Ludovic Courtès escreveu:
> Hello,
>
> Andy Wingo <[EMAIL PROTECTED]> writes:
>
>> 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))
>> [EMAIL PROTECTED] (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 [EMAIL PROTECTED]' here (single-binding
> `let's
> are memoized as [EMAIL PROTECTED]'):
>
> ((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
> ([EMAIL PROTECTED] (ct (#<variable b7d28110 value:
> #<primitive-procedure current-thread>>))
> ([EMAIL PROTECTED] (#<variable b7d2ad88 value:
> #<primitive-procedure hashq-ref>>
> #<variable 839df08 value:
> #<weak-key-hash-table 1/31>> [EMAIL PROTECTED])
> (#<variable b7d2adc0 value: #<primitive-procedure
> hashq-set!>> #<variable 839df08 value: #<weak-key-hash-table 1/31>> [EMAIL
> PROTECTED] (#<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.
>
> I can't think of any simple fix. `scm_m_letstar ()' could be made
> atomic by having it duplicate the input list instead of modifying it
> directly; it could then atomically update the input. However,
> allocating cells during memoization wouldn't be a good idea
> performance-wise.
I don't understand: memoization is only supposed to happen once for
each piece of code, right? So, the cost of it is not that interesting?
I remember seeing a very scary looking explanation in eval.c about the
evaluator being unlocked but still thread-safe since the result of memoizing
was supposed to be confluent (ie. duplicate runs would yield independent
results.)
/* The Lookup Car Race
- by Eva Luator
This was added by Marius Vollmer, but at the time, GUILE did not support
real posix threads, so any problem may not have manifested itself before.
--
Han-Wen Nienhuys - [EMAIL PROTECTED] - http://www.xs4all.nl/~hanwen
