Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161058AbbBCTKj (ORCPT ); Tue, 3 Feb 2015 14:10:39 -0500 Received: from mail-wi0-f173.google.com ([209.85.212.173]:57680 "EHLO mail-wi0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S966218AbbBCTKf (ORCPT ); Tue, 3 Feb 2015 14:10:35 -0500 From: Daniel Thompson To: Thomas Gleixner Cc: Daniel Thompson , Jason Cooper , Russell King , Will Deacon , Catalin Marinas , Marc Zyngier , Stephen Boyd , John Stultz , Steven Rostedt , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, patches@linaro.org, linaro-kernel@lists.linaro.org, Sumit Semwal , Dirk Behme , Daniel Drake , Dmitry Pervushin , Tim Sander Subject: [PATCH 3.19-rc6 v16 4/6] printk: Simple implementation for NMI backtracing Date: Tue, 3 Feb 2015 19:06:55 +0000 Message-Id: <1422990417-1783-5-git-send-email-daniel.thompson@linaro.org> X-Mailer: git-send-email 1.9.3 In-Reply-To: <1422990417-1783-1-git-send-email-daniel.thompson@linaro.org> References: <1422022952-31552-1-git-send-email-daniel.thompson@linaro.org> <1422990417-1783-1-git-send-email-daniel.thompson@linaro.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7179 Lines: 236 Currently there is a quite a pile of code sitting in arch/x86/kernel/apic/hw_nmi.c to support safe all-cpu backtracing from NMI. The code is inaccessible to backtrace implementations for other architectures, which is a shame because they would probably like to be safe too. Copy this code into printk. We'll port the x86 NMI backtrace to it in a later patch. Incidentally, technically I think it might be safe to call printk_nmi_prepare() from NMI, providing care were taken to honour the return code. printk_nmi_complete() cannot be called from NMI but could be scheduled using irq_work_queue(). However honouring the return code means sometimes it is impossible to get the message out so in most cases I'd say using this code in such a way should probably attract sympathy and/or derision rather than admiration. Signed-off-by: Daniel Thompson Cc: Steven Rostedt --- include/linux/printk.h | 18 ++++++ init/Kconfig | 3 + kernel/printk/printk.c | 149 +++++++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 170 insertions(+) diff --git a/include/linux/printk.h b/include/linux/printk.h index c8f170324e64..03c6921b3fcd 100644 --- a/include/linux/printk.h +++ b/include/linux/printk.h @@ -219,6 +219,24 @@ static inline void show_regs_print_info(const char *log_lvl) } #endif +#ifdef CONFIG_PRINTK_NMI +/* + * printk_nmi_prepare/complete are called to prepare the system for + * some or all cores to issue trace from NMI. printk_nmi_complete will + * print buffered output and cannot (safely) be called from NMI. + */ +extern int printk_nmi_prepare(void); +extern void printk_nmi_complete(void); + +/* + * printk_nmi_this_cpu_begin/end are used divert/restore printk on this + * cpu. The result is the output of printk() (by this CPU) will be + * stored in temporary buffers for later printing by printk_nmi_complete. + */ +extern void printk_nmi_this_cpu_begin(void); +extern void printk_nmi_this_cpu_end(void); +#endif + extern asmlinkage void dump_stack(void) __cold; #ifndef pr_fmt diff --git a/init/Kconfig b/init/Kconfig index 9afb971497f4..13b843e3b5f4 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1430,6 +1430,9 @@ config PRINTK very difficult to diagnose system problems, saying N here is strongly discouraged. +config PRINTK_NMI + bool + config BUG bool "BUG() support" if EXPERT default y diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 02d6b6d28796..fe1cd57ea349 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1829,6 +1829,155 @@ EXPORT_SYMBOL_GPL(vprintk_default); */ DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default; +#ifdef CONFIG_PRINTK_NMI + +#define NMI_BUF_SIZE 4096 + +struct nmi_seq_buf { + unsigned char buffer[NMI_BUF_SIZE]; + struct seq_buf seq; +}; + +/* Safe printing in NMI context */ +static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq); + +static