RFC: android logger feedback request

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Hi all,

I'm looking for feedback on the Android logger code, to see what
it would take to make this code acceptable for inclusion in
the mainline kernel.

Information about the features of Android logging system
can be found at: http://elinux.org/Android_Logging_System

This system creates a new system-wide logging service, in
the kernel, for user-space message.  It is more comparable
to syslog than to the kernel log buffer, as it holds only
user-space messages.  It is optimized for write
performance, since most of the time the log is written to
and never read.  It creates multiple log channels, to prevent
an abundance of log messages in one channel from overwriting
messages in another channel.  The log channels have sizes
fixed at kernel compile-time.

Log messages are stored in very simple in-kernel buffers, that
overflow old messages upon wrapping.  A fixed set of attributes
(pid, tid, timestamp and message), is kept for each message.
By convention, Android puts a message priority and context tag
into each message.

In Android, this system uses a fixed set of device nodes with
well-known names: /dev/log/main, /dev/log/events, /dev/log/radio
and /dev/log/system.

Operations on the log are done via a character device, using
standard file operations and some ioctls.

The code for this is below (I've moved it from linux-next
drivers/staging/android for my own testing).

Please let me know what issues you see with this code.

One specific question I have is where is the most appropriate
place for this code to live, in the kernel source tree?
Other embedded systems might want to use this system (it
is simpler than syslog, and superior in some ways), so I don't
think it should remain in an android-specific directory.

Thanks,
 -- Tim

P.S. Sorry to do this right before the holidays.  I'll be
away from my work machine for most of the next few weeks.  I
can answer informational questions, and gather feedback,
but I won't have a lot of time for testing new code until
after the New Year.


