From mboxrd@z Thu Jan  1 00:00:00 1970
From: buyitian@gmail.com (buyitian)
Date: Fri, 30 Jan 2015 21:52:07 +0800
Subject: spinlock variable protection
In-Reply-To: <mafu9s$mhn$1@ger.gmane.org>
References: <mafu9s$mhn$1@ger.gmane.org>
Message-ID: <A827E844-3767-4BE6-A45F-B3E1CC615E36@gmail.com>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org



> ? 2015?1?30??20:43?"Matwey V. Kornilov" <matwey.kornilov@gmail.com> ???
> 
> Hi,
> 
> I have the following code
> 
>         int ret = 0;
>         unsigned long irqflags;
> 
>         spin_lock_irqsave(&lock, irqflags);
> 
>         //...
>         ret = hdl->count;
>         //...
> 
>         spin_unlock_irqrestore(&lock, irqflags);
>         return ret;
> 
> I would like to be sure, that ret will not be optimized out. I think 
> compiler can convert the code to equivalent:
> 
>         unsigned long irqflags;
> 
>         spin_lock_irqsave(&lock, irqflags);
> 
>         //...
>         //...
> 
>         spin_unlock_irqrestore(&lock, irqflags);
>         return hdl->count;
> 
> But this is not what I want, because I use lock to protect hdl and want 
> to return hdl->count value as it was in protected section.
> 
> 
Please check the assembly code to double confirm the GCC behavior.
Why will GCC change the order as what you mentioned? Only assembly code can tell you.



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