[moderation] [block?] KCSAN: data-race in loop_assign_backing_file / shmem_write_begin

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 43e9ad0c55a3 Merge tag 'scsi-fixes' of git://git.kernel.or..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1753a3cd980000
kernel config: https://syzkaller.appspot.com/x/.config?x=68b6063e8fdfb838
dashboard link: https://syzkaller.appspot.com/bug?extid=78e8424155f5f43c1f51
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
CC: [ax...@kernel.dk linux...@vger.kernel.org linux-...@vger.kernel.org]

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/d7b1e012e5c1/disk-43e9ad0c.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/5e8b4899cd36/vmlinux-43e9ad0c.xz
kernel image: https://storage.googleapis.com/syzbot-assets/31503797fcfd/bzImage-43e9ad0c.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+78e842...@syzkaller.appspotmail.com

==================================================================
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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