Re: Race condition in threading code?

[Han-Wen Nienhuys <hanwen@xs4all.nl>]

Han-Wen Nienhuys escreveu:

>>> ERROR: srfi-18.test: thread-start!:
>>>   thread activates only after start
>>>    - arguments: ((syntax-error "memoization"
>>>                   "In file ~S, line ~S: ~A ~S in expression ~S."
>>>                   ("/home/lilydev/vc/guile/srfi/srfi-18.scm" 135
>>>                    "Bad binding" ct
>>>                     (let (ct (current-thread))
>>>                     #@let (or (hashq-ref thread-exception-handlers ct)
>>>                           (hashq-set! thread-exception-handlers ct
>>>                     (list initial-handler))))) #f))
>> I'm seeing this as well, but it's a `#@let*' here (single-binding `let's
>> are memoized as `#@let*'):
>>
>>   ((syntax-error "memoization"
>>                  "In file ~S, line ~S: ~A ~S in expression ~S."
>>                  ("/home/ludo/src/guile/srfi/srfi-18.scm" 138
>>                   "Bad binding"
>>                   ct
>>                   (#@let* (ct (#<variable b7d28110 value: #<primitive-procedure current-thread>>))
>>                     (#@or (#<variable b7d2ad88 value: #<primitive-procedure hashq-ref>>
>>                                       #<variable 839df08 value: #<weak-key-hash-table 1/31>> #@0-0)
>>                           (#<variable b7d2adc0 value: #<primitive-procedure hashq-set!>> #<variable 839df08 value: #<weak-key-hash-table 1/31>> #@0-0 (#<variable b7d2c498 value: #<primitive-procedure list>> #<variable 839d130 value: #<procedure initial-handler (obj)>>))
>>                           )))
>>                  #f))
>>
>> It can be reproduced, but very infrequently, with this program:
>>
>>   (use-modules (ice-9 threads))
>>
>>   (define (foo x y)
>>     (let ((z (+ x y)))
>>       (let ((a (+ z 1)))
>>         (let ((b (- a 2)))
>>           (let ((c (* b 3)))
>>             c)))))
>>
>>   (define (entry)
>>     (foo 1 2))
>>
>>   (for-each (lambda (i) (make-thread entry))
>>             (iota 123))
>>
>> My explanation is that the `let*' memoizer, aka. `scm_m_letstar ()', is
>> not thread-safe; it's clearly not atomic, and it's of course not
>> protected by a mutex or so.
> 
> Is that the only one?
> 
>   SCM
>   scm_m_let (SCM expr, SCM env)
>   ...
>       /* plain let */
>       SCM rvariables;
>       SCM inits;
>       transform_bindings (bindings, expr, &rvariables, &inits);
> 
>       {
>         const SCM new_body = m_body (SCM_IM_LET, SCM_CDR (cdr_expr));
>         const SCM new_tail = scm_cons2 (rvariables, inits, new_body);
>         SCM_SETCAR (expr, SCM_IM_LET); 
>         // ****!!!
>         SCM_SETCDR (expr, new_tail);
> 
> What happens if another thread tries to evaluate expr at the place marked ****!!! ?
> 
> At the very least, we should have an atomic SCM_SETCELL() which overwrites car and
> cdr atomically.

Anyone?  Does anyone still understand how the evaluator works?  (if not, let's move 
to the VM earlier than later.)

-- 
 Han-Wen Nienhuys - hanwen@xs4all.nl - http://www.xs4all.nl/~hanwen