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authorSergey Senozhatsky <sergey.senozhatsky@gmail.com>2016-12-27 23:16:05 +0900
committerPetr Mladek <pmladek@suse.com>2017-02-08 11:02:33 +0100
commitf92bac3b141b8233e34ddf32d227e12bfba07b48 (patch)
tree6f59d9dacd2b4eaf944e173aee136a6868389ff5 /kernel/printk/printk_safe.c
parentbd66a89249892acc9d938ba4956066b21403fa5f (diff)
printk: rename nmi.c and exported api
A preparation patch for printk_safe work. No functional change. - rename nmi.c to print_safe.c - add `printk_safe' prefix to some (which used both by printk-safe and printk-nmi) of the exported functions. Link: http://lkml.kernel.org/r/20161227141611.940-3-sergey.senozhatsky@gmail.com Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Jan Kara <jack@suse.cz> Cc: Tejun Heo <tj@kernel.org> Cc: Calvin Owens <calvinowens@fb.com> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Ingo Molnar <mingo@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Andy Lutomirski <luto@kernel.org> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: linux-kernel@vger.kernel.org Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Signed-off-by: Petr Mladek <pmladek@suse.com>
Diffstat (limited to 'kernel/printk/printk_safe.c')
-rw-r--r--kernel/printk/printk_safe.c291
1 files changed, 291 insertions, 0 deletions
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
new file mode 100644
index 000000000000..fc80359dcd78
--- /dev/null
+++ b/kernel/printk/printk_safe.c
@@ -0,0 +1,291 @@
+/*
+ * printk_safe.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_safe_irq_ready;
+atomic_t nmi_message_lost;
+
+#define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) - \
+ sizeof(atomic_t) - sizeof(struct irq_work))
+
+struct printk_safe_seq_buf {
+ atomic_t len; /* length of written data */
+ struct irq_work work; /* IRQ work that flushes the buffer */
+ unsigned char buffer[SAFE_LOG_BUF_LEN];
+};
+static DEFINE_PER_CPU(struct printk_safe_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 printk_safe_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
+ int add = 0;
+ size_t len;
+
+again:
+ len = atomic_read(&s->len);
+
+ /* The trailing '\0' is not counted into len. */
+ if (len >= sizeof(s->buffer) - 1) {
+ 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 = vscnprintf(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_safe_irq_ready) {
+ /* Make sure that IRQ work is really initialized. */
+ smp_rmb();
+ irq_work_queue(&s->work);
+ }
+
+ return add;
+}
+
+static void printk_safe_flush_line(const char *text, int len)
+{
+ /*
+ * 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", len, text);
+ else
+ printk("%.*s", len, text);
+}
+
+/* printk part of the temporary buffer line by line */
+static int printk_safe_flush_buffer(const char *start, size_t len)
+{
+ const char *c, *end;
+ bool header;
+
+ c = start;
+ end = start + len;
+ header = true;
+
+ /* Print line by line. */
+ while (c < end) {
+ if (*c == '\n') {
+ printk_safe_flush_line(start, c - start + 1);
+ start = ++c;
+ header = true;
+ continue;
+ }
+
+ /* Handle continuous lines or missing new line. */
+ if ((c + 1 < end) && printk_get_level(c)) {
+ if (header) {
+ c = printk_skip_level(c);
+ continue;
+ }
+
+ printk_safe_flush_line(start, c - start);
+ start = c++;
+ header = true;
+ continue;
+ }
+
+ header = false;
+ c++;
+ }
+
+ /* Check if there was a partial line. Ignore pure header. */
+ if (start < end && !header) {
+ static const char newline[] = KERN_CONT "\n";
+
+ printk_safe_flush_line(start, end - start);
+ printk_safe_flush_line(newline, strlen(newline));
+ }
+
+ return len;
+}
+
+/*
+ * Flush data from the associated per_CPU buffer. The function
+ * can be called either via IRQ work or independently.
+ */
+static void __printk_safe_flush(struct irq_work *work)
+{
+ static raw_spinlock_t read_lock =
+ __RAW_SPIN_LOCK_INITIALIZER(read_lock);
+ struct printk_safe_seq_buf *s =
+ container_of(work, struct printk_safe_seq_buf, work);
+ unsigned long flags;
+ size_t len;
+ int 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. Also it should never overflow the
+ * buffer size.
+ */
+ if ((i && i >= len) || len > sizeof(s->buffer)) {
+ const char *msg = "printk_safe_flush: internal error\n";
+
+ printk_safe_flush_line(msg, strlen(msg));
+ len = 0;
+ }
+
+ if (!len)
+ goto out; /* Someone else has already flushed the buffer. */
+
+ /* Make sure that data has been written up to the @len */
+ smp_rmb();
+ i += printk_safe_flush_buffer(s->buffer + i, len - i);
+
+ /*
+ * 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_safe_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_safe_flush(void)
+{
+ int cpu;
+
+ for_each_possible_cpu(cpu)
+ __printk_safe_flush(&per_cpu(nmi_print_seq, cpu).work);
+}
+
+/**
+ * printk_safe_flush_on_panic - flush all per-cpu nmi buffers when the system
+ * goes down.
+ *
+ * Similar to printk_safe_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_safe_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_safe_flush();
+}
+
+void __init printk_safe_init(void)
+{
+ int cpu;
+
+ for_each_possible_cpu(cpu) {
+ struct printk_safe_seq_buf *s = &per_cpu(nmi_print_seq, cpu);
+
+ init_irq_work(&s->work, __printk_safe_flush);
+ }
+
+ /* Make sure that IRQ works are initialized before enabling. */
+ smp_wmb();
+ printk_safe_irq_ready = 1;
+
+ /* Flush pending messages that did not have scheduled IRQ works. */
+ printk_safe_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);
+}