[moderation] [block?] KCSAN: data-race in loop_assign_backing_file / shmem_write_begin (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: cf38b2340c0e Merge tag 'soc-fixes-6.19-2' of git://git.ker..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12c33f9a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=6c9e86480ac654d3
dashboard link: https://syzkaller.appspot.com/bug?extid=147ede2b25c9a0f1780f
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
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/241b9944a06a/disk-cf38b234.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/91148017de63/vmlinux-cf38b234.xz
kernel image: https://storage.googleapis.com/syzbot-assets/65689f426657/bzImage-cf38b234.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+147ede...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in loop_assign_backing_file / shmem_write_begin

write to 0xffff88811b036510 of 4 bytes by task 4620 on cpu 0:
mapping_set_gfp_mask include/linux/pagemap.h:377 [inline]
loop_assign_backing_file+0x80/0x210 drivers/block/loop.c:513
loop_configure+0x720/0x9c0 drivers/block/loop.c:1060
lo_ioctl+0x5ac/0x13a0 drivers/block/loop.c:1563
blkdev_ioctl+0x387/0x460 block/ioctl.c:792
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+0x14b0/0x3000 arch/x86/include/generated/asm/syscalls_64.h:17
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 0xffff88811b036510 of 4 bytes by task 4615 on cpu 1:
mapping_gfp_mask include/linux/pagemap.h:361 [inline]
shmem_get_folio mm/shmem.c:2663 [inline]
shmem_write_begin+0xdc/0x1f0 mm/shmem.c:3315
generic_perform_write+0x183/0x490 mm/filemap.c:4314
shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3490
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

value changed: 0x00100cca -> 0x00100c0a

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 4615 Comm: syz.3.370 Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================


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