Please excuse me.  I had a major typo.  Corrected:

But if you read the variable in one particular thread, there is no
promise that its value will persist (unaltered by another thread)
while you execute the branch of a conditional that choses a path based
on the variable's value.

On Mon, Oct 26, 2009 at 1:22 PM, David L. Rager <ragerdl@gmail.com> wrote:
> Pretty sure the answer to your question is "no."
>
> I think common practice (in CCL) is that a single setf is assumed to
> write a value to an address "atomically" at the assembly level (it
> should just be a move and it should be the case that if two threads
> write to a pointer, that you end up with either the first or second
> value, not a mashup of the two values), but if you read the variable
> in a thread and there is no promise that its value to persist while
> you execute a branch of an if (or other conditional that choses a path
> based on a variable's value).
>

> (defvar *val* 0)
>
> (run-thread (lambda () (setf * val* (1+ * val*)))
> (run-thread (labmda () (setf * val* (1+ * val*)))
>
> After the threads finish, *tmp* will be either 1 or 2 at the end of
> the execution, depending on whether the race conditions triggered.
>
>
> Instead you'd need, the following, which results in *val* always containing 2.
>
> (defvar * val* 0)
> (defvar *lock* (make-lock))
> (run-thread (lambda () (with-lock *lock* (setf * val* (1+ * val*))))
> (run-thread (lambda () (with-lock *lock* (setf * val* (1+ * val*))))
>
>
>
> If you just want an atomic counter, you could consider the function
> sb-ext:atomic-incf.
>
> BTW, the notation is a bit abstract, because I have my own wrappers
> for the SBCL and CCL multi-threading libraries and prefer to use said
> abstractions.
>
>
> On Mon, Oct 26, 2009 at 1:07 PM, Bhaskara Marthi
> <bhaskara@willowgarage.com> wrote:
>> In other words, is it safe to have multiple threads reading a special
>> variable and/or using setq to change it, without any locking?
>> - Bhaskara
>>
>> --
>> Bhaskara Marthi
>> Research Scientist
>> Willow Garage Inc.
>> 650-475-2856
>>
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