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I was hitting the below panic continuously when attaching kprobes to scheduler functions [ 159.045212] Unexpected kernel BRK exception at EL1 [ 159.053753] Internal error: BRK handler: f2000006 [#1] PREEMPT SMP [ 159.059954] Modules linked in: [ 159.063025] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 5.11.0-rc4-00008-g1e2a199f6ccd #56 [rt-app] <notice> [1] Exiting.[ 159.071166] Hardware name: ARM Juno development board (r2) (DT) [ 159.079689] pstate: 600003c5 (nZCv DAIF -PAN -UAO -TCO BTYPE=--) [ 159.085723] pc : 0xffff80001624501c [ 159.089377] lr : attach_entity_load_avg+0x2ac/0x350 [ 159.094271] sp : ffff80001622b640 [rt-app] <notice> [0] Exiting.[ 159.097591] x29: ffff80001622b640 x28: 0000000000000001 [ 159.105515] x27: 0000000000000049 x26: ffff000800b79980 [ 159.110847] x25: ffff00097ef37840 x24: 0000000000000000 [ 159.116331] x23: 00000024eacec1ec x22: ffff00097ef12b90 [ 159.121663] x21: ffff00097ef37700 x20: ffff800010119170 [rt-app] <notice> [11] Exiting.[ 159.126995] x19: ffff00097ef37840 x18: 000000000000000e [ 159.135003] x17: 0000000000000001 x16: 0000000000000019 [ 159.140335] x15: 0000000000000000 x14: 0000000000000000 [ 159.145666] x13: 0000000000000002 x12: 0000000000000002 [ 159.150996] x11: ffff80001592f9f0 x10: 0000000000000060 [ 159.156327] x9 : ffff8000100f6f9c x8 : be618290de0999a1 [ 159.161659] x7 : ffff80096a4b1000 x6 : 0000000000000000 [ 159.166990] x5 : ffff00097ef37840 x4 : 0000000000000000 [ 159.172321] x3 : ffff000800328948 x2 : 0000000000000000 [ 159.177652] x1 : 0000002507d52fec x0 : ffff00097ef12b90 [ 159.182983] Call trace: [ 159.185433] 0xffff80001624501c [ 159.188581] update_load_avg+0x2d0/0x778 [ 159.192516] enqueue_task_fair+0x134/0xe20 [ 159.196625] enqueue_task+0x4c/0x2c8 [ 159.200211] ttwu_do_activate+0x70/0x138 [ 159.204147] sched_ttwu_pending+0xbc/0x160 [ 159.208253] flush_smp_call_function_queue+0x16c/0x320 [ 159.213408] generic_smp_call_function_single_interrupt+0x1c/0x28 [ 159.219521] ipi_handler+0x1e8/0x3c8 [ 159.223106] handle_percpu_devid_irq+0xd8/0x460 [ 159.227650] generic_handle_irq+0x38/0x50 [ 159.231672] __handle_domain_irq+0x6c/0xc8 [ 159.235781] gic_handle_irq+0xcc/0xf0 [ 159.239452] el1_irq+0xb4/0x180 [ 159.242600] rcu_is_watching+0x28/0x70 [ 159.246359] rcu_read_lock_held_common+0x44/0x88 [ 159.250991] rcu_read_lock_any_held+0x30/0xc0 [ 159.255360] kretprobe_dispatcher+0xc4/0xf0 [ 159.259555] __kretprobe_trampoline_handler+0xc0/0x150 [ 159.264710] trampoline_probe_handler+0x38/0x58 [ 159.269255] kretprobe_trampoline+0x70/0xc4 [ 159.273450] run_rebalance_domains+0x54/0x80 [ 159.277734] __do_softirq+0x164/0x684 [ 159.281406] irq_exit+0x198/0x1b8 [ 159.284731] __handle_domain_irq+0x70/0xc8 [ 159.288840] gic_handle_irq+0xb0/0xf0 [ 159.292510] el1_irq+0xb4/0x180 [ 159.295658] arch_cpu_idle+0x18/0x28 [ 159.299245] default_idle_call+0x9c/0x3e8 [ 159.303265] do_idle+0x25c/0x2a8 [ 159.306502] cpu_startup_entry+0x2c/0x78 [ 159.310436] secondary_start_kernel+0x160/0x198 [ 159.314984] Code: d42000c0 aa1e03e9 d42000c0 aa1e03e9 (d42000c0) After a bit of head scratching and debugging it turned out that it is due to kprobe handler being interrupted by a tick that causes us to go into (I think another) kprobe handler. The culprit was kprobe_breakpoint_ss_handler() returning DBG_HOOK_ERROR which leads to the Unexpected kernel BRK exception. Reverting commitba090f9caf("arm64: kprobes: Remove redundant kprobe_step_ctx") seemed to fix the problem for me. Further analysis showed that kcb->kprobe_status is set to KPROBE_REENTER when the error occurs. By teaching kprobe_breakpoint_ss_handler() to handle this status I can no longer reproduce the problem. Fixes:ba090f9caf("arm64: kprobes: Remove redundant kprobe_step_ctx") Signed-off-by: Qais Yousef <qais.yousef@arm.com> Acked-by: Will Deacon <will@kernel.org> Acked-by: Masami Hiramatsu <mhiramat@kernel.org> Link: https://lore.kernel.org/r/20210122110909.3324607-1-qais.yousef@arm.com Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
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.