4 files changed, 154 insertions, 26 deletions

diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
index 6f8d0b77006b..13a54d0bdfa8 100644
--- a/include/linux/ftrace_event.h
+++ b/include/linux/ftrace_event.h
@@ -83,6 +83,9 @@ struct trace_iterator {
         long                    idx;
 
         cpumask_var_t           started;
+
+        /* it's true when current open file is snapshot */
+        bool                    snapshot;
 };
 
 enum trace_iter_flags {
diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index cdc9d284d24e..36567564e221 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -253,6 +253,16 @@ config FTRACE_SYSCALLS
         help
           Basic tracer to catch the syscall entry and exit events.
 
+config TRACER_SNAPSHOT
+        bool "Create a snapshot trace buffer"
+        select TRACER_MAX_TRACE
+        help
+          Allow tracing users to take snapshot of the current buffer using the
+          ftrace interface, e.g.:
+
+              echo 1 > /sys/kernel/debug/tracing/snapshot
+              cat snapshot
+
 config TRACE_BRANCH_PROFILING
         bool
         select GENERIC_TRACER
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 2c724662a3e8..70dce64b9ecf 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -710,12 +710,11 @@ update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu)
 
         WARN_ON_ONCE(!irqs_disabled());
 
-        /* If we disabled the tracer, stop now */
-        if (current_trace == &nop_trace)
-                return;
-
-        if (WARN_ON_ONCE(!current_trace->use_max_tr))
+        if (!current_trace->allocated_snapshot) {
+                /* Only the nop tracer should hit this when disabling */
+                WARN_ON_ONCE(current_trace != &nop_trace);
                 return;
+        }
 
         arch_spin_lock(&ftrace_max_lock);
 
@@ -743,10 +742,8 @@ update_max_tr_single(struct trace_array *tr, struct task_struct *tsk, int cpu)
                 return;
 
         WARN_ON_ONCE(!irqs_disabled());
-        if (!current_trace->use_max_tr) {
-                WARN_ON_ONCE(1);
+        if (WARN_ON_ONCE(!current_trace->allocated_snapshot))
                 return;
-        }
 
         arch_spin_lock(&ftrace_max_lock);
 
@@ -866,10 +863,13 @@ int register_tracer(struct tracer *type)
 
                 current_trace = type;
 
-                /* If we expanded the buffers, make sure the max is expanded too */
-                if (ring_buffer_expanded && type->use_max_tr)
-                        ring_buffer_resize(max_tr.buffer, trace_buf_size,
-                                                RING_BUFFER_ALL_CPUS);
+                if (type->use_max_tr) {
+                        /* If we expanded the buffers, make sure the max is expanded too */
+                        if (ring_buffer_expanded)
+                                ring_buffer_resize(max_tr.buffer, trace_buf_size,
+                                                   RING_BUFFER_ALL_CPUS);
+                        type->allocated_snapshot = true;
+                }
 
                 /* the test is responsible for initializing and enabling */
                 pr_info("Testing tracer %s: ", type->name);
@@ -885,10 +885,14 @@ int register_tracer(struct tracer *type)
                 /* Only reset on passing, to avoid touching corrupted buffers */
                 tracing_reset_online_cpus(tr);
 
-                /* Shrink the max buffer again */
-                if (ring_buffer_expanded && type->use_max_tr)
-                        ring_buffer_resize(max_tr.buffer, 1,
-                                                RING_BUFFER_ALL_CPUS);
+                if (type->use_max_tr) {
+                        type->allocated_snapshot = false;
+
+                        /* Shrink the max buffer again */
+                        if (ring_buffer_expanded)
+                                ring_buffer_resize(max_tr.buffer, 1,
+                                                   RING_BUFFER_ALL_CPUS);
+                }
 
                 printk(KERN_CONT "PASSED\n");
         }
@@ -1964,7 +1968,11 @@ static void *s_start(struct seq_file *m, loff_t *pos)
                 *iter->trace = *current_trace;
         mutex_unlock(&trace_types_lock);
 