Here's the code:
diff --git a/drivers/misc/logger.c b/drivers/misc/logger.c
new file mode 100644
index 0000000..fa76ce7
--- /dev/null
+++ b/drivers/misc/logger.c
@@ -0,0 +1,616 @@
+/*
+ * drivers/misc/logger.c
+ *
+ * A Logging Subsystem
+ *
+ * Copyright (C) 2007-2008 Google, Inc.
+ *
+ * Robert Love <rlove@xxxxxxxxxx>
+ *
+ * This software is licensed under the terms of the GNU General Public
+ * License version 2, as published by the Free Software Foundation, and
+ * may be copied, distributed, and modified under those terms.
+ *
+ * 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.
+ */
+
+#include <linux/sched.h>
+#include <linux/module.h>
+#include <linux/fs.h>
+#include <linux/miscdevice.h>
+#include <linux/uaccess.h>
+#include <linux/poll.h>
+#include <linux/slab.h>
+#include <linux/time.h>
+#include "logger.h"
+
+#include <asm/ioctls.h>
+
+/*
+ * struct logger_log - represents a specific log, such as 'main' or 'radio'
+ *
+ * This structure lives from module insertion until module removal, so it does
+ * not need additional reference counting. The structure is protected by the
+ * mutex 'mutex'.
+ */
+struct logger_log {
+	unsigned char 		*buffer;/* the ring buffer itself */
+	struct miscdevice	misc;	/* misc device representing the log */
+	wait_queue_head_t	wq;	/* wait queue for readers */
+	struct list_head	readers; /* this log's readers */
+	struct mutex		mutex;	/* mutex protecting buffer */
+	size_t			w_off;	/* current write head offset */
+	size_t			head;	/* new readers start here */
+	size_t			size;	/* size of the log */
+};
+
+/*
+ * struct logger_reader - a logging device open for reading
+ *
+ * This object lives from open to release, so we don't need additional
+ * reference counting. The structure is protected by log->mutex.
+ */
+struct logger_reader {
+	struct logger_log	*log;	/* associated log */
+	struct list_head	list;	/* entry in logger_log's list */
+	size_t			r_off;	/* current read head offset */
+};
+
+/* logger_offset - returns index 'n' into the log via (optimized) modulus */
+#define logger_offset(n)	((n) & (log->size - 1))
+
+/*
+ * file_get_log - Given a file structure, return the associated log
+ *
+ * This isn't aesthetic. We have several goals:
+ *
+ * 	1) Need to quickly obtain the associated log during an I/O operation
+ * 	2) Readers need to maintain state (logger_reader)
+ * 	3) Writers need to be very fast (open() should be a near no-op)
+ *
+ * In the reader case, we can trivially go file->logger_reader->logger_log.
+ * For a writer, we don't want to maintain a logger_reader, so we just go
+ * file->logger_log. Thus what file->private_data points at depends on whether
+ * or not the file was opened for reading. This function hides that dirtiness.
+ */
+static inline struct logger_log *file_get_log(struct file *file)
+{
+	if (file->f_mode & FMODE_READ) {
+		struct logger_reader *reader = file->private_data;
+		return reader->log;
+	} else
+		return file->private_data;
+}
+
+/*
+ * get_entry_len - Grabs the length of the payload of the next entry starting
+ * from 'off'.
+ *
+ * Caller needs to hold log->mutex.
+ */
+static __u32 get_entry_len(struct logger_log *log, size_t off)
+{
+	__u16 val;
+
+	switch (log->size - off) {
+	case 1:
+		memcpy(&val, log->buffer + off, 1);
+		memcpy(((char *) &val) + 1, log->buffer, 1);
+		break;
+	default:
+		memcpy(&val, log->buffer + off, 2);
+	}
+
+	return sizeof(struct logger_entry) + val;
+}
+
+/*
+ * do_read_log_to_user - reads exactly 'count' bytes from 'log' into the
+ * user-space buffer 'buf'. Returns 'count' on success.
+ *
+ * Caller must hold log->mutex.
+ */
+static ssize_t do_read_log_to_user(struct logger_log *log,
+				   struct logger_reader *reader,
+				   char __user *buf,
+				   size_t count)
+{
+	size_t len;
+
+	/*
+	 * We read from the log in two disjoint operations. First, we read from
+	 * the current read head offset up to 'count' bytes or to the end of
+	 * the log, whichever comes first.
+	 */
+	len = min(count, log->size - reader->r_off);
+	if (copy_to_user(buf, log->buffer + reader->r_off, len))
+		return -EFAULT;
+
+	/*
+	 * Second, we read any remaining bytes, starting back at the head of
+	 * the log.
+	 */
+	if (count != len)
+		if (copy_to_user(buf + len, log->buffer, count - len))
+			return -EFAULT;
+
+	reader->r_off = logger_offset(reader->r_off + count);
+
+	return count;
+}
+
+/*
+ * logger_read - our log's read() method
+ *
+ * Behavior:
+ *
+ * 	- O_NONBLOCK works
+ * 	- If there are no log entries to read, blocks until log is written to
+ * 	- Atomically reads exactly one log entry
+ *
