linux-kernel - Re: [PATCH v3 2/2] make buffer_locked provide an acquire semantics

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Message-ID: <Yub+vPb53zt6dDpn@casper.infradead.org>
Date:   Sun, 31 Jul 2022 23:14:20 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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 v3 2/2] make buffer_locked provide an acquire semantics

On Sun, Jul 31, 2022 at 04:43:08PM -0400, Mikulas Patocka wrote:
> Let's have a look at this piece of code in __bread_slow:
> 	get_bh(bh);
> 	bh->b_end_io = end_buffer_read_sync;
> 	submit_bh(REQ_OP_READ, 0, bh);
> 	wait_on_buffer(bh);
> 	if (buffer_uptodate(bh))
> 		return bh;
> Neither wait_on_buffer nor buffer_uptodate contain a memory barrier.
> Consequently, if someone calls sb_bread and then reads the buffer data,
> the read of buffer data may be executed before wait_on_buffer(bh) on
> architectures with weak memory ordering and it may return invalid data.

I think we should be consistent between PageUptodate() and
buffer_uptodate().  Here's how it's done for pages currently:

static inline bool folio_test_uptodate(struct folio *folio)
        bool ret = test_bit(PG_uptodate, folio_flags(folio, 0));
        /*
         * Must ensure that the data we read out of the folio is loaded
         * _after_ we've loaded folio->flags to check the uptodate bit.
         * We can skip the barrier if the folio is not uptodate, because
         * we wouldn't be reading anything from it.
         *
         * See folio_mark_uptodate() for the other side of the story.
         */
        if (ret)
                smp_rmb();

        return ret;

...

static __always_inline void folio_mark_uptodate(struct folio *folio)
        /*
         * Memory barrier must be issued before setting the PG_uptodate bit,
         * so that all previous stores issued in order to bring the folio
         * uptodate are actually visible before folio_test_uptodate becomes true.
         */
        smp_wmb();
        set_bit(PG_uptodate, folio_flags(folio, 0));

I'm happy for these to also be changed to use acquire/release; no
attachment to the current code.  But bufferheads & pages should have the
same semantics, or we'll be awfully confused.