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BUG: KCSAN: data-race in get_task_mm / page_cache_readahead_unbounded

write to 0xffff88812b786064 of 4 bytes by task 1 on cpu 1:
 memalloc_nofs_save include/linux/sched/mm.h:253 [inline]
 page_cache_readahead_unbounded+0xa6/0x470 mm/readahead.c:199
 ondemand_readahead+0x4e4/0x6e0 mm/readahead.c:273
 page_cache_sync_readahead+0x178/0x1a0 mm/readahead.c:585
 generic_file_buffered_read+0x2ec/0x1f60 mm/filemap.c:2101
 generic_file_read_iter+0x7d/0x3e0 mm/filemap.c:2405
 ext4_file_read_iter+0x2d8/0x420 fs/ext4/file.c:74
 call_read_iter include/linux/fs.h:1876 [inline]
 new_sync_read fs/read_write.c:415 [inline]
 __kernel_read+0x3cf/0x600 fs/read_write.c:438
 integrity_kernel_read+0x4c/0x70 security/integrity/iint.c:191
 ima_calc_file_hash_tfm security/integrity/ima/ima_crypto.c:484 [inline]
 ima_calc_file_shash security/integrity/ima/ima_crypto.c:515 [inline]
 ima_calc_file_hash+0x9bf/0x13e0 security/integrity/ima/ima_crypto.c:580
 ima_collect_measurement+0x168/0x310 security/integrity/ima/ima_api.c:250
 process_measurement+0xa65/0x10d0 security/integrity/ima/ima_main.c:324
 ima_file_check+0x75/0xa0 security/integrity/ima/ima_main.c:492
 do_open fs/namei.c:3253 [inline]
 path_openat+0x1887/0x20a0 fs/namei.c:3368
 do_filp_open+0xbd/0x1d0 fs/namei.c:3395
 do_sys_openat2+0x33b/0x500 fs/open.c:1168
 do_sys_open fs/open.c:1184 [inline]
 __do_sys_open fs/open.c:1192 [inline]
 __se_sys_open fs/open.c:1188 [inline]
 __x64_sys_open+0xe2/0x110 fs/open.c:1188
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88812b786064 of 4 bytes by task 5068 on cpu 0:
 get_task_mm+0x3e/0x90 kernel/fork.c:1206
 get_task_cmdline fs/proc/base.c:348 [inline]
 proc_pid_cmdline_read+0x82/0x670 fs/proc/base.c:368
 vfs_read+0x16d/0x5e0 fs/read_write.c:479
 ksys_read+0xce/0x180 fs/read_write.c:607
 __do_sys_read fs/read_write.c:617 [inline]
 __se_sys_read fs/read_write.c:615 [inline]
 __x64_sys_read+0x3e/0x50 fs/read_write.c:615
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 5068 Comm: systemd-journal Not tainted 5.9.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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