some newbie questions about __attribute__((context,...))

["Robert P. J. Day" <rpjday@crashcourse.ca>]

  (if there's a good writeup on this, or a previous mailing post
explaining this, a pointer to that will do just fine.)

  i'm trying to understand how this context checking is actually done
by sparse.  from the sparse man page:

"Functions with the extended attribute
__attribute__((context(expression,in_context,out_context)) require the
context expression (for instance, a lock) to have the value in_context
(a constant nonnegative integer) when called, and return with the
value out_context (a constant nonnegative integer)."

  fair enough, but what are the possibilities for that "expression"
and what exactly is being compared to the values of 0 or 1?  sure, a
lock is an obvious candidate, but you can't really simply be comparing
the value of a lock to 0 or 1 -- a lock is a *structure* which doesn't
have a simple value of 0 or 1.

  so, to try to keep things simple, what is happening in the
background with code like this:

=====
static void *aarp_seq_start(struct seq_file *seq, loff_t *pos)
        __acquires(aarp_lock)
{
        struct aarp_iter_state *iter = seq->private;

        read_lock_bh(&aarp_lock);
        iter->table     = resolved;
        iter->bucket    = 0;

        return *pos ? iter_next(iter, pos) : SEQ_START_TOKEN;
}
=====

  and how is that call to read_lock_bh() changing the "context" value
of the object aarp_lock?  thanks for any enlightenment.

rday
--


========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry:
    Have classroom, will lecture.

http://crashcourse.ca                          Waterloo, Ontario, CANADA
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