kernel/linux.git/lib/nmi_backtrace.c, branch v7.3-rc1

nmi: Export CPU backtrace APIs for loadable modules

2026-08-10T16:00:05+00:00

Currently, CPU backtrace functions cannot be called from loadable modules because the underlying helper arch_trigger_cpumask_backtrace() is not exported. Instead of exporting arch_trigger_cpumask_backtrace() individually across every supported architecture, introduce and export a common helper, cpumask_backtrace(), in lib/nmi_backtrace.c. Update the four inline CPU backtrace macros in include/linux/nmi.h to route through this centralized helper. Signed-off-by: Mayank Rungta Reviewed-by: Douglas Anderson Reviewed-by: Andrew Morton Link: https://lore.kernel.org/r/20260730-export-cpu-backtrace-apis-v1-1-bace8e1cb817@google.com Signed-off-by: Guenter Roeck

lib/nmi_backtrace: print out the CPUs which fail to respond to NMI

2026-05-29T04:24:59+00:00

When debugging RCU stall cases, usually all CPUs will respond to the NMI and print out the backtrace. But in some nasty or hardware related cases, some CPUs may fail to respond in 10 seconds, and very likely this is sign of severe issues. Paul McKenney has implemented the NMI backtrace stall check for x86, and for other architectures, it should be also helpful to at least print out those CPUs which failed to repond to the NMI, so that users can get an early heads-up for possible CPU hard stall. [feng.tang@linux.alibaba.com: avoid hard-coding "10" in two places and in a comment] Link: https://lore.kernel.org/ag-1ciG0FSomBf7q@U-2FWC9VHC-2323.local [akpm@linux-foundation.org: use __stringify()] Link: https://lore.kernel.org/20260521030336.92172-1-feng.tang@linux.alibaba.com Signed-off-by: Feng Tang Reviewed-by: Petr Mladek Cc: "Paul E . McKenney" Signed-off-by: Andrew Morton

nmi_backtrace: allow excluding an arbitrary CPU

2023-08-18T17:19:00+00:00

The APIs that allow backtracing across CPUs have always had a way to exclude the current CPU. This convenience means callers didn't need to find a place to allocate a CPU mask just to handle the common case. Let's extend the API to take a CPU ID to exclude instead of just a boolean. This isn't any more complex for the API to handle and allows the hardlockup detector to exclude a different CPU (the one it already did a trace for) without needing to find space for a CPU mask. Arguably, this new API also encourages safer behavior. Specifically if the caller wants to avoid tracing the current CPU (maybe because they already traced the current CPU) this makes it more obvious to the caller that they need to make sure that the current CPU ID can't change. [akpm@linux-foundation.org: fix trigger_allbutcpu_cpu_backtrace() stub] Link: https://lkml.kernel.org/r/20230804065935.v4.1.Ia35521b91fc781368945161d7b28538f9996c182@changeid Signed-off-by: Douglas Anderson Acked-by: Michal Hocko Cc: kernel test robot Cc: Lecopzer Chen Cc: Petr Mladek Cc: Pingfan Liu Signed-off-by: Andrew Morton

x86/nmi: Print reasons why backtrace NMIs are ignored

2023-01-19T23:55:12+00:00

Instrument nmi_trigger_cpumask_backtrace() to dump out diagnostics based on evidence accumulated by exc_nmi(). These diagnostics are dumped for CPUs that ignored an NMI backtrace request for more than 10 seconds. [ paulmck: Apply Ingo Molnar feedback. ] Signed-off-by: Paul E. McKenney Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Borislav Petkov Cc: Dave Hansen Cc: "H. Peter Anvin" Cc: Reviewed-by: Ingo Molnar

printk: rename cpulock functions

2022-04-22T19:30:57+00:00

Since the printk cpulock is CPU-reentrant and since it is used in all contexts, its usage must be carefully considered and most likely will require programming locklessly. To avoid mistaking the printk cpulock as a typical lock, rename it to cpu_sync. The main functions then become: printk_cpu_sync_get_irqsave(flags); printk_cpu_sync_put_irqrestore(flags); Add extra notes of caution in the function description to help developers understand the requirements for correct usage. Signed-off-by: John Ogness Reviewed-by: Petr Mladek Signed-off-by: Petr Mladek Link: https://lore.kernel.org/r/20220421212250.565456-2-john.ogness@linutronix.de

