[Spread-users] Question about thread-safety
jschultz at d-fusion.net
Fri Jun 13 14:31:05 EDT 2003
NOTE: if anyone knows how to instruct Unix/Linux systems not to reuse
file descriptor IDs in a process, could you please email me or the list
a good reference (with page #)? Thanks!
What you are proposing is sound provided that you #define _REENTRANT
when compiling and link with Spread's thread safe library libtspread.a.
Currently, you can get spurious errors from Spread for the following
reason: whenever there is a non-user error (socket failure, etc.) on a
mailbox/socket, the Spread user library immediately closes and
invalidates the mailbox/socket and returns CONNECTION_CLOSED.
Any subsequent SP call's on that mailbox/socket will return
ILLEGAL_SESSION. So, if your sender thread gets a CONNECTION_CLOSED
your recv'er thread would very likely get an ILLEGAL_SESSION (or maybe a
CONNECTION_CLOSED), and vice versa. Just treat any such ILLEGAL_SESSION
error as if it were a CONNECTION_CLOSED error.
Personally, I think that the Spread library should be modified to record
any such error and return it for all subsequent SP calls on that
mailbox. Furthermore, the mailbox/socket should be invalidated/closed
only upon the user calling SP_disconnect on it.
If your program is opening and closing multiple Spread connections then
there is also a OS file descriptor reuse race condition that could be
causing problems. This race condition is best explained by example:
Imagine you have a sender thread (x) and a receiver thread (y) for
mailbox/socket A and another thread (z) which is going to call
SP_connect to create a mailbox/socket B. Just before x starts writing a
msg on A, y receives an error on A and therefore immediately
closes/invalidates it. Next, z successfully performs SP_connect and is
assigned mailbox/socket B, which happens to have the same value as A due
to the OS reusing file descriptor IDs. Finally, y happily (and
successfully) writes its msg for A on B not realizing that it is
actually writing to a different Spread connection!
This behavior is obviously not correct! I'm not sure if this race
condition exists on Windows but it definitely exists in Unix/Linux. I
don't know if this problem can be reliably handled on the daemon side
and I doubt if currently the daemon even tries to detect it.
The only way I can think of to avoid this race condition with the
current Spread library is to instruct your OS not to reuse file
descriptors IDs (see NOTE above).
If the Spread library is modified as I suggested above, then I think the
race condition could be avoided by synchronizing calls to SP_connect and
Co-Founder, Lead Engineer
D-Fusion, Inc. (http://www.d-fusion.net)
Phn: 443-838-2200 Fax: 707-885-1055
White Stuart - stwhit wrote:
> Hello all,
> I'm a spread newbie - just downloaded a few days ago. My question is
> regarding using Spread in a threaded C application.
> I would like to write an application that can simultaneously send and
> receive messages. In my main thread, I spawn another thread (the "message
> handler" thread) and he loops forever calling SP_receive(). The main thread
> loops, sending his messages via SP_multicast().
> My question is this: I'm getting spurrious failures, (getting errors like
> ILLEGAL_SESSION from SP_receive) and I'm wondering what synchronization is
> appropriate for using spread in a threaded application? Do I need to make
> sure only 1 thread is calling an SP_* function at-a-time?
> Thanks much!
> Stuart White
> Acxiom Corporation
> AbiliTec Architecture
> Email: stuart.white at acxiom.com
> Phone: 501.329.5975
More information about the Spread-users