On Thu, Mar 20, 2025 at 04:39:11PM +0000, David Howells wrote:
> Jarkko Sakkinen <jarkko@xxxxxxxxxx> wrote:
>
> > > + if (test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) {
> > > + smp_mb(); /* Clobber key->user after FINAL_PUT seen. */
> >
> > test_bit() is already atomic.
>
> Atomiticity doesn't apply to test_bit() - it only matters when it does two (or
> more) accesses that must be perceptually indivisible (e.g. set_bit doing RMW).
>
> But atomiticity isn't the issue here, hence the barrier. You need to be
> looking at memory-barriers.txt, not atomic_bitops.txt.
>
> We have two things to correctly order and set_bit() does not imply sufficient
> barriering; test_and_set_bit() does, but not set_bit(), hence Linus's comment
> about really wanting a set_bit_release().
Oops, I was hallucinating here. And yeah, test_and_set_bit() does
imply full mb as you said.
I was somehow remembering what I did in SGX driver incorrectly and
that led me into misconclusions, sorry.
if (test_and_set_bit(SGX_ENCL_IOCTL, &encl->flags))
return -EBUSY;
>
> > > + smp_mb(); /* key->user before FINAL_PUT set. */
> > > + set_bit(KEY_FLAG_FINAL_PUT, &key->flags);
> >
> > Ditto.
>
> Ditto. ;-)
Duh, no need poke with the stick further (or deeper) ;-)
>
> > Nit: I'm just thinking should the name imply more like that "now
> > key_put() is actually done". E.g., even something like KEY_FLAG_PUT_DONE
> > would be more self-descriptive.
>
> KEY_FLAG_PUT_DONE isn't right. There can be lots of puts on a single key -
> only the one that reduces it to 0 matters for this. You could call it
> KEY_FLAG_CAN_NOW_GC or KEY_FLAG_GC_ABLE.
Well all alternatives are fine but my thinking was that one that finally
zeros the refcount, "finalizes put" (pick whatever you want anyway).
>
> David
BR, Jarkko
