		old.tickets.head = head;
>> +		new.tickets.head = head & ~TICKET_SLOWPATH_FLAG;
>> +		old.tickets.tail = new.tickets.head + TICKET_LOCK_INC;
>> +		new.tickets.tail = old.tickets.tail;
>> +
>> +		/* try to clear slowpath flag when there are no contenders */
>> +		cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
>> +	}
>> +}
>
> ...
>
>> +clear_slowpath:
>> +	if (TICKET_SLOWPATH_FLAG)
>> +		__ticket_check_and_clear_slowpath(lock, inc.head);
>
> I think you can remove this "if (TICKET_SLOWPATH_FLAG)" check. If it is
> defined as zero, gcc should optimize out the code under "if (head & 0)".
>

right, the above if ( ) is unnecesary, though we would have same code
at the end, getting rid of that makes code more clean.

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