[RFC][CFT] GEOM direct dispatch and fine-grained CAM locking

[Ryan Stone]

On Wed, Sep 4, 2013 at 8:45 AM, Nathan Whitehorn <nwhitehorn at freebsd.org> wrote:
> Could you describe what this macro is supposed to do so that we can do the
> porting work?
> -Nathan

#define GET_STACK_USAGE(total, used)

GET_STACK_USAGE sets the variable passed in total to the total amount
of stack space available to the current thread.  used is set to the
amount of stack space currently used (this does not have to have
byte-precision).  Netgraph uses this to decide when to stop recursing
and instead defer to a work queue (to prevent stack overflow).  I
presume that Alexander is using it in a similar way.  It looks like
the amd64 version could be ported to other architectures quite easily
if you were to account for stacks that grow up and stacks that grow
down:

http://svnweb.freebsd.org/base/head/sys/amd64/include/proc.h?revision=233291&view=markup

/* Get the current kernel thread stack usage. */
#define GET_STACK_USAGE(total, used) do {                \
    struct thread    *td = curthread;                \
    (total) = td->td_kstack_pages * PAGE_SIZE;            \
    (used) = (char *)td->td_kstack +                \
        td->td_kstack_pages * PAGE_SIZE -                \
        (char *)&td;                        \
} while (0)