mainlining shenanigans
b55bd58555
The TASKS03 and TREE04 rcutorture scenarios produce the following lockdep complaint: ------------------------------------------------------------------------ ================================ WARNING: inconsistent lock state 5.2.0-rc1+ #513 Not tainted -------------------------------- inconsistent {IN-HARDIRQ-W} -> {HARDIRQ-ON-W} usage. migration/1/14 [HC0[0]:SC0[0]:HE1:SE1] takes: (____ptrval____) (tick_broadcast_lock){?...}, at: tick_broadcast_offline+0xf/0x70 {IN-HARDIRQ-W} state was registered at: lock_acquire+0xb0/0x1c0 _raw_spin_lock_irqsave+0x3c/0x50 tick_broadcast_switch_to_oneshot+0xd/0x40 tick_switch_to_oneshot+0x4f/0xd0 hrtimer_run_queues+0xf3/0x130 run_local_timers+0x1c/0x50 update_process_times+0x1c/0x50 tick_periodic+0x26/0xc0 tick_handle_periodic+0x1a/0x60 smp_apic_timer_interrupt+0x80/0x2a0 apic_timer_interrupt+0xf/0x20 _raw_spin_unlock_irqrestore+0x4e/0x60 rcu_nocb_gp_kthread+0x15d/0x590 kthread+0xf3/0x130 ret_from_fork+0x3a/0x50 irq event stamp: 171 hardirqs last enabled at (171): [<ffffffff8a201a37>] trace_hardirqs_on_thunk+0x1a/0x1c hardirqs last disabled at (170): [<ffffffff8a201a53>] trace_hardirqs_off_thunk+0x1a/0x1c softirqs last enabled at (0): [<ffffffff8a264ee0>] copy_process.part.56+0x650/0x1cb0 softirqs last disabled at (0): [<0000000000000000>] 0x0 other info that might help us debug this: Possible unsafe locking scenario: CPU0 ---- lock(tick_broadcast_lock); <Interrupt> lock(tick_broadcast_lock); *** DEADLOCK *** 1 lock held by migration/1/14: #0: (____ptrval____) (clockevents_lock){+.+.}, at: tick_offline_cpu+0xf/0x30 stack backtrace: CPU: 1 PID: 14 Comm: migration/1 Not tainted 5.2.0-rc1+ #513 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS Bochs 01/01/2011 Call Trace: dump_stack+0x5e/0x8b print_usage_bug+0x1fc/0x216 ? print_shortest_lock_dependencies+0x1b0/0x1b0 mark_lock+0x1f2/0x280 __lock_acquire+0x1e0/0x18f0 ? __lock_acquire+0x21b/0x18f0 ? _raw_spin_unlock_irqrestore+0x4e/0x60 lock_acquire+0xb0/0x1c0 ? tick_broadcast_offline+0xf/0x70 _raw_spin_lock+0x33/0x40 ? tick_broadcast_offline+0xf/0x70 tick_broadcast_offline+0xf/0x70 tick_offline_cpu+0x16/0x30 take_cpu_down+0x7d/0xa0 multi_cpu_stop+0xa2/0xe0 ? cpu_stop_queue_work+0xc0/0xc0 cpu_stopper_thread+0x6d/0x100 smpboot_thread_fn+0x169/0x240 kthread+0xf3/0x130 ? sort_range+0x20/0x20 ? kthread_cancel_delayed_work_sync+0x10/0x10 ret_from_fork+0x3a/0x50 ------------------------------------------------------------------------ To reproduce, run the following rcutorture test: tools/testing/selftests/rcutorture/bin/kvm.sh --duration 5 --kconfig "CONFIG_DEBUG_LOCK_ALLOC=y CONFIG_PROVE_LOCKING=y" --configs "TASKS03 TREE04" It turns out that tick_broadcast_offline() was an innocent bystander. After all, interrupts are supposed to be disabled throughout take_cpu_down(), and therefore should have been disabled upon entry to tick_offline_cpu() and thus to tick_broadcast_offline(). This suggests that one of the CPU-hotplug notifiers was incorrectly enabling interrupts, and leaving them enabled on return. Some debugging code showed that the culprit was sched_cpu_dying(). It had irqs enabled after return from sched_tick_stop(). Which in turn had irqs enabled after return from cancel_delayed_work_sync(). Which is a wrapper around __cancel_work_timer(). Which can sleep in the case where something else is concurrently trying to cancel the same delayed work, and as Thomas Gleixner pointed out on IRC, sleeping is a decidedly bad idea when you are invoked from take_cpu_down(), regardless of the state you leave interrupts in upon return. Code inspection located no reason why the delayed work absolutely needed to be canceled from sched_tick_stop(): The work is not bound to the outgoing CPU by design, given that the whole point is to collect statistics without disturbing the outgoing CPU. This commit therefore simply drops the cancel_delayed_work_sync() from sched_tick_stop(). Instead, a new ->state field is added to the tick_work structure so that the delayed-work handler function sched_tick_remote() can avoid reposting itself. A cpu_is_offline() check is also added to sched_tick_remote() to avoid mucking with the state of an offlined CPU (though it does appear safe to do so). The sched_tick_start() and sched_tick_stop() functions also update ->state, and sched_tick_start() also schedules the delayed work if ->state indicates that it is not already in flight. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Reviewed-by: Frederic Weisbecker <frederic@kernel.org> [ paulmck: Apply Peter Zijlstra and Frederic Weisbecker atomics feedback. ] Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.