svn commit: r202889 - head/sys/kern

[John Baldwin]

On Tuesday 26 January 2010 3:09:32 pm M. Warner Losh wrote:
> In message: <C6A8F7A7-F0A9-4F63-B61E-DDC5332DC495 at mac.com>
>             Marcel Moolenaar <xcllnt at mac.com> writes:
> : Maybe what is in order right now is a description (using pseudo
> : code if you like) of what exactly needs to happen with the 3rd
> : argument, when and how (i.e. what must be atomic and what does
> : not have to be atomic).
> 
> I believe the proper pseudo code should be:
> 
> cpu_switch(struct thread *old, struct thread *new, struct mutext *mtx)
> {
> 	/* Save the registers to the pcb */
> 	old->td_lock = mtx;
> #if defined(SMP) && defined(SCHED_ULE)
> 	/* s/long/int/ if sizeof(long) != sizeof(void *) */
> 	/* as we have no 'void *' version of the atomics */
> 	while (atomic_load_acq_long(&new->td_lock) == (long)&blocked_lock)
> 		continue;
> #endif
> 	/* Switch to new context */
> }

FYI, that is what the '_ptr' variants of atomic ops are for.  I do think that 
the 'acq' membar on the load is needed to ensure the CPU doesn't try to 
speculatively read any of the 'new' thread's PCB until it sees the update to 
td_lock.

-- 
John Baldwin