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BUG: KCSAN: data-race in mmb_mark_buffer_dirty / mmb_sync

write to 0xffff88811b22bfb0 of 8 bytes by task 14925 on cpu 1:
 __remove_assoc_queue fs/buffer.c:502 [inline]
 mmb_sync+0x14d/0x5f0 fs/buffer.c:573
 fat_file_fsync+0xba/0x110 fs/fat/file.c:198
 vfs_fsync_range+0xc5/0xe0 fs/sync.c:186
 generic_write_sync include/linux/fs.h:2666 [inline]
 generic_file_write_iter+0x1a3/0x1f0 mm/filemap.c:4505
 iter_file_splice_write+0x6a0/0xa60 fs/splice.c:736
 do_splice_from fs/splice.c:936 [inline]
 direct_splice_actor+0x156/0x280 fs/splice.c:1159
 splice_direct_to_actor+0x2f9/0x680 fs/splice.c:1103
 do_splice_direct_actor fs/splice.c:1202 [inline]
 do_splice_direct+0x119/0x1a0 fs/splice.c:1228
 do_sendfile+0x37d/0x640 fs/read_write.c:1371
 __do_sys_sendfile64 fs/read_write.c:1432 [inline]
 __se_sys_sendfile64 fs/read_write.c:1418 [inline]
 __x64_sys_sendfile64+0x105/0x150 fs/read_write.c:1418
 x64_sys_call+0x2dc4/0x3020 arch/x86/include/generated/asm/syscalls_64.h:41
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811b22bfb0 of 8 bytes by task 14942 on cpu 0:
 mmb_mark_buffer_dirty+0x2d/0x140 fs/buffer.c:727
 fat16_ent_put+0x52/0x60 fs/fat/fatent.c:186
 fat_alloc_clusters+0x55e/0xc40 fs/fat/fatent.c:508
 fat_add_cluster fs/fat/inode.c:108 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x252/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2123
 block_write_begin fs/buffer.c:2234 [inline]
 cont_write_begin+0x5bf/0x920 fs/buffer.c:2596
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 cont_expand_zero fs/buffer.c:2524 [inline]
 cont_write_begin+0x18d/0x920 fs/buffer.c:2586
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2487
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:227
 fat_fallocate+0x177/0x1c0 fs/fat/file.c:305
 vfs_fallocate+0x3ac/0x400 fs/open.c:338
 ksys_fallocate fs/open.c:362 [inline]
 __do_sys_fallocate fs/open.c:367 [inline]
 __se_sys_fallocate fs/open.c:365 [inline]
 __x64_sys_fallocate+0x7a/0xd0 fs/open.c:365
 x64_sys_call+0x298e/0x3020 arch/x86/include/generated/asm/syscalls_64.h:286
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff88811b0531f8 -> 0x0000000000000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 14942 Comm: syz.6.3434 Tainted: G        W           syzkaller #0 PREEMPT(lazy) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
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