printk: restore flushing of NMI buffers on remote CPUs after NMI backtraces

2021-11-10T15:12:00+00:00

printk from NMI context relies on irq work being raised on the local CPU to print to console. This can be a problem if the NMI was raised by a lockup detector to print lockup stack and regs, because the CPU may not enable irqs (because it is locked up). Introduce printk_trigger_flush() that can be called another CPU to try to get those messages to the console, call that where printk_safe_flush was previously called. Fixes: 93d102f094be ("printk: remove safe buffers") Cc: stable@vger.kernel.org # 5.15 Signed-off-by: Nicholas Piggin Reviewed-by: Petr Mladek Reviewed-by: John Ogness Signed-off-by: Petr Mladek Link: https://lore.kernel.org/r/20211107045116.1754411-1-npiggin@gmail.com

lib/nmi_backtrace: Serialize even messages about idle CPUs

2021-07-30T07:25:34+00:00

The commit 55d6af1d66885059ffc2a ("lib/nmi_backtrace: explicitly serialize banner and regs") serialized backtraces from more CPUs using the re-entrant printk_printk_cpu lock. It was a preparation step for removing the obsolete nmi_safe buffers. The single-line messages about idle CPUs were not serialized against other CPUs and might appear in the middle of backtrace from another CPU, for example: [56394.590068] NMI backtrace for cpu 2 [56394.590069] CPU: 2 PID: 444 Comm: systemd-journal Not tainted 5.14.0-rc1-default+ #268 [56394.590071] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59-gc9ba527-rebuilt.opensuse.org 04/01/2014 [56394.590072] RIP: 0010:lock_is_held_type+0x0/0x120 [56394.590071] NMI backtrace for cpu 0 skipped: idling at native_safe_halt+0xb/0x10 [56394.590076] Code: a2 38 ff 0f 0b 8b 44 24 04 eb bd 48 8d ... [56394.590077] RSP: 0018:ffffab02c07c7e68 EFLAGS: 00000246 [56394.590079] RAX: 0000000000000000 RBX: ffff9a7bc0ec8a40 RCX: ffffffffaab8eb40 It might cause confusion what CPU the following lines belongs to and whether the backtraces are really serialized. Prevent the confusion and serialize also the single line message against other CPUs. Fixes: 55d6af1d66885059ffc2a ("lib/nmi_backtrace: explicitly serialize banner and regs") Reviewed-by: John Ogness Reviewed-by: Sergey Senozhatsky Signed-off-by: Petr Mladek Link: https://lore.kernel.org/r/20210727080939.27193-1-pmladek@suse.com

printk: remove safe buffers

2021-07-26T13:09:34+00:00

With @logbuf_lock removed, the high level printk functions for storing messages are lockless. Messages can be stored from any context, so there is no need for the NMI and safe buffers anymore. Remove the NMI and safe buffers. Although the safe buffers are removed, the NMI and safe context tracking is still in place. In these contexts, store the message immediately but still use irq_work to defer the console printing. Since printk recursion tracking is in place, safe context tracking for most of printk is not needed. Remove it. Only safe context tracking relating to the console and console_owner locks is left in place. This is because the console and console_owner locks are needed for the actual printing. Signed-off-by: John Ogness Reviewed-by: Petr Mladek Signed-off-by: Petr Mladek Link: https://lore.kernel.org/r/20210715193359.25946-4-john.ogness@linutronix.de

lib/nmi_backtrace: explicitly serialize banner and regs

2021-07-26T13:07:15+00:00

Currently the nmi_backtrace is serialized against other CPUs because the messages are sent to the NMI buffers. Once these buffers are removed, only the dumped stack will be serialized against other CPUs (via the printk_cpu_lock). Also serialize the nmi_backtrace banner and regs using the printk_cpu_lock so that per-CPU serialization will be preserved even after the NMI buffers are removed. Signed-off-by: John Ogness Reviewed-by: Petr Mladek Signed-off-by: Petr Mladek Link: https://lore.kernel.org/r/20210715193359.25946-2-john.ogness@linutronix.de

lib: Add backtrace_idle parameter to force backtrace of idle CPUs

2020-08-24T21:24:25+00:00

Currently, the nmi_cpu_backtrace() declines to produce backtraces for idle CPUs. This is a good choice in the common case in which problems are caused only by non-idle CPUs. However, there are occasionally situations in which idle CPUs are helping to cause problems. This commit therefore adds an nmi_backtrace.backtrace_idle kernel boot parameter that causes nmi_cpu_backtrace() to dump stacks even of idle CPUs. Signed-off-by: Paul E. McKenney Cc: Jonathan Corbet Cc: Thomas Gleixner Cc: Andrew Morton Cc: Greg Kroah-Hartman Cc: