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

[Mikulas Patocka]

On Sun, 31 Jul 2022, Linus Torvalds wrote:

> [ Will and Paul, question for you below ]
> 
> On Sun, Jul 31, 2022 at 8:08 AM Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
> >
> > Also, there is this pattern present several times:
> >         wait_on_buffer(bh);
> >         if (!buffer_uptodate(bh))
> >                 err = -EIO;
> > It may be possible that buffer_uptodate is executed before wait_on_buffer
> > and it may return spurious error.
> 
> I'm not convinced that's actually valid.
> 
> They are testing the same memory location, and I don't think our
> memory ordering model allows for _that_ to be out-of-order. Memory
> barriers are for accesses to different memory locations.

You are right. And the bit tests are volatile, so the compiler can't 
reorder them.

But the compiler can reorder non-volatile accesses around volatile 
accesses (gcc does this, clang afaik doesn't), so the bit tests need at 
least a compiler barrier after them.

> But the patch looks fine, though I agree that the ordering in
> __wait_on_buffer should probably be moved into
> wait_on_bit/wait_on_bit_io.

Yes, there are more bugs where the code does wait_on_bit and then reads 
some data without any barrier. Adding the barrier to wait_on_bit fixes 
that.

I'll send two patches, one for wait_on_bit and the other for 
buffer_locked.

Do you think that wait_event also needs a read memory barrier? It is 
defined as:
#define wait_event(wq_head, condition)                                          \
do {                                                                            \
        might_sleep();                                                          \
        if (condition)                                                          \
                break;                                                          \
        __wait_event(wq_head, condition);                                       \
} while (0)

Mikulas

> And would we perhaps want the bitops to have the different ordering
> versions? Like "set_bit_release()" and "test_bit_acquire()"? That
> would seem to be (a) cleaner and (b) possibly generate better code for
> architectures where that makes a difference?
> 
>                Linus
>