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Date:   Thu, 11 Mar 2021 16:08:30 +0100
From:   Marco Elver <elver@google.com>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Jan Kara <jack@suse.cz>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        syzbot <syzbot+30774a6acf6a2cf6d535@syzkaller.appspotmail.com>,
        Jan Kara <jack@suse.com>, linux-ext4@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Theodore Ts'o <tytso@mit.edu>
Subject: Re: [syzbot] KCSAN: data-race in start_this_handle /
 start_this_handle
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References: <0000000000008de88005bd40ac36@google.com>
 <20210311142503.GA31816@quack2.suse.cz>
 <CACT4Y+ZtBwv1aXUumTXnWzAi7LEpJ6CZemGyVR2FC6_YO2E4EQ@mail.gmail.com>
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On Thu, Mar 11, 2021 at 03:53PM +0100, Dmitry Vyukov wrote:
> On Thu, Mar 11, 2021 at 3:25 PM Jan Kara <jack@suse.cz> wrote:
> >
> > On Thu 11-03-21 02:59:14, syzbot wrote:
> > > HEAD commit:    a74e6a01 Merge tag 's390-5.12-3' of git://git.kernel.org/p..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=159f69ecd00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=de394bbaade74fb7
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=30774a6acf6a2cf6d535
> > > compiler:       Debian clang version 11.0.1-2
> > >
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+30774a6acf6a2cf6d535@syzkaller.appspotmail.com
> > >
> > > ==================================================================
> > > BUG: KCSAN: data-race in start_this_handle / start_this_handle
> > >
> > > write to 0xffff888103880870 of 8 bytes by task 29956 on cpu 1:
> > >  jbd2_get_transaction fs/jbd2/transaction.c:125 [inline]
> > >  start_this_handle+0xceb/0x1010 fs/jbd2/transaction.c:400
> > >  jbd2__journal_start+0x1fc/0x3f0 fs/jbd2/transaction.c:503
> > >  __ext4_journal_start_sb+0x159/0x310 fs/ext4/ext4_jbd2.c:105
> > >  __ext4_journal_start fs/ext4/ext4_jbd2.h:320 [inline]
> > >  ext4_da_write_begin+0x460/0xaf0 fs/ext4/inode.c:2998
> > >  generic_perform_write+0x196/0x3a0 mm/filemap.c:3575
> > >  ext4_buffered_write_iter+0x2e5/0x3e0 fs/ext4/file.c:269
> > >  ext4_file_write_iter+0x48a/0x10b0 fs/ext4/file.c:502
> > >  call_write_iter include/linux/fs.h:1977 [inline]
> > >  do_iter_readv_writev+0x2cb/0x360 fs/read_write.c:740
> > >  do_iter_write+0x112/0x4c0 fs/read_write.c:866
> > >  vfs_iter_write+0x4c/0x70 fs/read_write.c:907
> > >  iter_file_splice_write+0x40a/0x750 fs/splice.c:689
> > >  do_splice_from fs/splice.c:767 [inline]
> > >  direct_splice_actor+0x80/0xa0 fs/splice.c:936
> > >  splice_direct_to_actor+0x345/0x650 fs/splice.c:891
> > >  do_splice_direct+0xf5/0x170 fs/splice.c:979
> > >  do_sendfile+0x7a6/0xe20 fs/read_write.c:1260
> > >  __do_sys_sendfile64 fs/read_write.c:1319 [inline]
> > >  __se_sys_sendfile64 fs/read_write.c:1311 [inline]
> > >  __x64_sys_sendfile64+0xa9/0x130 fs/read_write.c:1311
> > >  do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
> > >  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > >
> > > read to 0xffff888103880870 of 8 bytes by task 29936 on cpu 0:
> > >  start_this_handle+0x1c1/0x1010 fs/jbd2/transaction.c:352
> > >  jbd2__journal_start+0x1fc/0x3f0 fs/jbd2/transaction.c:503
> > >  __ext4_journal_start_sb+0x159/0x310 fs/ext4/ext4_jbd2.c:105
> > >  __ext4_journal_start fs/ext4/ext4_jbd2.h:320 [inline]
> > >  ext4_da_write_begin+0x460/0xaf0 fs/ext4/inode.c:2998
> > >  generic_perform_write+0x196/0x3a0 mm/filemap.c:3575
> > >  ext4_buffered_write_iter+0x2e5/0x3e0 fs/ext4/file.c:269
> > >  ext4_file_write_iter+0x48a/0x10b0 fs/ext4/file.c:502
> > >  call_write_iter include/linux/fs.h:1977 [inline]
> > >  do_iter_readv_writev+0x2cb/0x360 fs/read_write.c:740
> > >  do_iter_write+0x112/0x4c0 fs/read_write.c:866
> > >  vfs_iter_write+0x4c/0x70 fs/read_write.c:907
> > >  iter_file_splice_write+0x40a/0x750 fs/splice.c:689
> > >  do_splice_from fs/splice.c:767 [inline]
> > >  direct_splice_actor+0x80/0xa0 fs/splice.c:936
> > >  splice_direct_to_actor+0x345/0x650 fs/splice.c:891
> > >  do_splice_direct+0xf5/0x170 fs/splice.c:979
> > >  do_sendfile+0x7a6/0xe20 fs/read_write.c:1260
> > >  __do_sys_sendfile64 fs/read_write.c:1319 [inline]
> > >  __se_sys_sendfile64 fs/read_write.c:1311 [inline]
> > >  __x64_sys_sendfile64+0xa9/0x130 fs/read_write.c:1311
> > >  do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
> > >  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > >
> > > Reported by Kernel Concurrency Sanitizer on:
> > > CPU: 0 PID: 29936 Comm: syz-executor.5 Not tainted 5.12.0-rc2-syzkaller #0
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > > ==================================================================
> >
> > So this case is harmless. start_this_handle() does indeed check
> > journal->j_running_transaction without any protection and this is only a
> > racy check to opportunistically preallocate a transaction if we are likely
> > to need it. There was some macro to instruct KCSAN that the read is
> > actually fine, wasn't there?

If the outcome of the check does not affect correctness and the code is
entirely fault tolerant to the precise value being read, then a
data_race(!journal->j_running_transaction) marking here would be fine.

If in doubt, this is the latest summary for concurrent access markings:
https://lkml.kernel.org/r/20210220051000.GA457@paulmck-ThinkPad-P72

> +Marco
> +Tetsuo, did you want to fix it?

Thanks,
-- Marco