+ * Optimal read size is LOGGER_ENTRY_MAX_LEN. Will set errno to EINVAL if read
+ * buffer is insufficient to hold next entry.
+ */
+static ssize_t logger_read(struct file *file, char __user *buf,
+			   size_t count, loff_t *pos)
+{
+	struct logger_reader *reader = file->private_data;
+	struct logger_log *log = reader->log;
+	ssize_t ret;
+	DEFINE_WAIT(wait);
+
+start:
+	while (1) {
+		prepare_to_wait(&log->wq, &wait, TASK_INTERRUPTIBLE);
+
+		mutex_lock(&log->mutex);
+		ret = (log->w_off == reader->r_off);
+		mutex_unlock(&log->mutex);
+		if (!ret)
+			break;
+
+		if (file->f_flags & O_NONBLOCK) {
+			ret = -EAGAIN;
+			break;
+		}
+
+		if (signal_pending(current)) {
+			ret = -EINTR;
+			break;
+		}
+
+		schedule();
+	}
+
+	finish_wait(&log->wq, &wait);
+	if (ret)
+		return ret;
+
+	mutex_lock(&log->mutex);
+
+	/* is there still something to read or did we race? */
+	if (unlikely(log->w_off == reader->r_off)) {
+		mutex_unlock(&log->mutex);
+		goto start;
+	}
+
+	/* get the size of the next entry */
+	ret = get_entry_len(log, reader->r_off);
+	if (count < ret) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	/* get exactly one entry from the log */
+	ret = do_read_log_to_user(log, reader, buf, ret);
+
+out:
+	mutex_unlock(&log->mutex);
+
+	return ret;
+}
+
+/*
+ * get_next_entry - return the offset of the first valid entry at least 'len'
+ * bytes after 'off'.
+ *
+ * Caller must hold log->mutex.
+ */
+static size_t get_next_entry(struct logger_log *log, size_t off, size_t len)
+{
+	size_t count = 0;
+
+	do {
+		size_t nr = get_entry_len(log, off);
+		off = logger_offset(off + nr);
+		count += nr;
+	} while (count < len);
+
+	return off;
+}
+
+/*
+ * clock_interval - is a < c < b in mod-space? Put another way, does the line
+ * from a to b cross c?
+ */
+static inline int clock_interval(size_t a, size_t b, size_t c)
+{
+	if (b < a) {
+		if (a < c || b >= c)
+			return 1;
+	} else {
+		if (a < c && b >= c)
+			return 1;
+	}
+
+	return 0;
+}
+
+/*
+ * fix_up_readers - walk the list of all readers and "fix up" any who were
+ * lapped by the writer; also do the same for the default "start head".
+ * We do this by "pulling forward" the readers and start head to the first
+ * entry after the new write head.
+ *
+ * The caller needs to hold log->mutex.
+ */
+static void fix_up_readers(struct logger_log *log, size_t len)
+{
+	size_t old = log->w_off;
+	size_t new = logger_offset(old + len);
+	struct logger_reader *reader;
+
+	if (clock_interval(old, new, log->head))
+		log->head = get_next_entry(log, log->head, len);
+
+	list_for_each_entry(reader, &log->readers, list)
+		if (clock_interval(old, new, reader->r_off))
+			reader->r_off = get_next_entry(log, reader->r_off, len);
+}
+
+/*
+ * do_write_log - writes 'len' bytes from 'buf' to 'log'
+ *
+ * The caller needs to hold log->mutex.
+ */
+static void do_write_log(struct logger_log *log, const void *buf, size_t count)
+{
+	size_t len;
+
+	len = min(count, log->size - log->w_off);
+	memcpy(log->buffer + log->w_off, buf, len);
+
+	if (count != len)
+		memcpy(log->buffer, buf + len, count - len);
+
+	log->w_off = logger_offset(log->w_off + count);
+
+}
+
+/*
+ * do_write_log_user - writes 'len' bytes from the user-space buffer 'buf' to
+ * the log 'log'
+ *
+ * The caller needs to hold log->mutex.
+ *
+ * Returns 'count' on success, negative error code on failure.
+ */
+static ssize_t do_write_log_from_user(struct logger_log *log,
+				      const void __user *buf, size_t count)
+{
+	size_t len;
+
+	len = min(count, log->size - log->w_off);
+	if (len && copy_from_user(log->buffer + log->w_off, buf, len))
+		return -EFAULT;
+
+	if (count != len)
+		if (copy_from_user(log->buffer, buf + len, count - len))
+			return -EFAULT;
+
+	log->w_off = logger_offset(log->w_off + count);
+
+	return count;
+}
+
+/*
+ * logger_aio_write - our write method, implementing support for write(),
+ * writev(), and aio_write(). Writes are our fast path, and we try to optimize
+ * them above all else.
+ */
+ssize_t logger_aio_write(struct kiocb *iocb, const struct iovec *iov,
+			 unsigned long nr_segs, loff_t ppos)
+{
+	struct logger_log *log = file_get_log(iocb->ki_filp);
+	size_t orig = log->w_off;
+	struct logger_entry header;
+	struct timespec now;
+	ssize_t ret = 0;
+
+	now = current_kernel_time();
+
+	header.pid = current->tgid;
+	header.tid = current->pid;
+	header.sec = now.tv_sec;
+	header.nsec = now.tv_nsec;
+	header.len = min_t(size_t, iocb->ki_left, LOGGER_ENTRY_MAX_PAYLOAD);
+
+	/* null writes succeed, return zero */
+	if (unlikely(!header.len))
+		return 0;
+
+	mutex_lock(&log->mutex);
+
+	/*
