linux-kernel - Re: [PATCH] eventfd: change int to __u64 in eventfd

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Date:	Fri, 13 Apr 2012 10:52:35 +0800
From:	Sha Zhengju <handai.szj@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, davidel@...ilserver.org,
	Sha Zhengju <handai.szj@...bao.com>
Subject: Re: [PATCH] eventfd: change int to __u64 in eventfd_signal()

On 04/13/2012 07:09 AM, Andrew Morton wrote:
> On Thu, 12 Apr 2012 18:01:20 +0800
> handai.szj@...il.com wrote:
>
>> From: Sha Zhengju<handai.szj@...bao.com>
>>
>> From: Sha Zhengju<handai.szj@...bao.com>
>>
>> eventfd_ctx->count is an __u64 counter which is allowed to reach ULLONG_MAX.
>> Now eventfd_write() add an __u64 value to "count", but kernel side
>> eventfd_signal() only add an int value to it. So make them consistent here.
>>
>> ...
>>
>> --- a/fs/eventfd.c
>> +++ b/fs/eventfd.c
>> @@ -51,7 +51,7 @@ struct eventfd_ctx {
>>    *
>>    * -EINVAL    : The value of @n is negative.
>>    */
>> -int eventfd_signal(struct eventfd_ctx *ctx, int n)
>> +__u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
>>   {
>>   	unsigned long flags;
>>
>> @@ -59,7 +59,7 @@ int eventfd_signal(struct eventfd_ctx *ctx, int n)
>>   		return -EINVAL;
>>   	spin_lock_irqsave(&ctx->wqh.lock, flags);
>>   	if (ULLONG_MAX - ctx->count<  n)
>> -		n = (int) (ULLONG_MAX - ctx->count);
>> +		n = ULLONG_MAX - ctx->count;
>>   	ctx->count += n;
>>   	if (waitqueue_active(&ctx->wqh))
>>   		wake_up_locked_poll(&ctx->wqh, POLLIN);
> Changing `n' to an unsigned type makes the "if (n<  0)" test a no-op.
>
yeah, it's really not necessary. I slipped up..
> Every in-kernel caller of eventfd_signal() passes n=1.  All of them.
> Perhaps we can just remove that argument and hard-wire the +1
> assumption into eventfd_signal().
A userspace write(2) performing on an eventfd can pass an u64 value to
"count" and issue a wakeup, I think the kernel side can just be in 
accord with it.
Though all the current in-kernel callers just pass 1 to eventfd_signal(), it
doesn't mean there is no possibility of other usages in future. 
Actually, we're
considering to encode some usefully info to "count" and sent it to 
userspace. :-)

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