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bh=ExCyh8f2qvxA4qvVKJp1KepYAznGBkpQOkRSD2+vZvQ=; b=KKe6HP3yeOsOH2Z6hyyByxYI/La5T7Ntn7QnI+4nRbF1+xpGl1IkDF7rTCwaH6MtEY BHi6ytiuYdjoal2v5aa/nKmDoJaZ88jzPjNi8OyasJjyMzSDMSE/hQlMHyqzMRj1zpZZ X2FzYqV3xenGydl4B5NUUS7Ig1SX2z4rwmTb2gH++HVmPkdwqcDu08B3v2NM0zK/PWE4 yIrOkfbBdY9i+MHluPFfHnSVqPcAkSwipxtSI4WLny2+aBPngKwR+1DhC/ortTwj9w1p +hUSAaZ54vX7974jet17n3ptaESIn0AGso3+pmZZJu/VppjlwIL7Fa9H/Ffo1pISY4QI jeEA== X-Gm-Message-State: APjAAAUWmNBfUr0wJvf2aQcOVy2m0J8Hyui9RfocYsMHhg/3Rw6aJ9Jm oQrLZupFo/iPrT+mYFjGcw3Sb9g3HV27Rvrj+eDh3Q== X-Google-Smtp-Source: APXvYqxG4k94Rp9wpScirL1ahnOi9g6nzM0xsAK3Yck0YQ/25V8gZJ9uO2yEZWPMQlzo4j2/IEx9w4SJakYf46lj4FQ= X-Received: by 2002:a9d:66d1:: with SMTP id t17mr56428otm.233.1581091737268; Fri, 07 Feb 2020 08:08:57 -0800 (PST) MIME-Version: 1.0 References: <1581085751-31793-1-git-send-email-cai@lca.pw> <1581089930.7365.20.camel@lca.pw> In-Reply-To: <1581089930.7365.20.camel@lca.pw> From: Marco Elver Date: Fri, 7 Feb 2020 17:08:46 +0100 Message-ID: Subject: Re: [PATCH v2] ext4: fix a data race in EXT4_I(inode)->i_disksize To: Qian Cai Cc: "Theodore Ts'o" , Andreas Dilger , linux-ext4@vger.kernel.org, LKML Content-Type: text/plain; charset="UTF-8" Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Fri, 7 Feb 2020 at 16:38, Qian Cai wrote: > > On Fri, 2020-02-07 at 16:12 +0100, Marco Elver wrote: > > On Fri, 7 Feb 2020 at 15:29, Qian Cai wrote: > > > > > > EXT4_I(inode)->i_disksize could be accessed concurrently as noticed by > > > KCSAN, > > > > > > BUG: KCSAN: data-race in ext4_write_end [ext4] / ext4_writepages [ext4] > > > > > > write to 0xffff91c6713b00f8 of 8 bytes by task 49268 on cpu 127: > > > ext4_write_end+0x4e3/0x750 [ext4] > > > ext4_update_i_disksize at fs/ext4/ext4.h:3032 > > > (inlined by) ext4_update_inode_size at fs/ext4/ext4.h:3046 > > > (inlined by) ext4_write_end at fs/ext4/inode.c:1287 > > > generic_perform_write+0x208/0x2a0 > > > ext4_buffered_write_iter+0x11f/0x210 [ext4] > > > ext4_file_write_iter+0xce/0x9e0 [ext4] > > > new_sync_write+0x29c/0x3b0 > > > __vfs_write+0x92/0xa0 > > > vfs_write+0x103/0x260 > > > ksys_write+0x9d/0x130 > > > __x64_sys_write+0x4c/0x60 > > > do_syscall_64+0x91/0xb47 > > > entry_SYSCALL_64_after_hwframe+0x49/0xbe > > > > > > read to 0xffff91c6713b00f8 of 8 bytes by task 24872 on cpu 37: > > > ext4_writepages+0x10ac/0x1d00 [ext4] > > > mpage_map_and_submit_extent at fs/ext4/inode.c:2468 > > > (inlined by) ext4_writepages at fs/ext4/inode.c:2772 > > > do_writepages+0x5e/0x130 > > > __writeback_single_inode+0xeb/0xb20 > > > writeback_sb_inodes+0x429/0x900 > > > __writeback_inodes_wb+0xc4/0x150 > > > wb_writeback+0x4bd/0x870 > > > wb_workfn+0x6b4/0x960 > > > process_one_work+0x54c/0xbe0 > > > worker_thread+0x80/0x650 > > > kthread+0x1e0/0x200 > > > ret_from_fork+0x27/0x50 > > > > > > Reported by Kernel Concurrency Sanitizer on: > > > CPU: 37 PID: 24872 Comm: kworker/u261:2 Tainted: G W O L 5.5.0-next-20200204+ #5 > > > Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019 > > > Workqueue: writeback wb_workfn (flush-7:0) > > > > > > Since only the read is operating as lockless (outside of the > > > "i_data_sem"), load tearing could introduce a logic bug. Fix it by > > > adding READ_ONCE() for the read and WRITE_ONCE() for the write. > > > > > > Signed-off-by: Qian Cai > > > --- > > > > > > v2: also add WRITE_ONCE() which is recommended even for fixing load tearing. > > > > Just a note: I keep seeing 'load tearing' mentioned as the only reason: > > > > - The WRITE_ONCE avoids store-tearing (and other optimizations). > > > > - We're not only interested in avoiding load/store tearing. There > > are plenty other compiler optimizations that can break concurrent > > code: https://lwn.net/Articles/793253/ > > I also realized that from that article, store tearing is strictly from multiple > concurrent writers. However, in the sense of without the WRITE_ONCE() here, > compilers could still have 2 store instructions, so > > CPU0: CPU1: > store #1 > read > store #2 > > which was not mentioned in that article. I called it also load tearing, but > maybe you will call that store tearing. Do I understand correctly? The effect is the same, so yes. If you have the writer side split the write, but have a concurrent load, the observed value will appear "teared". Similar if the reader side splits the reads (the more obvious case). > > > > Thanks, > > -- Marco > > > > > > > fs/ext4/ext4.h | 2 +- > > > fs/ext4/inode.c | 2 +- > > > 2 files changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h > > > index 9a2ee2428ecc..8329ccc82fa9 100644 > > > --- a/fs/ext4/ext4.h > > > +++ b/fs/ext4/ext4.h > > > @@ -3029,7 +3029,7 @@ static inline void ext4_update_i_disksize(struct inode *inode, loff_t newsize) > > > !inode_is_locked(inode)); > > > down_write(&EXT4_I(inode)->i_data_sem); > > > if (newsize > EXT4_I(inode)->i_disksize) > > > - EXT4_I(inode)->i_disksize = newsize; > > > + WRITE_ONCE(EXT4_I(inode)->i_disksize, newsize); > > > up_write(&EXT4_I(inode)->i_data_sem); > > > } > > > > > > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c > > > index 3313168b680f..6f9862bf63f1 100644 > > > --- a/fs/ext4/inode.c > > > +++ b/fs/ext4/inode.c > > > @@ -2465,7 +2465,7 @@ static int mpage_map_and_submit_extent(handle_t *handle, > > > * truncate are avoided by checking i_size under i_data_sem. > > > */ > > > disksize = ((loff_t)mpd->first_page) << PAGE_SHIFT; > > > - if (disksize > EXT4_I(inode)->i_disksize) { > > > + if (disksize > READ_ONCE(EXT4_I(inode)->i_disksize)) { > > > int err2; > > > loff_t i_size; > > > > > > -- > > > 1.8.3.1 > > >