syzbot

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BUG: KCSAN: data-race in loop_assign_backing_file / shmem_write_begin

write to 0xffff88811b29cdd0 of 4 bytes by task 4278 on cpu 0:
 mapping_set_gfp_mask include/linux/pagemap.h:369 [inline]
 loop_assign_backing_file+0x80/0x200 drivers/block/loop.c:514
 loop_configure+0x720/0x9c0 drivers/block/loop.c:1061
 lo_ioctl+0x5aa/0x12b0 drivers/block/loop.c:1544
 blkdev_ioctl+0x356/0x440 block/ioctl.c:705
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xce/0x140 fs/ioctl.c:583
 __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:583
 x64_sys_call+0x1816/0x3000 arch/x86/include/generated/asm/syscalls_64.h:17
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811b29cdd0 of 4 bytes by task 4273 on cpu 1:
 mapping_gfp_mask include/linux/pagemap.h:353 [inline]
 shmem_get_folio mm/shmem.c:2640 [inline]
 shmem_write_begin+0x88/0x190 mm/shmem.c:3289
 generic_perform_write+0x184/0x490 mm/filemap.c:4242
 shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3464
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x52a/0x960 fs/read_write.c:686
 ksys_write+0xda/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+0x2802/0x3000 arch/x86/include/generated/asm/syscalls_64.h:2
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00100cca -> 0x00100c0a

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 4273 Comm: syz.0.185 Not tainted syzkaller #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
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