Re: Atomic Reference Counting - Do you think this would be useful to Boost?

"kanze" <>
16 Oct 2006 04:58:32 -0400
Peter Dimov wrote:

James Kanze wrote:

Chris Thomasson wrote:

I don't think the following contrived scenario is valid with
shared_ptr<...>. Please correct me if I am wrong:


static shared_ptr<app> g_app(new app(...))
static shared_ptr<foo> g_foo;

void a_number_of_threads() {
  for(...) {

    // I don't think the following is atomic in current shared_ptr<...>:
    shared_ptr<foo> l_foo(g_foo);

I can't see any reason why it should be. I can't imagine any
use where it wouldn't lead to a programming error later.

This is the reader-writer problem. One scenario is

void reader()
     shared_ptr<foo> local( g_foo );
     // use local to compute something

void writer()
     shared_ptr<foo> local( g_foo );
     shared_ptr<foo> next( new foo( *local ) );
     g_foo = next; // race here

One could put a fast reader-writer lock around g_foo, of
course, but it wouldn't be truly lock-free.

Well, the context requires memory synchronization, and something
to ensure the atomicity of the update of g_foo.

But it begs the point. I can easily image abstract, constructed
cases where you'd need some sort of locking or synchronization.
I can't find any practical application for them, however.

Consider, for example, what you might want if g_foo pointed to
configuration data, read from a file. Your writer function
detects that the configuration file has changed (i.e. a newer
time stamp, or there was a signal telling the program to
rereaded it). At this point, however, it is important that any
given transaction see a coherent view of the configuration. And
within a transaction, it's quite possible that there are several
"readers". So you'll probably want to synchronize the writer
with the transaction logic, at a higher level. And this is the
case for every scenario that I can think of. My impression is
that when dealing with global data, shared_ptr is a bit too low
level; you need something higher level (and once you've got it,
shared_ptr becomes caduc).

Also, of course, global variables don't normally hold critical
resources that must be freed in a timely manner. So you can
just use ordinary pointers, and let the Boehm collector do the
cleaning up. No reason to introduce extra complexity.

Where a shared_ptr is useful for managing global data, IMHO, is
where you have a singleton. Calling instance() acquires a lock,
and returns a shared_ptr whose "destructor" frees the lock. It
is, perhaps, somewhat dangerous, in that anyone keeping a
pointer blocks the entire system, but then, anyone keeping a
pointer needs the lock, no matter how you manage it, and so will
block the system anyway.

James Kanze GABI Software
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