-        atomic_inc(&trace_record_cmdline_disabled);
+        if (iter->snapshot && iter->trace->use_max_tr)
+                return ERR_PTR(-EBUSY);
+
+        if (!iter->snapshot)
+                atomic_inc(&trace_record_cmdline_disabled);
 
         if (*pos != iter->pos) {
                 iter->ent = NULL;
@@ -2003,7 +2011,11 @@ static void s_stop(struct seq_file *m, void *p)
 {
         struct trace_iterator *iter = m->private;
 
-        atomic_dec(&trace_record_cmdline_disabled);
+        if (iter->snapshot && iter->trace->use_max_tr)
+                return;
+
+        if (!iter->snapshot)
+                atomic_dec(&trace_record_cmdline_disabled);
         trace_access_unlock(iter->cpu_file);
         trace_event_read_unlock();
 }
@@ -2438,7 +2450,7 @@ static const struct seq_operations tracer_seq_ops = {
 };
 
 static struct trace_iterator *
-__tracing_open(struct inode *inode, struct file *file)
+__tracing_open(struct inode *inode, struct file *file, bool snapshot)
 {
         long cpu_file = (long) inode->i_private;
         struct trace_iterator *iter;
@@ -2471,10 +2483,11 @@ __tracing_open(struct inode *inode, struct file *file)
         if (!zalloc_cpumask_var(&iter->started, GFP_KERNEL))
                 goto fail;
 
-        if (current_trace && current_trace->print_max)
+        if ((current_trace && current_trace->print_max) || snapshot)
                 iter->tr = &max_tr;
         else
                 iter->tr = &global_trace;
+        iter->snapshot = snapshot;
         iter->pos = -1;
         mutex_init(&iter->mutex);
         iter->cpu_file = cpu_file;
@@ -2491,8 +2504,9 @@ __tracing_open(struct inode *inode, struct file *file)
         if (trace_clocks[trace_clock_id].in_ns)
                 iter->iter_flags |= TRACE_FILE_TIME_IN_NS;
 
-        /* stop the trace while dumping */
-        tracing_stop();
+        /* stop the trace while dumping if we are not opening "snapshot" */
+        if (!iter->snapshot)
+                tracing_stop();
 
         if (iter->cpu_file == TRACE_PIPE_ALL_CPU) {
                 for_each_tracing_cpu(cpu) {
@@ -2555,8 +2569,9 @@ static int tracing_release(struct inode *inode, struct file *file)
         if (iter->trace && iter->trace->close)
                 iter->trace->close(iter);
 
-        /* reenable tracing if it was previously enabled */
-        tracing_start();
+        if (!iter->snapshot)
+                /* reenable tracing if it was previously enabled */
+                tracing_start();
         mutex_unlock(&trace_types_lock);
 
         mutex_destroy(&iter->mutex);
@@ -2584,7 +2599,7 @@ static int tracing_open(struct inode *inode, struct file *file)
         }
 
         if (file->f_mode & FMODE_READ) {
-                iter = __tracing_open(inode, file);
+                iter = __tracing_open(inode, file, false);
                 if (IS_ERR(iter))
                         ret = PTR_ERR(iter);
                 else if (trace_flags & TRACE_ITER_LATENCY_FMT)
@@ -3219,7 +3234,7 @@ static int tracing_set_tracer(const char *buf)
         if (current_trace && current_trace->reset)
                 current_trace->reset(tr);
 
-        had_max_tr = current_trace && current_trace->use_max_tr;
+        had_max_tr = current_trace && current_trace->allocated_snapshot;
         current_trace = &nop_trace;
 
         if (had_max_tr && !t->use_max_tr) {
@@ -3238,6 +3253,8 @@ static int tracing_set_tracer(const char *buf)
                  */
                 ring_buffer_resize(max_tr.buffer, 1, RING_BUFFER_ALL_CPUS);
                 set_buffer_entries(&max_tr, 1);
+                tracing_reset_online_cpus(&max_tr);
+                current_trace->allocated_snapshot = false;
         }
         destroy_trace_option_files(topts);
 
@@ -3248,6 +3265,7 @@ static int tracing_set_tracer(const char *buf)
                                                    RING_BUFFER_ALL_CPUS);
                 if (ret < 0)
                         goto out;
+                t->allocated_snapshot = true;
         }
 
         if (t->init) {
@@ -4066,6 +4084,87 @@ static int tracing_clock_open(struct inode *inode, struct file *file)
         return single_open(file, tracing_clock_show, NULL);
 }
 
