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

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Date:   Sun, 31 Jul 2022 16:39:26 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Will Deacon <will@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2] make buffer_locked provide an acquire semantics



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@...hat.com> 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
>