DEFINE_PER_CPU(printk_func_t, nmi_print_saved_print_func); + +/* "in progress" flag of NMI printing */ +static unsigned long nmi_print_flag; + +static int __init printk_nmi_init(void) +{ + struct nmi_seq_buf *s; + int cpu; + + for_each_possible_cpu(cpu) { + s = &per_cpu(nmi_print_seq, cpu); + seq_buf_init(&s->seq, s->buffer, NMI_BUF_SIZE); + } + + return 0; +} +pure_initcall(printk_nmi_init); + +/* + * It is not safe to call printk() directly from NMI handlers. + * It may be fine if the NMI detected a lock up and we have no choice + * but to do so, but doing a NMI on all other CPUs to get a back trace + * can be done with a sysrq-l. We don't want that to lock up, which + * can happen if the NMI interrupts a printk in progress. + * + * Instead, we redirect the vprintk() to this nmi_vprintk() that writes + * the content into a per cpu seq_buf buffer. Then when the NMIs are + * all done, we can safely dump the contents of the seq_buf to a printk() + * from a non NMI context. + * + * This is not a generic printk() implementation and must be used with + * great care. In particular there is a static limit on the quantity of + * data that may be emitted during NMI, only one client can be active at + * one time (arbitrated by the return value of printk_nmi_begin() and + * it is required that something at task or interrupt context be scheduled + * to issue the output. + */ +static int nmi_vprintk(const char *fmt, va_list args) +{ + struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq); + unsigned int len = seq_buf_used(&s->seq); + + seq_buf_vprintf(&s->seq, fmt, args); + return seq_buf_used(&s->seq) - len; +} + +/* + * Reserve the NMI printk mechanism. Return an error if some other component + * is already using it. + */ +int printk_nmi_prepare(void) +{ + if (test_and_set_bit(0, &nmi_print_flag)) { + /* + * If something is already using the NMI print facility we + * can't allow a second one... + */ + return -EBUSY; + } + + return 0; +} + +static void print_seq_line(struct nmi_seq_buf *s, int start, int end) +{ + const char *buf = s->buffer + start; + + printk("%.*s", (end - start) + 1, buf); +} + +void printk_nmi_complete(void) +{ + struct nmi_seq_buf *s; + int len, cpu, i, last_i; + + /* + * Now that all the NMIs have triggered, we can dump out their + * back traces safely to the console. + */ + for_each_possible_cpu(cpu) { + s = &per_cpu(nmi_print_seq, cpu); + last_i = 0; + + len = seq_buf_used(&s->seq); + if (!len) + continue; + + /* Print line by line. */ + for (i = 0; i < len; i++) { + if (s->buffer[i] == '\n') { + print_seq_line(s, last_i, i); + last_i = i + 1; + } + } + /* Check if there was a partial line. */ + if (last_i < len) { + print_seq_line(s, last_i, len - 1); + pr_cont("\n"); + } + + /* Wipe out the buffer ready for the next time around. */ + seq_buf_clear(&s->seq); + } + + clear_bit(0, &nmi_print_flag); + smp_mb__after_atomic(); +} + +void printk_nmi_this_cpu_begin(void) +{ + /* + * Detect double-begins and report them. This code is unsafe (because + * it will print from NMI) but things are pretty badly damaged if the + * NMI re-enters and is somehow granted permission to use NMI printk, + * so how much worse can it get? Also since this code interferes with + * the operation of printk it is unlikely that any consequential + * failures will be able to log anything making this our last + * opportunity to tell anyone that something is wrong. + */ + if (this_cpu_read(nmi_print_saved_print_func)) { + this_cpu_write(printk_func, vprintk_default); + BUG(); + } + + this_cpu_write(nmi_print_saved_print_func, this_cpu_read(printk_func)); + this_cpu_write(printk_func, nmi_vprintk); +} + +void printk_nmi_this_cpu_end(void) +{ + this_cpu_write(printk_func, this_cpu_read(nmi_print_saved_print_func)); + this_cpu_write(nmi_print_saved_print_func, NULL); +} + +#endif /* CONFIG_PRINTK_NMI */ + /** * printk - print a kernel message * @fmt: format string -- 1.9.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/