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bug#40681: srfi-18 mutex-unlock! breaks mutex on timeout
From: |
d4ryus |
Subject: |
bug#40681: srfi-18 mutex-unlock! breaks mutex on timeout |
Date: |
Fri, 17 Apr 2020 16:21:52 +0200 |
hi,
when mutex-unlock! is called (master: module/srfi/srfi-18.scm +306) with
a condition and a timeout, it first resets the owner and then waits for
the condition. If the timeout is reached
(threads:wait-condition-variable returns #f) the cond's else condition
returns #f. Now the owner is #f, which prevents one from unlocking the
mutex, due to the early return if owner is #f.
Reproducer (also attached to the email):
(use-modules (srfi srfi-18))
(let ((condition (make-condition-variable))
(mutex (make-mutex))
(timeout (seconds->time
(+ (time->seconds (current-time)) 1)))) ;; 1 second timeout
(mutex-lock! mutex)
(when (mutex-unlock! mutex condition timeout)
;; Mutex is unlocked, everything is fine. This is not reached
;; though, as nobody signals the condition
(error "Not reached"))
;; Timeout was reached
;; Now the mutex has no owner but is stilled locked
;; Calling mutex-unlock! has no effect
(mutex-unlock! mutex)
;; And a call to lock will now block forever
(mutex-lock! mutex))
This affects guile 2.2 and master.
A fix would be to only reset the owner when the mutex is truly unlocked,
e.g. something along the lines of (also attached to the email):
diff --git a/module/srfi/srfi-18.scm b/module/srfi/srfi-18.scm
index 79aedb8d1..7da6254d4 100644
--- a/module/srfi/srfi-18.scm
+++ b/module/srfi/srfi-18.scm
@@ -307,17 +307,19 @@ object (absolute point in time), or #f."
(timeout %unlock-sentinel))
(let ((timeout (timeout->absolute-time timeout)))
(when (mutex-owner mutex)
- (set-mutex-owner! mutex #f)
(cond
((eq? cond-var %unlock-sentinel)
+ (set-mutex-owner! mutex #f)
(threads:unlock-mutex (mutex-prim mutex)))
((eq? timeout %unlock-sentinel)
(threads:wait-condition-variable (condition-variable-prim cond-var)
(mutex-prim mutex))
+ (set-mutex-owner! mutex #f)
(threads:unlock-mutex (mutex-prim mutex)))
((threads:wait-condition-variable (condition-variable-prim cond-var)
(mutex-prim mutex)
timeout)
+ (set-mutex-owner! mutex #f)
(threads:unlock-mutex (mutex-prim mutex)))
(else #f)))))
If you need any additional information let me know.
Thanks for your great work!
- d4ryus
reproducer.scm
Description: Lotus Screencam
potential-fix.patch
Description: Text document
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