Re: [patch] Re: using long instead of atomic_t when only set/readis required

[Linus Torvalds]

On Mon, 3 Mar 2008, Alan Stern wrote:
> 
> Consider a routine like the following:
> 
> 	static task_struct *the_task;
> 
> 	void store_task(void)
> 	{
> 		the_task = current;
> 	}
> 
> Is it possible to say whether readers examining "the_task" are 
> guaranteed to see a coherent value?

Yes, we do depend on this.  All the RCU stuff (and in general *anything* 
that depends on memory ordering as opposed to full locking, and we have 
quite a lot of it) is very fundamentally dependent on the fact that things 
like pointers get read and written atomically.

HOWEVER, it is worth pointing out that it's generally true in a 
"different" sense than the actual atomic accesses. For example, if you 
test a single bit of a word, it's still quite possible that gcc will have 
turned that "atomic" read into a single byte read, so it's not necessarily 
the case that we'll actually even read the whole word. 

(Writes are different: if you do things like bitwise updates they simply 
*will*not* be atomic, but that's simply not what we depend on anyway).

So in that sense, the atomicity guarantees are a lot weaker than the ones 
we do for IO accesses, but that's all fine. Memory isn't IO, and doesn't 
have side effects.

			Linus
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