[moderation] KCSAN: data-race in mmb_mark_buffer_dirty / mmb_sync (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 24a19b953e2a Linux 7.3-rc7
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
console output: https://syzkaller.appspot.com/x/log.txt?x=17cd8f15580000
kernel config: https://syzkaller.appspot.com/x/.config?x=de3702ecab64292d
dashboard link: https://syzkaller.appspot.com/bug?extid=fcf9c67a6e09a80570cc
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1441883c580000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+fcf9c6...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in mmb_mark_buffer_dirty / mmb_sync

write to 0xffff888105e1c350 of 8 bytes by task 3644 on cpu 0:
__remove_assoc_queue fs/buffer.c:473 [inline]
mmb_sync+0x14d/0x5f0 fs/buffer.c:544
fat_file_fsync+0xa0/0x100 fs/fat/file.c:197
vfs_fsync_range fs/sync.c:186 [inline]
vfs_fsync fs/sync.c:200 [inline]
do_fsync fs/sync.c:211 [inline]
__do_sys_fsync fs/sync.c:216 [inline]
__se_sys_fsync fs/sync.c:214 [inline]
__x64_sys_fsync+0xc3/0x120 fs/sync.c:214
x64_sys_call+0x1d2b/0x2550 arch/x86/include/generated/asm/syscalls_64.h:75
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x112/0x360 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888105e1c350 of 8 bytes by task 3643 on cpu 1:
mmb_mark_buffer_dirty+0x2d/0x140 fs/buffer.c:625
fat12_ent_put+0x118/0x180 fs/fat/fatent.c:173
fat_ent_write+0x6c/0xe0 fs/fat/fatent.c:422
fat_chain_add+0x170/0x4c0 fs/fat/misc.c:138
fat_add_cluster fs/fat/inode.c:113 [inline]
__fat_get_block fs/fat/inode.c:155 [inline]
fat_get_block+0x468/0x5e0 fs/fat/inode.c:190
__block_write_begin_int+0x400/0xf90 fs/buffer.c:2030
block_write_begin fs/buffer.c:2144 [inline]
cont_write_begin+0x5bf/0x920 fs/buffer.c:2506
fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
generic_perform_write+0x186/0x490 mm/filemap.c:4374
__generic_file_write_iter+0x9e/0x120 mm/filemap.c:4491
generic_file_write_iter+0x77/0x1f0 mm/filemap.c:4517
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x57f/0x9a0 fs/read_write.c:687
ksys_write+0xdc/0x1a0 fs/read_write.c:739
__do_sys_write fs/read_write.c:750 [inline]
__se_sys_write fs/read_write.c:747 [inline]
__x64_sys_write+0x40/0x50 fs/read_write.c:747
x64_sys_call+0x154e/0x2550 arch/x86/include/generated/asm/syscalls_64.h:2
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x112/0x360 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff8881181c8f18 -> 0x0000000000000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 3643 Comm: syz-executor205 Not tainted syzkaller #0 PREEMPT(lazy)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/16/2026
==================================================================


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