On Fri, Nov 22, 2013 at 3:40 PM, Jeff Trawick <trawick@gmail.com> wrote:
On Fri, Nov 22, 2013 at 3:24 PM, Jeff Trawick <trawick@gmail.com> wrote:
On Fri, Nov 22, 2013 at 2:52 PM, Jim Jagielski <jim@jagunet.com> wrote:
Note, the only think changed in event now (via https://svn.apache.org/viewvc?view=revision&revision=1542560)
is that event *checks* that atomics work as required for
event... if the check fails, it means that event has
been "broken" on that system, assuming it ever hit
blocked idlers, for a *long* time...

Got it...  fdqueue.c is asking for trouble...

I'm using atomic/unix/ia32.c with icc too.

Need to compare generated code...  I hate stuff like "int foo() { unsigned char x;  ... return x;  }"


APR is documented as returning "zero if the value becomes zero on decrement, otherwise non-zero".

With gcc we use get __sync_sub_and_fetch(), which returns the new value after the decrement.

With icc we use the assembler implementation in APR.  I think that's returning 1 instead of -1.

It does a decrement long, which sets the zero flag as appropriate.  Then it does set-not-equal to set what becomes the return value to 0 if the result of the decrement was 0 and 1 otherwise.

It "should be easy" to make it return the new value like the __sync_sub_and_fetch() version does :)
 

Here is where fdqueue needs to be able to see a negative return code:

apr_status_t ap_queue_info_try_get_idler(fd_queue_info_t * queue_info)
{
    int prev_idlers;
    prev_idlers = apr_atomic_dec32((apr_uint32_t *)&(queue_info->idlers));
    if (prev_idlers <= 0) {
        apr_atomic_inc32((apr_uint32_t *)&(queue_info->idlers));    /* back out dec */
        return APR_EAGAIN;
    }
    return APR_SUCCESS;
}

("prev" in the ia32.c version of apr_atomic_dec32() and "prev_idlers" here is deceiving.)

 

You should be seeing it in trunk as well...


On Nov 22, 2013, at 2:43 PM, Jeff Trawick <trawick@gmail.com> wrote:

> On Fri, Nov 22, 2013 at 2:39 PM, Jim Jagielski <jim@jagunet.com> wrote:
>
> On Nov 22, 2013, at 2:22 PM, Jeff Trawick <trawick@gmail.com> wrote:
>
> > On Sat, Nov 17, 2012 at 6:00 AM, Ruediger Pluem <rpluem@apache.org> wrote:
> >
> >
> > jim@apache.org wrote:
> > > +        i = apr_atomic_dec32(&foo);
> > > +        if (i >= 0) {
> >
> > Why can we expect i < 0? apr_atomic_dec32 returns 0 if the dec causes foo to become zero and it returns non zero
> > otherwise. Shouldn't this behavior the same across all platforms? And if not should that be fixed in APR?
> >
> > icc (Intel) builds of httpd 2.4.7 event MPM (with apr-1.5.0) bomb here.
> >
> > --enable-nonportable-atomics is specified for apr, though I haven't checked what that does with icc.
> >
>
> As noted back with the orig update, this test is due to the
> fdqueue code in the new event:
>
> apr_status_t ap_queue_info_set_idle(fd_queue_info_t * queue_info,
>                                     apr_pool_t * pool_to_recycle)
> {
>     apr_status_t rv;
>     int prev_idlers;
>
>     ap_push_pool(queue_info, pool_to_recycle);
>
>     /* Atomically increment the count of idle workers */
>     /*
>      * TODO: The atomics expect unsigned whereas we're using signed.
>      *       Need to double check that they work as expected or else
>      *       rework how we determine blocked.
>      * UPDATE: Correct operation is performed during open_logs()
>      */
>     prev_idlers = apr_atomic_inc32((apr_uint32_t *)&(queue_info->idlers));
>
>     /* If other threads are waiting on a worker, wake one up */
>     if (prev_idlers < 0) {
>
>
> See the comments ("The atomics expect unsigned whereas...") for
> the reason, etc.
>
> When you say "icc (Intel) builds of httpd 2.4.7 event MPM (with apr-1.5.0) bomb here."
> do you mean that you get the 'atomics not working as expected' error
> (and the internal server error) or that it core dumps?
>
>
> "atomics not working as expected"
>
> Let me see what code is used...
>
> --
> Born in Roswell... married an alien...
> http://emptyhammock.com/




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http://emptyhammock.com/