+#ifdef CONFIG_TRACER_SNAPSHOT
+static int tracing_snapshot_open(struct inode *inode, struct file *file)
+{
+        struct trace_iterator *iter;
+        int ret = 0;
+
+        if (file->f_mode & FMODE_READ) {
+                iter = __tracing_open(inode, file, true);
+                if (IS_ERR(iter))
+                        ret = PTR_ERR(iter);
+        }
+        return ret;
+}
+
+static ssize_t
+tracing_snapshot_write(struct file *filp, const char __user *ubuf, size_t cnt,
+                       loff_t *ppos)
+{
+        unsigned long val;
+        int ret;
+
+        ret = tracing_update_buffers();
+        if (ret < 0)
+                return ret;
+
+        ret = kstrtoul_from_user(ubuf, cnt, 10, &val);
+        if (ret)
+                return ret;
+
+        mutex_lock(&trace_types_lock);
+
+        if (current_trace && current_trace->use_max_tr) {
+                ret = -EBUSY;
+                goto out;
+        }
+
+        switch (val) {
+        case 0:
+                if (current_trace->allocated_snapshot) {
+                        /* free spare buffer */
+                        ring_buffer_resize(max_tr.buffer, 1,
+                                           RING_BUFFER_ALL_CPUS);
+                        set_buffer_entries(&max_tr, 1);
+                        tracing_reset_online_cpus(&max_tr);
+                        current_trace->allocated_snapshot = false;
+                }
+                break;
+        case 1:
+                if (!current_trace->allocated_snapshot) {
+                        /* allocate spare buffer */
+                        ret = resize_buffer_duplicate_size(&max_tr,
+                                        &global_trace, RING_BUFFER_ALL_CPUS);
+                        if (ret < 0)
+                                break;
+                        current_trace->allocated_snapshot = true;
+                }
+
+                local_irq_disable();
+                /* Now, we're going to swap */
+                update_max_tr(&global_trace, current, smp_processor_id());
+                local_irq_enable();
+                break;
+        default:
+                if (current_trace->allocated_snapshot)
+                        tracing_reset_online_cpus(&max_tr);
+                else
+                        ret = -EINVAL;
+                break;
+        }
+
+        if (ret >= 0) {
+                *ppos += cnt;
+                ret = cnt;
+        }
+out:
+        mutex_unlock(&trace_types_lock);
+        return ret;
+}
+#endif /* CONFIG_TRACER_SNAPSHOT */
+
+
 static const struct file_operations tracing_max_lat_fops = {
         .open           = tracing_open_generic,
         .read           = tracing_max_lat_read,
@@ -4122,6 +4221,16 @@ static const struct file_operations trace_clock_fops = {
         .write          = tracing_clock_write,
 };
 
+#ifdef CONFIG_TRACER_SNAPSHOT
+static const struct file_operations snapshot_fops = {
+        .open           = tracing_snapshot_open,
+        .read           = seq_read,
+        .write          = tracing_snapshot_write,
+        .llseek         = tracing_seek,
+        .release        = tracing_release,
+};
+#endif /* CONFIG_TRACER_SNAPSHOT */
+
 struct ftrace_buffer_info {
         struct trace_array      *tr;
         void                    *spare;
@@ -4921,6 +5030,11 @@ static __init int tracer_init_debugfs(void)
                         &ftrace_update_tot_cnt, &tracing_dyn_info_fops);
 #endif
 
+#ifdef CONFIG_TRACER_SNAPSHOT
+        trace_create_file("snapshot", 0644, d_tracer,
+                          (void *) TRACE_PIPE_ALL_CPU, &snapshot_fops);
+#endif
+
         create_trace_options_dir();
 
         for_each_tracing_cpu(cpu)
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 04a2c7ab1735..57d7e5397d56 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -287,6 +287,7 @@ struct tracer {
         struct tracer_flags     *flags;
         bool                    print_max;
         bool                    use_max_tr;
+        bool                    allocated_snapshot;
 };