[BUG BISECTED] Missing RCU reader in perf_event_setup_cpumask()

[Paul E. McKenney]

Hello!

On next-20240912 running rcutorture scenario TREE05, I see this
deterministically:

[   32.603233] =============================
[   32.604594] WARNING: suspicious RCU usage
[   32.605928] 6.11.0-rc5-00040-g4ba4f1afb6a9 #55238 Not tainted
[   32.607812] -----------------------------
[   32.609140] kernel/events/core.c:13946 RCU-list traversed in non-reader section!!
[   32.611595]
[   32.611595] other info that might help us debug this:
[   32.611595]
[   32.614247]
[   32.614247] rcu_scheduler_active = 2, debug_locks = 1
[   32.616392] 3 locks held by cpuhp/4/35:
[   32.617687]  #0: ffffffffb666a650 (cpu_hotplug_lock){++++}-{0:0}, at: cpuhp_thread_fun+0x4e/0x200
[   32.620563]  #1: ffffffffb666cd20 (cpuhp_state-down){+.+.}-{0:0}, at: cpuhp_thread_fun+0x4e/0x200
[   32.623412]  #2: ffffffffb677c288 (pmus_lock){+.+.}-{3:3}, at: perf_event_exit_cpu_context+0x32/0x2f0
[   32.626399]
[   32.626399] stack backtrace:
[   32.627848] CPU: 4 UID: 0 PID: 35 Comm: cpuhp/4 Not tainted 6.11.0-rc5-00040-g4ba4f1afb6a9 #55238
[   32.628832] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[   32.628832] Call Trace:
[   32.628832]  <TASK>
[   32.628832]  dump_stack_lvl+0x83/0xa0
[   32.628832]  lockdep_rcu_suspicious+0x143/0x1a0
[   32.628832]  perf_event_exit_cpu_context+0x2e5/0x2f0
[   32.628832]  ? __pfx_perf_event_exit_cpu+0x10/0x10
[   32.628832]  perf_event_exit_cpu+0x9/0x10
[   32.628832]  cpuhp_invoke_callback+0x130/0x2a0
[   32.628832]  ? lock_release+0xc7/0x290
[   32.628832]  ? cpuhp_thread_fun+0x4e/0x200
[   32.628832]  cpuhp_thread_fun+0x183/0x200
[   32.628832]  smpboot_thread_fn+0xd8/0x1d0
[   32.628832]  ? __pfx_smpboot_thread_fn+0x10/0x10
[   32.628832]  kthread+0xd4/0x100
[   32.628832]  ? __pfx_kthread+0x10/0x10
[   32.628832]  ret_from_fork+0x2f/0x50
[   32.628832]  ? __pfx_kthread+0x10/0x10
[   32.628832]  ret_from_fork_asm+0x1a/0x30
[   32.628832]  </TASK>

I bisected this to:

4ba4f1afb6a9 ("perf: Generic hotplug support for a PMU with a scope")

This adds a perf_event_setup_cpumask() function that uses
list_for_each_entry_rcu() without an obvious RCU read-side critical
section, so the fix might be as simple as adding rcu_read_lock() and
rcu_read_unlock().  In the proper places, of course.  ;-)

						Thanx, Paul