==================================================================
BUG: KCSAN: data-race in fat16_ent_put / fat_mirror_bhs

write to 0xffff888130a59882 of 2 bytes by task 11385 on cpu 0:
 fat16_ent_put+0x28/0x60 fs/fat/fatent.c:183
 fat_alloc_clusters+0x57b/0xc50 fs/fat/fatent.c:503
 fat_add_cluster fs/fat/inode.c:108 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x258/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
 block_write_begin fs/buffer.c:2256 [inline]
 cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
 generic_perform_write+0x183/0x490 mm/filemap.c:4314
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
 generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x5a6/0x9f0 fs/read_write.c:686
 ksys_write+0xdc/0x1a0 fs/read_write.c:738
 __do_sys_write fs/read_write.c:749 [inline]
 __se_sys_write fs/read_write.c:746 [inline]
 __x64_sys_write+0x40/0x50 fs/read_write.c:746
 x64_sys_call+0x2847/0x3000 arch/x86/include/generated/asm/syscalls_64.h:2
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888130a59800 of 2048 bytes by task 11387 on cpu 1:
 fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
 fat_ent_write+0xd0/0xe0 fs/fat/fatent.c:423
 fat_chain_add+0x16c/0x490 fs/fat/misc.c:136
 fat_add_cluster fs/fat/inode.c:113 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x46c/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
 block_write_begin fs/buffer.c:2256 [inline]
 cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
 generic_perform_write+0x183/0x490 mm/filemap.c:4314
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
 generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x5a6/0x9f0 fs/read_write.c:686
 ksys_write+0xdc/0x1a0 fs/read_write.c:738
 __do_sys_write fs/read_write.c:749 [inline]
 __se_sys_write fs/read_write.c:746 [inline]
 __x64_sys_write+0x40/0x50 fs/read_write.c:746
 x64_sys_call+0x2847/0x3000 arch/x86/include/generated/asm/syscalls_64.h:2
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 11387 Comm: syz.3.2663 Tainted: G        W           syzkaller #0 PREEMPT(voluntary) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================
bio_check_eod: 31 callbacks suppressed
syz.3.2663: attempt to access beyond end of device
loop3: rw=2049, sector=1728, nr_sectors = 96 limit=256
syz.3.2663: attempt to access beyond end of device
loop3: rw=2049, sector=1856, nr_sectors = 32 limit=256
syz.3.2663: attempt to access beyond end of device
loop3: rw=2049, sector=1920, nr_sectors = 64 limit=256
syz.3.2663: attempt to access beyond end of device
loop3: rw=2049, sector=2016, nr_sectors = 32 limit=256
syz.3.2663: attempt to access beyond end of device
loop3: rw=2049, sector=2080, nr_sectors = 64 limit=256
syz.3.2663: attempt to access beyond end of device
loop3: rw=2049, sector=2176, nr_sectors = 64 limit=256
syz.3.2663: attempt to access beyond end of device
loop3: rw=2049, sector=2272, nr_sectors = 64 limit=256