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Date:   Sat, 9 Dec 2023 07:51:30 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     kernel test robot <lkp@intel.com>
Cc:     llvm@lists.linux.dev, oe-kbuild-all@lists.linux.dev,
        linux-kernel@vger.kernel.org, Josef Bacik <josef@toxicpanda.com>,
        julia.lawall@inria.fr, clm@fb.com, dsterba@suse.com,
        baptiste.lepers@gmail.com
Subject: Re: [paulmck-rcu:frederic.2023.12.08a 29/37]
 fs/btrfs/transaction.c:496:6: error: call to '__compiletime_assert_329'
 declared with 'error' attribute: Need native word sized stores/loads for
 atomicity.
Message-ID: <0487d7cc-b906-4a4a-b284-9c79700b4ede@paulmck-laptop>
Reply-To: paulmck@kernel.org
References: <202312091837.cKaPw0Tf-lkp@intel.com>
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On Sat, Dec 09, 2023 at 06:20:37PM +0800, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git frederic.2023.12.08a
> head:   37843b5f561a08ae899fb791eeeb5abd992eabe2
> commit: 7dd87072d40809e26503f04b79d63290288dbbac [29/37] btrfs: Adjust ->last_trans ordering in btrfs_record_root_in_trans()
> config: riscv-rv32_defconfig (https://download.01.org/0day-ci/archive/20231209/202312091837.cKaPw0Tf-lkp@intel.com/config)
> compiler: clang version 17.0.0 (https://github.com/llvm/llvm-project.git 4a5ac14ee968ff0ad5d2cc1ffa0299048db4c88a)
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20231209/202312091837.cKaPw0Tf-lkp@intel.com/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202312091837.cKaPw0Tf-lkp@intel.com/
> 
> All errors (new ones prefixed by >>):
> 
>    warning: unknown warning option '-Wpacked-not-aligned'; did you mean '-Wpacked-non-pod'? [-Wunknown-warning-option]
>    warning: unknown warning option '-Wstringop-truncation'; did you mean '-Wstring-concatenation'? [-Wunknown-warning-option]
>    warning: unknown warning option '-Wmaybe-uninitialized'; did you mean '-Wuninitialized'? [-Wunknown-warning-option]
> >> fs/btrfs/transaction.c:496:6: error: call to '__compiletime_assert_329' declared with 'error' attribute: Need native word sized stores/loads for atomicity.
>      496 |         if (smp_load_acquire(&root->last_trans) == trans->transid && /* ^^^ */
>          |             ^

Ooooh!!!  :-/

From what I can see, the current code can tear this load on 32-bit
systems, which can result in bad comparisons and then in failure to wait
for a partially complete transaction.

So is btrfs actually supported on 32-bit systems?  If not, would the
following patch be appropriate?

If btrfs is to be supported on 32-bit systems, from what I can see some
major surgery is required, even if a 32-bit counter is wrap-safe for
this particular type of transaction.  (But SSDs?  In-memory btrfs
filesystems?)

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/fs/btrfs/Kconfig b/fs/btrfs/Kconfig
index 4fb925e8c981..4d56158c34f9 100644
--- a/fs/btrfs/Kconfig
+++ b/fs/btrfs/Kconfig
@@ -19,6 +19,7 @@ config BTRFS_FS
 	select RAID6_PQ
 	select XOR_BLOCKS
 	depends on PAGE_SIZE_LESS_THAN_256KB
+	depends on 64BIT
 
 	help
 	  Btrfs is a general purpose copy-on-write filesystem with extents,