threading issues in 1.8?

[Ken Raeburn <raeburn@raeburn.org>]

Hi.  I've been starting to look at the 1.8 code, and I'm concerned  
that there may still be thread safety issues.

1) See my old mail about scm_leave_guile -- in fact, it's now in a  
comment in threads.c. :-)  We're still calling setjmp, returning, and  
then calling the user's function.  I think setjmp and the user's  
function need to be called from the same function, or at least the  
function calling setjmp should not return before the user's function  
is called.  (And possibly any args and local variables should be  
"volatile".)

2) Access to newly created objects: I've been trying to figure out...  
if one thread allocates a new object (cons cell, say, or vector, or  
smob), and does a set-car! or something to make it accessible to  
another thread running on another processor, and the system  
implements a weak memory order model (so memory writes done without  
synchronization primitives can be seen as happening out of order),  
what's to cause the contents of the new pair to get written before  
the second thread can examine them?

Sure, we've more or less adopted the rule that without  
synchronization, one processor may see the "before" or "after"  
results of another thread's modifications, or two threads modifying  
an object may cause it to end up with some unpredictable but valid  
value.  But does that work in the case of a free cell being turned  
into a cons cell or smob, or random heap storage being turned into a  
vector?

I've been doing a little reading.  Not enough to have any answers  
yet, but enough to see that Java ran into similar issues.  Bill  
Pugh's paper, "Implementing OO Languages under a Weak Memory  
Order" (http://www.cs.umd.edu/~pugh/java/memoryModel/weak.pdf)  
discusses much the same issue as was bothering me in Guile -- object  
initialization in multiprocessor systems with weak memory ordering,  
particularly on the Alpha.  (From other stuff I've read, Alpha seems  
to provide the weakest coherency model of modern processors, so if we  
want a robust solution, it pretty much needs to target the Alpha.)   
His paper is largely about Java, but it seems to me that Guile needs  
a more precise memory model as well, if it's to be multiprocessor- 
multithreaded and robust.  Unfortunately, I don't follow the Java  
world closely, but it looks like the revision of the Java memory  
model and thread spec is worth reading up on.  I'm still looking....

Ken

P.S.  For those not familiar with the Alpha, a tiny bit of  
background.  Digital made some architectural design decisions  
intended to allow for aggressive performance work.  One of them was  
the weak memory coherency model.  The processor has "memory barrier"  
instructions which can force delayed writes to be performed, or cause  
changes to main memory by other processors to be seen; naturally,  
these are somewhat expensive, so you don't want to do them all the  
time.  But without the memory barrier, this sort of situation is  
*explicitly* permitted in the architecture reference manual:

   initial state: X=1, Y=1
   processor I instruction stream: write X=2, write Y=2
   processor J instruction stream: read Y => 2, read X => 1

Even if one processor (not both) uses a memory barrier instruction in  
between the two operations, the reordered results are allowed.   
Without a barrier, processor I can perform the second write first,  
and without a barrier, processor J can perform the second read first.

Another interesting change in early Alpha processors was the absence  
of byte memory I/O operations.  (Later processors, since about the  
EV56, have added them in as an extension.)  If you wanted to modify a  
byte, you'd read the containing 32- or 64-bit word, modify it, and  
write it back; likewise for 16-bit shorts.

There are instructions to provide coordinated changes to specific  
locations.  The "load locked" instructions load a word from storage  
and "locks" the address; the "store conditional" instruction stores a  
value if the lock is still set, and sets a flag indicating whether it  
worked.  The lock flag can be reset by various conditions, including  
other reads or writes from the same processor (I think?), interrupts  
(including system clock), branches, and writes by other processors to  
the same location (or within a 2**N block containing it, where the  
block size is architecture-dependent, but must be at least 16 and no  
more than one page).  If the other processor is using these  
instructions too, both processors can try to modify a location, but  
only one (at most) will succeed.


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