Re: ECPG thread success (kind of) on Linux

According to POSIX 1003.1c-1995, no such mutex-altering function exists.

pthread_mutexattr_get/settype(...) functions are defined by X/Open XSH5
(Unix98). I would suggest writing a wrapper for OSs that don't
implement recursive locks (it's easy enough to make your own
implementation- just check pthread_self() before deciding whether to
lock the mutex- potentially again). Either that or the recursive locks
can be eliminated.

Just for the record, OS X, Solaris 5.8, FreeBSD 4.8, and LinuxThreads
support the UNIX98 version, so perhaps this isn't so important after
all.

On Thursday, June 26, 2003, at 08:45 PM, Philip Yarra wrote:

> On Thu, 26 Jun 2003 11:19 am, Philip Yarra wrote:
>
>> there appears to still be a problem
>> occurring at "EXEC SQL DISCONNECT con_name". I'll look into it
>> tonight if I
>> can.
>
> I did some more poking around last night, and believe I have found the
> issue:
> RedHat Linux 7.3 (the only distro I have access to currently) ships
> with a
> fairly challenged pthreads inplementation. The default mutex type
> (which you
> get from PTHREAD_MUTEX_INITIALIZER) is, according the the man page,
> PTHREAD_MUTEX_FAST_NP which is not a recursive mutex. If a thread owns
> a
> mutex and attempts to lock the mutex again, it will hang.
>
> By replacing PTHREAD_MUTEX_INITIALIZER with PTHREAD_MUTEX_RECURSIVE_NP
> for the
> two mutexes that are used recursively (debug_mutex and
> connections_mutex) I
> got my sample app to work flawlessly on Linux RedHat 7.3
>
> Sadly, the _NP suffix is used to indicate non-portable, so of course my
> FreeBSD box steadfastly refused to compile it. Darn.
>
> The correct way to do this appears to be:
>
> pthread_mutexattr_t *mattr;
> pthread_mutexattr_settype(mattr, PTHREAD_MUTEX_RECURSIVE);
>
> (will verify this against FreeBSD when I get home, and Tru64 man page
> indicates support for this too, so I'll test that later). It won't
> work on
> RedHat Linux 7.3... I guess something like:
>
> #ifdef DODGY_PTHREADS
> #define PTHREAD_MUTEX_RECURSIVE = PTHREAD_MUTEX_RECURSIVE_NP
> #endif
>
> might do it... if we could detect the problem during configure. How is
> this
> sort of detection handled in other cases (such as long long, etc)?
>
> The other solution I can think of is to eradicate the two recursive
> locks I
> found.
>
> One is simple: ECPGlog calls ECPGdebug, which share debug_mutex - it
> ought to
> be okay to use different mutexes for each of these functions (there's
> a risk
> someone might call ECPGdebug while someone else is running through
> ECPGlog,
> but I think it is less likely, since it is a debug mechanism.)
>
> The second recursive lock I found is ECPGdisconnect calling
> ECPGget_connection, both of which share a mutex. Would it be okay if
> we did
> the following:
>
> ECPGdisconnect() still locks connections_mutex, but calls
> ECPGget_connection_nr() instead of ECPGget_connection()
>
> ECPGget_connection() becomes a locking wrapper, which locks
> connections_mutex
> then calls ECPGget_connection_nr()
>
> ECPGget_connection_nr() is a non-locking function which implements what
> ECPGget_connection() currently does.
>
> I'm not sure if this sort of thing is okay (and there may be other
> recursive
> locking scenarios that I haven't exercised yet).
>
> What approach should I take? I'm leaning towards eradicating recursive
> locks,
> unless someone has a good reason not to.
>
>> All this does kinda raise the interesting question of why it worked
>> at all
>> on FreeBSD... probably different scheduling and blind luck, I suppose.
>
> FreeBSD 4.8 must have PTHREAD_MUTEX_RECURSIVE as default mutex type.
> I'm a bit
> concerned about FreeBSD 4.2 though - I noticed (before I blew it away
> in
> favour of 4.8) that its pthreads implementation came from a package
> called
> linuxthreads.tgz - it might have inherited the same problematic
> behaviour.
> Could someone with access to or knowledge of FreeBSD 4.2 check what the
> default mutex type is there?
>
> Regards, Philip.
>
> I can just see the ad for 7.3's pthreads impementation
> "Fast mutexes: zero to deadlock in 6.9 milliseconds!"
>
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