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BUG: KCSAN: data-race in copy_process / get_task_exe_file

write to 0xffff88810ecd602c of 4 bytes by task 28299 on cpu 0:
 copy_process+0x614/0x2e60 kernel/fork.c:1987
 kernel_clone+0x14b/0x690 kernel/fork.c:2500
 __do_sys_clone kernel/fork.c:2617 [inline]
 __se_sys_clone kernel/fork.c:2601 [inline]
 __x64_sys_clone+0xb4/0xe0 kernel/fork.c:2601
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88810ecd602c of 4 bytes by task 28340 on cpu 1:
 get_task_exe_file+0x44/0x120 kernel/fork.c:1196
 proc_exe_link+0x44/0x100 fs/proc/base.c:1725
 proc_pid_get_link+0x50/0xb0 fs/proc/base.c:1750
 pick_link fs/namei.c:1737 [inline]
 step_into+0x974/0xec0 fs/namei.c:1799
 open_last_lookups fs/namei.c:3314 [inline]
 path_openat+0x105e/0x21b0 fs/namei.c:3497
 do_filp_open+0xd9/0x1f0 fs/namei.c:3527
 do_sys_openat2+0xa3/0x250 fs/open.c:1187
 do_sys_open fs/open.c:1203 [inline]
 __do_sys_openat fs/open.c:1219 [inline]
 __se_sys_openat fs/open.c:1214 [inline]
 __x64_sys_openat+0xef/0x110 fs/open.c:1214
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 28340 Comm: syz-executor.5 Not tainted 5.12.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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