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+/*
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+ * nmi.c - Safe printk in NMI context
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+ *
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+ * This program is free software; you can redistribute it and/or
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+ * modify it under the terms of the GNU General Public License
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+ * as published by the Free Software Foundation; either version 2
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+ * of the License, or (at your option) any later version.
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+ *
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+ * This program is distributed in the hope that it will be useful,
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+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
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+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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+ * GNU General Public License for more details.
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+ *
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+ * You should have received a copy of the GNU General Public License
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+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
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+ */
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+
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+#include <linux/preempt.h>
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+#include <linux/spinlock.h>
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+#include <linux/smp.h>
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+#include <linux/cpumask.h>
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+#include <linux/irq_work.h>
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+#include <linux/printk.h>
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+
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+#include "internal.h"
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+
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+/*
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+ * printk() could not take logbuf_lock in NMI context. Instead,
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+ * it uses an alternative implementation that temporary stores
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+ * the strings into a per-CPU buffer. The content of the buffer
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+ * is later flushed into the main ring buffer via IRQ work.
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+ *
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+ * The alternative implementation is chosen transparently
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+ * via @printk_func per-CPU variable.
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+ *
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+ * The implementation allows to flush the strings also from another CPU.
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+ * There are situations when we want to make sure that all buffers
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+ * were handled or when IRQs are blocked.
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+ */
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+DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
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+static int printk_nmi_irq_ready;
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+
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+#define NMI_LOG_BUF_LEN (4096 - sizeof(atomic_t) - sizeof(struct irq_work))
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+
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+struct nmi_seq_buf {
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+ atomic_t len; /* length of written data */
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+ struct irq_work work; /* IRQ work that flushes the buffer */
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+ unsigned char buffer[NMI_LOG_BUF_LEN];
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+};
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+static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
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+
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+/*
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+ * Safe printk() for NMI context. It uses a per-CPU buffer to
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+ * store the message. NMIs are not nested, so there is always only
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+ * one writer running. But the buffer might get flushed from another
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+ * CPU, so we need to be careful.
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+ */
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+static int vprintk_nmi(const char *fmt, va_list args)
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+{
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+ struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
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+ int add = 0;
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+ size_t len;
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+
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+again:
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+ len = atomic_read(&s->len);
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+
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+ if (len >= sizeof(s->buffer))
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+ return 0;
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+
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+ /*
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+ * Make sure that all old data have been read before the buffer was
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+ * reseted. This is not needed when we just append data.
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+ */
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+ if (!len)
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+ smp_rmb();
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+
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+ add = vsnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);
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+
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+ /*
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+ * Do it once again if the buffer has been flushed in the meantime.
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+ * Note that atomic_cmpxchg() is an implicit memory barrier that
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+ * makes sure that the data were written before updating s->len.
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+ */
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+ if (atomic_cmpxchg(&s->len, len, len + add) != len)
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+ goto again;
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+
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+ /* Get flushed in a more safe context. */
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+ if (add && printk_nmi_irq_ready) {
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+ /* Make sure that IRQ work is really initialized. */
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+ smp_rmb();
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+ irq_work_queue(&s->work);
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+ }
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+
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+ return add;
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+}
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+
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+/*
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+ * printk one line from the temporary buffer from @start index until
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+ * and including the @end index.
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+ */
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+static void print_nmi_seq_line(struct nmi_seq_buf *s, int start, int end)
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+{
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+ const char *buf = s->buffer + start;
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+
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+ printk("%.*s", (end - start) + 1, buf);
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+}
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+
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+/*
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+ * Flush data from the associated per_CPU buffer. The function
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+ * can be called either via IRQ work or independently.
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+ */
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+static void __printk_nmi_flush(struct irq_work *work)
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+{
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+ static raw_spinlock_t read_lock =
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+ __RAW_SPIN_LOCK_INITIALIZER(read_lock);
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+ struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
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+ unsigned long flags;
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+ size_t len, size;
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+ int i, last_i;
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+
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+ /*
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+ * The lock has two functions. First, one reader has to flush all
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+ * available message to make the lockless synchronization with
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+ * writers easier. Second, we do not want to mix messages from
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+ * different CPUs. This is especially important when printing
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+ * a backtrace.
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+ */
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+ raw_spin_lock_irqsave(&read_lock, flags);
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+
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+ i = 0;
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+more:
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+ len = atomic_read(&s->len);
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+
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+ /*
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+ * This is just a paranoid check that nobody has manipulated
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+ * the buffer an unexpected way. If we printed something then
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+ * @len must only increase.
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+ */
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+ if (i && i >= len)
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+ pr_err("printk_nmi_flush: internal error: i=%d >= len=%zu\n",
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+ i, len);
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+
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+ if (!len)
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+ goto out; /* Someone else has already flushed the buffer. */
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+
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+ /* Make sure that data has been written up to the @len */
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+ smp_rmb();
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+
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+ size = min(len, sizeof(s->buffer));
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+ last_i = i;
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+
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+ /* Print line by line. */
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+ for (; i < size; i++) {
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+ if (s->buffer[i] == '\n') {
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+ print_nmi_seq_line(s, last_i, i);
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+ last_i = i + 1;
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+ }
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+ }
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+ /* Check if there was a partial line. */
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+ if (last_i < size) {
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+ print_nmi_seq_line(s, last_i, size - 1);
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+ pr_cont("\n");
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+ }
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+
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+ /*
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+ * Check that nothing has got added in the meantime and truncate
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+ * the buffer. Note that atomic_cmpxchg() is an implicit memory
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+ * barrier that makes sure that the data were copied before
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+ * updating s->len.
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+ */
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+ if (atomic_cmpxchg(&s->len, len, 0) != len)
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+ goto more;
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+
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+out:
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+ raw_spin_unlock_irqrestore(&read_lock, flags);
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+}
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+
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+/**
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+ * printk_nmi_flush - flush all per-cpu nmi buffers.
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+ *
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+ * The buffers are flushed automatically via IRQ work. This function
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+ * is useful only when someone wants to be sure that all buffers have
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+ * been flushed at some point.
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+ */
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+void printk_nmi_flush(void)
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+{
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+ int cpu;
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+
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+ for_each_possible_cpu(cpu)
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+ __printk_nmi_flush(&per_cpu(nmi_print_seq, cpu).work);
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+}
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+
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+void __init printk_nmi_init(void)
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+{
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+ int cpu;
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+
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+ for_each_possible_cpu(cpu) {
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+ struct nmi_seq_buf *s = &per_cpu(nmi_print_seq, cpu);
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+
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+ init_irq_work(&s->work, __printk_nmi_flush);
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+ }
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+
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+ /* Make sure that IRQ works are initialized before enabling. */
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+ smp_wmb();
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+ printk_nmi_irq_ready = 1;
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+
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+ /* Flush pending messages that did not have scheduled IRQ works. */
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+ printk_nmi_flush();
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+}
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+
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+void printk_nmi_enter(void)
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+{
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+ this_cpu_write(printk_func, vprintk_nmi);
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+}
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+
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+void printk_nmi_exit(void)
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+{
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+ this_cpu_write(printk_func, vprintk_default);
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+}
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Petr Mladek