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Date: Thu, 23 Aug 2001 09:57:21 -0400
From: Christopher Faylor <cgf AT redhat DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: bug in pthread_mutex_init (cygwin-src-20010813)
Message-ID: <20010823095721.F15420@redhat.com>
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References: <15230 DOT 62731 DOT 676056 DOT 47490 AT wallaroo DOT lum DOT org> <998194382 DOT 16382 DOT 47 DOT camel AT lifelesswks>
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In-Reply-To: <998194382.16382.47.camel@lifelesswks>; from robert.collins@itdomain.com.au on Sun, Aug 19, 2001 at 02:13:45PM +1000

Was there a ruling on this bug?  I'd like to make a new release soon and
it sounds like the patch below should be in it?

cgf

On Sun, Aug 19, 2001 at 02:13:45PM +1000, Robert Collins wrote:
>On 18 Aug 2001 18:06:51 -0500, David Lum wrote:
>> Hi.
>> 
>> I think there's a problem with pthread_mutex_init in Cygwin-- I looked
>> at the source code (from 8/13/01) and it verified my suspicions.
>> 
>> The implementation of pthread_mutex_init in "winsup/cygwin/thread.cc"
>> does this:
>> 
>> > int
>> > __pthread_mutex_init (pthread_mutex_t *mutex,
>> >                       const pthread_mutexattr_t *attr)
>> > {
>> >   if ((((pshared_mutex *)(mutex))->flags & SYS_BASE == SYS_BASE))
>> >     // a pshared mutex
>> >     return EBUSY;
>> >   ...
>> > }
>> 
>> This is bogus!  The pthread specification permits the passed-in mutex
>> pointer to refer to completely uninitialized memory.  However, the
>> first line of code here accesses a flag from that region of random
>> memory.  If the bit happens to be zero, all is well.  But if it
>> happens to be 1 then the call fails with EBUSY.
>
>Good catch.
> 
>> It fails on the second call with the expected EBUSY return code.
>> 
>> I don't really have any suggestions on how to fix this, since it was
>> probably caused by the introduction of "pshared" mutexes (which I do
>> not understand).  But I'll bet someone on the list does.
>
>pshared mutexs are mutex shared by several process's. In you have done
>win32 programming compare critical  sections(single process only) with
>win32 mutexs (multiple process's either by naming or handle passing).
>
>Clean pshared mutexs requires a daemon process, something that is on the
>wishlist. So for now its going to stay rather ugly :[. Variants of this
>bug will affect pthread_cond_init as well.. and the fix will be similar
>:]
>
>Try this David... if it works please confirm via the mailing list and
>I'll fixup the cond_init as well.
>
>Index: thread.cc
>===================================================================
>RCS file: /cvs/src/src/winsup/cygwin/thread.cc,v
>retrieving revision 1.42
>diff -u -p -r1.42 thread.cc
>--- thread.cc	2001/08/04 21:10:52	1.42
>+++ thread.cc	2001/08/19 04:10:52
>@@ -1848,8 +1848,14 @@ __pthread_mutex_init (pthread_mutex_t *m
> 		      const pthread_mutexattr_t *attr)
> {
>   if ((((pshared_mutex *)(mutex))->flags & SYS_BASE == SYS_BASE))
>-    // a pshared mutex
>-    return EBUSY;
>+    {
>+      // a pshared mutex or random data?
>+      pshared_mutex *pmutex=(pshared_mutex *)(mutex);
>+      if ((MT_INTERFACE->pshared_mutexs[pmutex->id]) != NULL)
>+	return EBUSY;
>+      // not a pshared mutex.
>+      *mutex = NULL;
>+    }
>   if (attr && !verifyable_object_isvalid (*attr,
>PTHREAD_MUTEXATTR_MAGIC))
>     return EINVAL;
>
>
>PS (referring to your private mail) I prefer all cygwin related matters
>to be sent to the mailing list - there's no need to bring something up
>publically and privately. This serves serveral purposes - Archival,
>openness, mailbox organisation :].
>
>Rob
>
>> 
>> 				       -Dave
>
>
>
>
>--
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-- 
cgf AT cygnus DOT com                        Red Hat, Inc.
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