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Date:   Mon, 5 Apr 2021 09:26:14 -0700
From:   harshad shirwadkar <harshadshirwadkar@...il.com>
To:     Hao Sun <sunhao.th@...il.com>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: KCSAN: data-race in ext4_fc_commit / ext4_fc_commit

Thanks for the report Hao! I think following lines in fast_commit.c
are the culprit:

        if (likely(sbi->s_fc_avg_commit_time))
                sbi->s_fc_avg_commit_time = (commit_time +
                                sbi->s_fc_avg_commit_time * 3) / 4;
        else
                sbi->s_fc_avg_commit_time = commit_time;

This should have been done after taking sbi level lock, but this is
happening after releasing the lock.

I'll send a patch to fix this, thanks again!

- Harshad

On Sun, Apr 4, 2021 at 2:52 AM Hao Sun <sunhao.th@...il.com> wrote:
>
> Hi:
>
> When using Healer(https://github.com/SunHao-0/healer/tree/dev) to fuzz
> the Linux kernel, I found a data-race vulnerability in ext4_fc_commit
> / ext4_fc_commit, which looks strange.
> Sorry, data-race is usually difficult to reproduce. I cannot provide
> you with a reproducing program.
> I hope that the call stack information in the crash log can help you
> locate the problem.
> Kernel config and full log can be found in the attachment.
>
> Here is the detailed information:
> commit:   3b9cdafb5358eb9f3790de2f728f765fef100731
> version:   linux 5.11
> git tree:    upstream
> report:
> ==================================================================
> BUG: KCSAN: data-race in ext4_fc_commit / ext4_fc_commit
> write to 0xffff88800dd89f10 of 8 bytes by task 12063 on cpu 0:
>  ext4_fc_commit+0x1e6/0x1850 linux/fs/ext4/fast_commit.c:1205
>  ext4_fsync_journal linux/fs/ext4/fsync.c:115 [inline]
>  ext4_sync_file+0x38c/0x6d0 linux/fs/ext4/fsync.c:174
>  vfs_fsync_range linux/fs/sync.c:200 [inline]
>  vfs_fsync linux/fs/sync.c:214 [inline]
>  do_fsync linux/fs/sync.c:224 [inline]
>  __do_sys_fdatasync linux/fs/sync.c:237 [inline]
>  __se_sys_fdatasync linux/fs/sync.c:235 [inline]
>  __x64_sys_fdatasync+0x7a/0xd0 linux/fs/sync.c:235
>  do_syscall_64+0x39/0x80 linux/arch/x86/entry/common.c:46
>  entry_SYSCALL_64_after_hwframe+0x44/0xae

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