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-rw-r--r--kernel/printk/Makefile1
-rw-r--r--kernel/printk/internal.h44
-rw-r--r--kernel/printk/nmi.c219
-rw-r--r--kernel/printk/printk.c19
4 files changed, 265 insertions, 18 deletions
diff --git a/kernel/printk/Makefile b/kernel/printk/Makefile
index 85405bdcf2b3..abb0042a427b 100644
--- a/kernel/printk/Makefile
+++ b/kernel/printk/Makefile
@@ -1,2 +1,3 @@
obj-y = printk.o
+obj-$(CONFIG_PRINTK_NMI) += nmi.o
obj-$(CONFIG_A11Y_BRAILLE_CONSOLE) += braille.o
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
new file mode 100644
index 000000000000..2de99faedfc1
--- /dev/null
+++ b/kernel/printk/internal.h
@@ -0,0 +1,44 @@
+/*
+ * internal.h - printk internal definitions
+ *
+ * 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/percpu.h>
+
+typedef __printf(1, 0) int (*printk_func_t)(const char *fmt, va_list args);
+
+int __printf(1, 0) vprintk_default(const char *fmt, va_list args);
+
+#ifdef CONFIG_PRINTK_NMI
+
+/*
+ * printk() could not take logbuf_lock in NMI context. Instead,
+ * it temporary stores the strings into a per-CPU buffer.
+ * The alternative implementation is chosen transparently
+ * via per-CPU variable.
+ */
+DECLARE_PER_CPU(printk_func_t, printk_func);
+static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
+{
+ return this_cpu_read(printk_func)(fmt, args);
+}
+
+#else /* CONFIG_PRINTK_NMI */
+
+static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
+{
+ return vprintk_default(fmt, args);
+}
+
+#endif /* CONFIG_PRINTK_NMI */
diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
new file mode 100644
index 000000000000..303cf0d15e57
--- /dev/null
+++ b/kernel/printk/nmi.c
@@ -0,0 +1,219 @@
+/*
+ * 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/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;
+
+#define NMI_LOG_BUF_LEN (4096 - 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))
+ 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;
+
+ 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);
+}
+
+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);
+}
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index bfbf284e4218..71eba0607034 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -55,6 +55,7 @@
#include "console_cmdline.h"
#include "braille.h"
+#include "internal.h"
int console_printk[4] = {
CONSOLE_LOGLEVEL_DEFAULT, /* console_loglevel */
@@ -1807,14 +1808,6 @@ int vprintk_default(const char *fmt, va_list args)
}
EXPORT_SYMBOL_GPL(vprintk_default);
-/*
- * This allows printk to be diverted to another function per cpu.
- * This is useful for calling printk functions from within NMI
- * without worrying about race conditions that can lock up the
- * box.
- */
-DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
-
/**
* printk - print a kernel message
* @fmt: format string
@@ -1838,21 +1831,11 @@ DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
*/
asmlinkage __visible int printk(const char *fmt, ...)
{
- printk_func_t vprintk_func;
va_list args;
int r;
va_start(args, fmt);
-
- /*
- * If a caller overrides the per_cpu printk_func, then it needs
- * to disable preemption when calling printk(). Otherwise
- * the printk_func should be set to the default. No need to
- * disable preemption here.
- */
- vprintk_func = this_cpu_read(printk_func);
r = vprintk_func(fmt, args);
-
va_end(args);
return r;