On 18-May-2002 Kenneth D. Merry wrote:
> On Fri, May 17, 2002 at 23:02:55 -0700, Alfred Perlstein wrote:
>> * Kenneth D. Merry <[EMAIL PROTECTED]> [020517 22:40] wrote:
>> >
>> > I have released a new set of zero copy sockets patches, against -current
>> > from today (May 17th, 2002).
>> >
>> > The main change is to deal with the vfs_ioopt changes that Alan Cox made
>> > in
>> > kern_subr.c. (They conflicted a bit with the zero copy receive code.)
>> >
>> > The patches and the FAQ are available here:
>> >
>> > http://people.freebsd.org/~ken/zero_copy/
>> >
>> > Comments, questions and reviews are all welcome!
>>
>> jumbo_vm_init() has a bunch of bugs
>>
>> first it doesn't work right if called concurrently.
>> you need to protect the initialization of jumbo_vm_object otherwise
>> bad things can happen. my suggestion is to store the results of
>> vm_object_allocate into a temporary, grab the mutex and then check
>> to see if jumbo_vm_object has been initialized again if it has then
>> free the object, otherwise store the allocated object into the
>> global and continue.
>
> The problem here is that the mutex needs to be initialized before I can
> acquire it, and there's going to be a race between checking to see
> whether it has been initialized and actually initializing it.
>
> e.g.:
>
> if (!mtx_initialized(&jumbo_mutex))
> mtx_init(&jumbo_mutex, "jumbo mutex", NULL, MTX_DEF);
>
> mtx_lock(&jumbo_mutex);
>
> if (jumbo_vm_object != NULL) {
> mtx_unlock(&jumbo_mutex);
> return (1);
> }
>
> /* allocate our object */
> jumbo_vm_object = vm_object_allocate(OBJT_DEFAULT, JUMBO_MAX_PAGES);
>
> The above would work, I think, if it weren't for the race in the mutex
> initialization, and assuming I can allocate a vm object while holding
> the jumbo mutex.
>
> Suggestions?
Either use a sysinit or something gross like this:
static int jumbo_init = 0;
...
while (jumbo_init != 2) {
if (atomic_cmpset_acq_int(&jumbo_init, 0, 1) {
mtx_init(&jumbo_mutex, "jumbo_mutex", NULL, MTX_DEF);
atomic_store_rel_int(&jumbo_init, 2);
}
}
mtx_lock(&jumbo_mutex);
...
a sysinit is probably best though.
--
John Baldwin <[EMAIL PROTECTED]> <>< http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve!" - http://www.FreeBSD.org/
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