Re: [PATCH v2] make buffer_locked provide an acquire semantics

[Matthew Wilcox]

On Sun, Jul 31, 2022 at 10:30:11AM -0700, Paul E. McKenney wrote:
> That said, I confess that I am having a hard time finding the
> buffer_locked() definition.  So if buffer_locked() uses normal C-language
> accesses to sample the BH_Lock bit of the ->b_state field, then yes,
> there could be a problem.  The compiler would then be free to reorder
> calls to buffer_locked() because it could then assume that no other
> thread was touching that ->b_state field.

You're having a hard time finding it because it's constructed with the C
preprocessor.  I really wish we generated header files using CPP once
and then included the generated/ file.  That would make them greppable.

You're looking for include/linux/buffer_head.h and it's done like this:

enum bh_state_bits {
...
        BH_Lock,        /* Is locked */
...

#define BUFFER_FNS(bit, name)                                           \
...
static __always_inline int buffer_##name(const struct buffer_head *bh)  \
{                                                                       \
        return test_bit(BH_##bit, &(bh)->b_state);                      \
}

BUFFER_FNS(Lock, locked)

(fwiw, the page/folio versions of these functions don't autogenerate
the lock or uptodate ones because they need extra functions called)