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Fri, 31 Jul 2026 02:57:37 -0700 (PDT) Date: Fri, 31 Jul 2026 11:57:35 +0200 From: Petr Mladek To: Bradley Morgan Cc: Andrew Morton , Jinchao Wang , Feng Tang , Rio , Pnina Feder , Petr Pavlu , Sergey Senozhatsky , linux-kernel@vger.kernel.org, Sashiko , stable@vger.kernel.org Subject: Re: [PATCH v5 4/4] panic: allow force_cpu redirect from an NMI Message-ID: References: <20260726190412.10891-1-include@grrlz.net> <20260726190412.10891-5-include@grrlz.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260726190412.10891-5-include@grrlz.net> On Sun 2026-07-26 19:04:12, Bradley Morgan wrote: > nmi_panic() claims panic_cpu via panic_try_start() before calling > panic(). When the panic later reaches panic_try_force_cpu(), the > panic_in_progress() check sees panic_cpu set and refuses to redirect. > The crash kernel runs on the CPU that took the NMI instead of the CPU > requested with panic_force_cpu=: > > nmi_panic() > panic_try_start() wins, panic_cpu = X > panic("%s", msg) > vpanic() > panic_try_force_cpu() > panic_in_progress() true, panic_cpu is X > return false redirect bypassed > panic_try_start() already won > __crash_kexec() on X, not the requested CPU > > Try the redirect before claiming panic_cpu instead, as suggested by > Petr Mladek. nmi_panic() now calls panic_try_force_cpu() first and > claims panic_cpu only when no redirect happened. The requested CPU > claims panic_cpu itself when it runs panic(), so panic_cpu does not > need to be handed off. > > --- a/kernel/panic.c > +++ b/kernel/panic.c > @@ -364,18 +364,19 @@ int __weak panic_smp_redirect_cpu(int target_cpu, void *msg) > > /** > * panic_try_force_cpu - Redirect panic to a specific CPU for crash kernel > - * @fmt: panic message format string > - * @args: arguments for format string > + * @fmt: panic message format string, or the final message when @args is NULL > + * @args: arguments for the format string, or NULL when @fmt is final > * > * Some platforms require panic handling to occur on a specific CPU > * for the crash kernel to function correctly. This function redirects > * panic handling to the CPU specified via the panic_force_cpu= boot parameter. > * > - * Returns false if panic should proceed on current CPU. > - * Returns true if panic was redirected. > + * Returns true when this CPU must stop: the panic was redirected or is > + * already running on another CPU. > + * Returns false when panic() should proceed on this CPU. Nit: This a good improvement. But it would better fit into the 1st patch which fixed the true/false return values. > */ > __printf(1, 0) > -static bool panic_try_force_cpu(const char *fmt, va_list args) > +static bool panic_try_force_cpu(const char *fmt, va_list *args) > { > int this_cpu = raw_smp_processor_id(); > int old_cpu = PANIC_CPU_INVALID; > @@ -412,14 +413,18 @@ static bool panic_try_force_cpu(const char *fmt, va_list args) > return old_cpu != this_cpu; > > /* > - * Use dynamically allocated buffer if available, otherwise > - * fall back to static message for early boot panics or allocation failure. > + * A NULL @args means that @fmt is already the final message, for > + * example from nmi_panic(). Otherwise use the dynamically allocated > + * buffer if available, or fall back to a static message for early > + * boot panics or allocation failure. > */ > - if (panic_force_buf) { > + if (!args) { > + msg = fmt; Ah, I misunderstood the meaning of panic_try_force_cpu_fmt() in v4. I though that it was needed to _copy_ va_list() but it was needed to _create_ it. It looks like a cleaner solution after all. But I think that we could do even better, see below. > + } else if (panic_force_buf) { > va_list ap; > > /* Do not consume args, the caller reuses it if we fail */ > - va_copy(ap, args); > + va_copy(ap, *args); > vsnprintf(panic_force_buf, PANIC_MSG_BUFSZ, fmt, ap); > va_end(ap); > msg = panic_force_buf; > @@ -452,7 +457,7 @@ static bool panic_try_force_cpu(const char *fmt, va_list args) > } > #else > __printf(1, 0) > -static inline bool panic_try_force_cpu(const char *fmt, va_list args) > +static inline bool panic_try_force_cpu(const char *fmt, va_list *args) > { > return false; > } > @@ -512,13 +517,24 @@ bool panic_on_other_cpu(void) > EXPORT_SYMBOL(panic_on_other_cpu); > > /* > - * A variant of panic() called from NMI context. We return if we've already > - * panicked on this CPU. If another CPU already panicked, loop in > - * nmi_panic_self_stop() which can provide architecture dependent code such > - * as saving register state for crash dump. > + * A variant of panic() called from NMI context. The panic is first > + * redirected to the CPU requested via panic_force_cpu=, when configured. > + * We return if we've already panicked on this CPU. If another CPU already > + * panicked, loop in nmi_panic_self_stop() which can provide architecture > + * dependent code such as saving register state for crash dump. > */ > void nmi_panic(struct pt_regs *regs, const char *msg) > { > + /* Try to redirect to the requested CPU before claiming panic_cpu. */ > + if (panic_try_force_cpu(msg, NULL)) { > + /* > + * Mark ourselves offline so panic_other_cpus_shutdown() won't > + * wait for us on architectures that check num_online_cpus(). > + */ > + set_cpu_online(raw_smp_processor_id(), false); > + nmi_panic_self_stop(regs); > + } > + > if (panic_try_start()) > panic("%s", msg); > > @@ -590,6 +606,7 @@ void vpanic(const char *fmt, va_list args) > long i, i_next = 0, len; > int state = 0; > bool _crash_kexec_post_notifiers = crash_kexec_post_notifiers; > + va_list redirect_args; > > if (panic_on_warn) { > /* > @@ -610,8 +627,13 @@ void vpanic(const char *fmt, va_list args) > local_irq_disable(); > preempt_disable_notrace(); > > - /* Redirect panic to target CPU if configured via panic_force_cpu=. */ > - if (panic_try_force_cpu(fmt, args)) { > + /* > + * Redirect panic to the target CPU if configured via panic_force_cpu=. > + * Hand over a disposable copy of the arguments, the address of a > + * va_list parameter cannot be taken portably. > + */ > + va_copy(redirect_args, args); This is weird. panic_try_force_cpu() does its own copy when it prints the string. Also the comment says that it creates a copy because the address of a va_list could not be taken portably. But the code below passes the address of "redirect_args" which is "va_list" again. So, the copy does not make any difference. > + if (panic_try_force_cpu(fmt, &redirect_args)) { > /* > * Mark ourselves offline so panic_other_cpus_shutdown() won't wait > * for us on architectures that check num_online_cpus(). It would be better to avoid passing the pointer. The main problem here is that nmi_panic() does not support variable list of arguments. I see two solutions: Solution A: Create a wrapper for panic_try_force_cpu() like in v4. But it should follow the existing naming scheme panic() vs. vpanic(): 1. Rename panic_try_force_cpu() -> vpanic_try_force_cpu() for the variant with va_list. 2. Create the wrapper panic_try_force_cpu() with "..." variable list of parameters. Solution B: Allow to pass the variable list of arguments to nmi_panic(). Note that the original nmi_panic() accepted variable list of arguments. This ability was removed by the commit ebc41f20d77f6ad91 ("panic: change nmi_panic from macro to function"). But vpanic() did not exist at that time. I have updated this patch accordingly to see how it would look like: diff --git a/include/linux/panic.h b/include/linux/panic.h index f1dd417e54b2..a9128bf3c168 100644 --- a/include/linux/panic.h +++ b/include/linux/panic.h @@ -13,7 +13,8 @@ __printf(1, 2) void panic(const char *fmt, ...) __noreturn __cold; __printf(1, 0) void vpanic(const char *fmt, va_list args) __noreturn __cold; -void nmi_panic(struct pt_regs *regs, const char *msg); +__printf(2, 3) +void nmi_panic(struct pt_regs *regs, const char *fmt, ...); void check_panic_on_warn(const char *origin); extern void oops_enter(void); extern void oops_exit(void); diff --git a/kernel/panic.c b/kernel/panic.c index a483587fdd2f..7ce252268714 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -371,8 +371,9 @@ int __weak panic_smp_redirect_cpu(int target_cpu, void *msg) * for the crash kernel to function correctly. This function redirects * panic handling to the CPU specified via the panic_force_cpu= boot parameter. * - * Returns false if panic should proceed on current CPU. - * Returns true if panic was redirected. + * Returns true when this CPU must stop: the panic was redirected or is + * already running on another CPU. + * Returns false when panic() should proceed on this CPU. */ __printf(1, 0) static bool panic_try_force_cpu(const char *fmt, va_list args) @@ -452,7 +453,7 @@ static bool panic_try_force_cpu(const char *fmt, va_list args) } #else __printf(1, 0) -static inline bool panic_try_force_cpu(const char *fmt, va_list args) +static inline bool panic_try_force_cpu(const char *fmt, va_list *args) { return false; } @@ -512,18 +513,37 @@ bool panic_on_other_cpu(void) EXPORT_SYMBOL(panic_on_other_cpu); /* - * A variant of panic() called from NMI context. We return if we've already - * panicked on this CPU. If another CPU already panicked, loop in - * nmi_panic_self_stop() which can provide architecture dependent code such - * as saving register state for crash dump. + * A variant of panic() called from NMI context. The panic is first + * redirected to the CPU requested via panic_force_cpu=, when configured. + * We return if we've already panicked on this CPU. If another CPU already + * panicked, loop in nmi_panic_self_stop() which can provide architecture + * dependent code such as saving register state for crash dump. */ -void nmi_panic(struct pt_regs *regs, const char *msg) +__printf(2, 3) +void nmi_panic(struct pt_regs *regs, const char *fmt, ...) { + va_list args; + + va_start(args, fmt); + + /* Try to redirect to the requested CPU before claiming panic_cpu. */ + if (panic_try_force_cpu(fmt, args)) { + /* + * Mark ourselves offline so panic_other_cpus_shutdown() won't + * wait for us on architectures that check num_online_cpus(). + */ + set_cpu_online(raw_smp_processor_id(), false); + nmi_panic_self_stop(regs); + } + if (panic_try_start()) - panic("%s", msg); + vpanic(fmt, args); if (panic_on_other_cpu()) nmi_panic_self_stop(regs); + + va_end(args); + } EXPORT_SYMBOL(nmi_panic); I personally prefer the variant B. Note that the above POC patch would need some changes: 1. Move the comment changes above panic_try_force_cpu() into the 1st patch 2. Add the "..." into nmi_panic() in a separate patch. 3. The patch allowing redirection from nmi_panic() would just add the following into nmi_panic(). /* Try to redirect to the requested CPU before claiming panic_cpu. */ if (panic_try_force_cpu(fmt, args)) { /* * Mark ourselves offline so panic_other_cpus_shutdown() won't * wait for us on architectures that check num_online_cpus(). */ set_cpu_online(raw_smp_processor_id(), false); nmi_panic_self_stop(regs); } plus it could the comment above nmi_panic(). Best Regards, Petr