perf: WARNING in perf_event_read

[Dmitry Vyukov]

Hello,

I've got the following WARNING while running syzkaller fuzzer:

------------[ cut here ]------------
WARNING: CPU: 3 PID: 23874 at kernel/events/core.c:3554
CPU: 3 PID: 23874 Comm: syz-executor Not tainted 4.8.0-rc3+ #31
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
 ffffffff884b8280 ffff88003754f860 ffffffff82d1b239 ffffffff017a1000
 fffffbfff1097050 ffffffff86e8eec0 ffff88003754f938 ffffffff86ef6200
 dffffc0000000000 0000000000000009 ffff88003754f928 ffffffff816ab4e3
Call Trace:
 [<ffffffff81380adc>] warn_slowpath_null+0x2c/0x40 kernel/panic.c:552
 [<ffffffff81685d48>] perf_event_read+0x578/0x690 kernel/events/core.c:3554
 [<ffffffff81685ee4>] perf_event_read_value+0x84/0x520 kernel/events/core.c:4192
 [<     inline     >] perf_read_one kernel/events/core.c:4307
 [<     inline     >] __perf_read kernel/events/core.c:4358
 [<ffffffff816867b6>] perf_read+0x436/0x8a0 kernel/events/core.c:4371
 [<ffffffff8182dda2>] do_loop_readv_writev+0x152/0x200 fs/read_write.c:714
 [<ffffffff81832214>] do_readv_writev+0x604/0x6f0 fs/orangefs/file.c:335
 [<ffffffff8183238b>] vfs_readv+0x8b/0xc0 fs/read_write.c:869
 [<ffffffff818324a1>] do_readv+0xe1/0x240 fs/read_write.c:895
 [<     inline     >] SYSC_readv fs/read_write.c:982
 [<ffffffff81835667>] SyS_readv+0x27/0x30 fs/read_write.c:979
 [<ffffffff86c25480>] entry_SYSCALL_64_fastpath+0x23/0xc1
arch/x86/entry/entry_64.S:207

On commit 61c04572de404e52a655a36752e696bbcb483cf5 (Aug 25).

Unfortunately it's not reproducible. I've seen only 4 such WARNINGS
over 24 hours.
Here is one crash report for the record:
https://gist.githubusercontent.com/dvyukov/fec6ffb2e7061f571d50877dfe605ffa/raw/473aec98d2c0cb60c93bd1babb51e7781e43c3db/gistfile1.txt