/* * nmi.c - Safe printk in NMI context * * This program is free software; you can redistribute it and/or * modify it under the terms of the GNU General Public License * as published by the Free Software Foundation; either version 2 * of the License, or (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, see <http://www.gnu.org/licenses/>. */ #include <linux/preempt.h> #include <linux/spinlock.h> #include <linux/debug_locks.h> #include <linux/smp.h> #include <linux/cpumask.h> #include <linux/irq_work.h> #include <linux/printk.h> #include "internal.h" /* * printk() could not take logbuf_lock in NMI context. Instead, * it uses an alternative implementation that temporary stores * the strings into a per-CPU buffer. The content of the buffer * is later flushed into the main ring buffer via IRQ work. * * The alternative implementation is chosen transparently * via @printk_func per-CPU variable. * * The implementation allows to flush the strings also from another CPU. * There are situations when we want to make sure that all buffers * were handled or when IRQs are blocked. */ DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default; static int printk_nmi_irq_ready; atomic_t nmi_message_lost; #define NMI_LOG_BUF_LEN ((1 << CONFIG_NMI_LOG_BUF_SHIFT) - \ sizeof(atomic_t) - sizeof(struct irq_work)) struct nmi_seq_buf { atomic_t len; /* length of written data */ struct irq_work work; /* IRQ work that flushes the buffer */ unsigned char buffer[NMI_LOG_BUF_LEN]; }; static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq); /* * Safe printk() for NMI context. It uses a per-CPU buffer to * store the message. NMIs are not nested, so there is always only * one writer running. But the buffer might get flushed from another * CPU, so we need to be careful. */ static int vprintk_nmi(const char *fmt, va_list args) { struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq); int add = 0; size_t len; again: len = atomic_read(&s->len); if (len >= sizeof(s->buffer)) { atomic_inc(&nmi_message_lost); return 0; } /* * Make sure that all old data have been read before the buffer was * reseted. This is not needed when we just append data. */ if (!len) smp_rmb(); add = vsnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args); /* * Do it once again if the buffer has been flushed in the meantime. * Note that atomic_cmpxchg() is an implicit memory barrier that * makes sure that the data were written before updating s->len. */ if (atomic_cmpxchg(&s->len, len, len + add) != len) goto again; /* Get flushed in a more safe context. */ if (add && printk_nmi_irq_ready) { /* Make sure that IRQ work is really initialized. */ smp_rmb(); irq_work_queue(&s->work); } return add; } /* * printk one line from the temporary buffer from @start index until * and including the @end index. */ static void print_nmi_seq_line(struct nmi_seq_buf *s, int start, int end) { const char *buf = s->buffer + start; /* * The buffers are flushed in NMI only on panic. The messages must * go only into the ring buffer at this stage. Consoles will get * explicitly called later when a crashdump is not generated. */ if (in_nmi()) printk_deferred("%.*s", (end - start) + 1, buf); else printk("%.*s", (end - start) + 1, buf); } /* * Flush data from the associated per_CPU buffer. The function * can be called either via IRQ work or independently. */ static void __printk_nmi_flush(struct irq_work *work) { static raw_spinlock_t read_lock = __RAW_SPIN_LOCK_INITIALIZER(read_lock); struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work); unsigned long flags; size_t len, size; int i, last_i; /* * The lock has two functions. First, one reader has to flush all * available message to make the lockless synchronization with * writers easier. Second, we do not want to mix messages from * different CPUs. This is especially important when printing * a backtrace. */ raw_spin_lock_irqsave(&read_lock, flags); i = 0; more: len = atomic_read(&s->len); /* * This is just a paranoid check that nobody has manipulated * the buffer an unexpected way. If we printed something then * @len must only increase. */ if (i && i >= len) pr_err("printk_nmi_flush: internal error: i=%d >= len=%zu\n", i, len); if (!len) goto out; /* Someone else has already flushed the buffer. */ /* Make sure that data has been written up to the @len */ smp_rmb(); size = min(len, sizeof(s->buffer)); last_i = i; /* Print line by line. */ for (; i < size; i++) { if (s->buffer[i] == '\n') { print_nmi_seq_line(s, last_i, i); last_i = i + 1; } } /* Check if there was a partial line. */ if (last_i < size) { print_nmi_seq_line(s, last_i, size - 1); pr_cont("\n"); } /* * Check that nothing has got added in the meantime and truncate * the buffer. Note that atomic_cmpxchg() is an implicit memory * barrier that makes sure that the data were copied before * updating s->len. */ if (atomic_cmpxchg(&s->len, len, 0) != len) goto more; out: raw_spin_unlock_irqrestore(&read_lock, flags); } /** * printk_nmi_flush - flush all per-cpu nmi buffers. * * The buffers are flushed automatically via IRQ work. This function * is useful only when someone wants to be sure that all buffers have * been flushed at some point. */ void printk_nmi_flush(void) { int cpu; for_each_possible_cpu(cpu) __printk_nmi_flush(&per_cpu(nmi_print_seq, cpu).work); } /** * printk_nmi_flush_on_panic - flush all per-cpu nmi buffers when the system * goes down. * * Similar to printk_nmi_flush() but it can be called even in NMI context when * the system goes down. It does the best effort to get NMI messages into * the main ring buffer. * * Note that it could try harder when there is only one CPU online. */ void printk_nmi_flush_on_panic(void) { /* * Make sure that we could access the main ring buffer. * Do not risk a double release when more CPUs are up. */ if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) { if (num_online_cpus() > 1) return; debug_locks_off(); raw_spin_lock_init(&logbuf_lock); } printk_nmi_flush(); } void __init printk_nmi_init(void) { int cpu; for_each_possible_cpu(cpu) { struct nmi_seq_buf *s = &per_cpu(nmi_print_seq, cpu); init_irq_work(&s->work, __printk_nmi_flush); } /* Make sure that IRQ works are initialized before enabling. */ smp_wmb(); printk_nmi_irq_ready = 1; /* Flush pending messages that did not have scheduled IRQ works. */ printk_nmi_flush(); } void printk_nmi_enter(void) { this_cpu_write(printk_func, vprintk_nmi); } void printk_nmi_exit(void) { this_cpu_write(printk_func, vprintk_default); }