linux-kernel - Re: [PATCH] "volatile considered harmful" document

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Message-id: <4646082D.2080502@shaw.ca>
Date:	Sat, 12 May 2007 12:32:13 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	Bill Davidsen <davidsen@....com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Cc:	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH] "volatile considered harmful" document

Bill Davidsen wrote:
> Jonathan Corbet wrote:
> 
>> +There are still a few rare situations where volatile makes sense in the
>> +kernel:
>> +
>> +  - The above-mentioned accessor functions might use volatile on
>> +    architectures where direct I/O memory access does work.  
>> Essentially,
>> +    each accessor call becomes a little critical section on its own and
>> +    ensures that the access happens as expected by the programmer.
>> +
>> +  - Inline assembly code which changes memory, but which has no other
>> +    visible side effects, risks being deleted by GCC.  Adding the 
>> volatile
>> +    keyword to asm statements will prevent this removal.
>> +
>> +  - The jiffies variable is special in that it can have a different 
>> value
>> +    every time it is referenced, but it can be read without any special
>> +    locking.  So jiffies can be volatile, but the addition of other
>> +    variables of this type is frowned upon.  Jiffies is considered to 
>> be a
>> +    "stupid legacy" issue in this regard.
> 
> It would seem that any variable which is (a) subject to change by other 
> threads or hardware, and (b) the value of which is going to be used 
> without writing the variable, would be a valid use for volatile.

You don't need volatile in that case, rmb() can be used.

-- 
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@...pamshaw.ca
Home Page: http://www.roberthancock.com/

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