KCSAN: data-race in shmem_file_read_iter / shmem_write_end

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 245a4300 Merge branch 'rcu/kcsan' into tip/locking/kcsan
git tree: https://github.com/google/ktsan.git kcsan
console output: https://syzkaller.appspot.com/x/log.txt?x=16ac1059e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=a38292766f8efdaa
dashboard link: https://syzkaller.appspot.com/bug?extid=5e4613548335cf05bf0e
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
CC: [ak...@linux-foundation.org hu...@google.com
linux-...@vger.kernel.org linu...@kvack.org el...@google.com]

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

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+5e4613...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in shmem_file_read_iter / shmem_write_end

write to 0xffff8880b350c488 of 8 bytes by task 13369 on cpu 0:
i_size_write include/linux/fs.h:888 [inline]
shmem_write_end+0x83/0x4f0 mm/shmem.c:2498
generic_perform_write+0x1d3/0x320 mm/filemap.c:3320
__generic_file_write_iter+0x251/0x380 mm/filemap.c:3438
generic_file_write_iter+0x28c/0x38c mm/filemap.c:3470
call_write_iter include/linux/fs.h:1902 [inline]
new_sync_write+0x388/0x4a0 fs/read_write.c:483
__vfs_write+0xb1/0xc0 fs/read_write.c:496
vfs_write fs/read_write.c:558 [inline]
vfs_write+0x18a/0x390 fs/read_write.c:542
ksys_write+0xd5/0x1b0 fs/read_write.c:611
__do_sys_write fs/read_write.c:623 [inline]
__se_sys_write fs/read_write.c:620 [inline]
__x64_sys_write+0x4c/0x60 fs/read_write.c:620
do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880b350c488 of 8 bytes by task 13370 on cpu 1:
shmem_file_read_iter+0x18e/0x720 mm/shmem.c:2553
call_read_iter include/linux/fs.h:1896 [inline]
generic_file_splice_read+0x35c/0x500 fs/splice.c:313
do_splice_to+0xf2/0x130 fs/splice.c:892
splice_direct_to_actor+0x1b6/0x540 fs/splice.c:971
do_splice_direct+0x161/0x1e0 fs/splice.c:1080
do_sendfile+0x384/0x7f0 fs/read_write.c:1464
__do_sys_sendfile64 fs/read_write.c:1525 [inline]
__se_sys_sendfile64 fs/read_write.c:1511 [inline]
__x64_sys_sendfile64+0x12a/0x140 fs/read_write.c:1511
do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 13370 Comm: syz-executor.1 Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
==================================================================


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.