+	 * Fix up any readers, pulling them forward to the first readable
+	 * entry after (what will be) the new write offset. We do this now
+	 * because if we partially fail, we can end up with clobbered log
+	 * entries that encroach on readable buffer.
+	 */
+	fix_up_readers(log, sizeof(struct logger_entry) + header.len);
+
+	do_write_log(log, &header, sizeof(struct logger_entry));
+
+	while (nr_segs-- > 0) {
+		size_t len;
+		ssize_t nr;
+
+		/* figure out how much of this vector we can keep */
+		len = min_t(size_t, iov->iov_len, header.len - ret);
+
+		/* write out this segment's payload */
+		nr = do_write_log_from_user(log, iov->iov_base, len);
+		if (unlikely(nr < 0)) {
+			log->w_off = orig;
+			mutex_unlock(&log->mutex);
+			return nr;
+		}
+
+		iov++;
+		ret += nr;
+	}
+
+	mutex_unlock(&log->mutex);
+
+	/* wake up any blocked readers */
+	wake_up_interruptible(&log->wq);
+
+	return ret;
+}
+
+static struct logger_log *get_log_from_minor(int);
+
+/*
+ * logger_open - the log's open() file operation
+ *
+ * Note how near a no-op this is in the write-only case. Keep it that way!
+ */
+static int logger_open(struct inode *inode, struct file *file)
+{
+	struct logger_log *log;
+	int ret;
+
+	ret = nonseekable_open(inode, file);
+	if (ret)
+		return ret;
+
+	log = get_log_from_minor(MINOR(inode->i_rdev));
+	if (!log)
+		return -ENODEV;
+
+	if (file->f_mode & FMODE_READ) {
+		struct logger_reader *reader;
+
+		reader = kmalloc(sizeof(struct logger_reader), GFP_KERNEL);
+		if (!reader)
+			return -ENOMEM;
+
+		reader->log = log;
+		INIT_LIST_HEAD(&reader->list);
+
+		mutex_lock(&log->mutex);
+		reader->r_off = log->head;
+		list_add_tail(&reader->list, &log->readers);
+		mutex_unlock(&log->mutex);
+
+		file->private_data = reader;
+	} else
+		file->private_data = log;
+
+	return 0;
+}
+
+/*
+ * logger_release - the log's release file operation
+ *
+ * Note this is a total no-op in the write-only case. Keep it that way!
+ */
+static int logger_release(struct inode *ignored, struct file *file)
+{
+	if (file->f_mode & FMODE_READ) {
+		struct logger_reader *reader = file->private_data;
+		list_del(&reader->list);
+		kfree(reader);
+	}
+
+	return 0;
+}
+
+/*
+ * logger_poll - the log's poll file operation, for poll/select/epoll
+ *
+ * Note we always return POLLOUT, because you can always write() to the log.
+ * Note also that, strictly speaking, a return value of POLLIN does not
+ * guarantee that the log is readable without blocking, as there is a small
+ * chance that the writer can lap the reader in the interim between poll()
+ * returning and the read() request.
+ */
+static unsigned int logger_poll(struct file *file, poll_table *wait)
+{
+	struct logger_reader *reader;
+	struct logger_log *log;
+	unsigned int ret = POLLOUT | POLLWRNORM;
+
+	if (!(file->f_mode & FMODE_READ))
+		return ret;
+
+	reader = file->private_data;
+	log = reader->log;
+
+	poll_wait(file, &log->wq, wait);
+
+	mutex_lock(&log->mutex);
+	if (log->w_off != reader->r_off)
+		ret |= POLLIN | POLLRDNORM;
+	mutex_unlock(&log->mutex);
+
+	return ret;
+}
+
+static long logger_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+{
+	struct logger_log *log = file_get_log(file);
+	struct logger_reader *reader;
+	long ret = -ENOTTY;
+
+	mutex_lock(&log->mutex);
+
+	switch (cmd) {
+	case LOGGER_GET_LOG_BUF_SIZE:
+		ret = log->size;
+		break;
+	case LOGGER_GET_LOG_LEN:
+		if (!(file->f_mode & FMODE_READ)) {
+			ret = -EBADF;
+			break;
+		}
+		reader = file->private_data;
+		if (log->w_off >= reader->r_off)
+			ret = log->w_off - reader->r_off;
+		else
+			ret = (log->size - reader->r_off) + log->w_off;
+		break;
+	case LOGGER_GET_NEXT_ENTRY_LEN:
+		if (!(file->f_mode & FMODE_READ)) {
+			ret = -EBADF;
+			break;
+		}
+		reader = file->private_data;
+		if (log->w_off != reader->r_off)
+			ret = get_entry_len(log, reader->r_off);
+		else
+			ret = 0;
+		break;
+	case LOGGER_FLUSH_LOG:
+		if (!(file->f_mode & FMODE_WRITE)) {
+			ret = -EBADF;
+			break;
+		}
+		list_for_each_entry(reader, &log->readers, list)
+			reader->r_off = log->w_off;
+		log->head = log->w_off;
+		ret = 0;
+		break;
+	}
+
+	mutex_unlock(&log->mutex);
+
+	return ret;
+}
+
+static const struct file_operations logger_fops = {
+	.owner = THIS_MODULE,
+	.read = logger_read,
+	.aio_write = logger_aio_write,
+	.poll = logger_poll,
+	.unlocked_ioctl = logger_ioctl,
+	.compat_ioctl = logger_ioctl,
+	.open = logger_open,
+	.release = logger_release,
+};
+
+/*
+ * Defines a log structure with name 'NAME' and a size of 'SIZE' bytes, which
+ * must be a power of two, greater than LOGGER_ENTRY_MAX_LEN, and less than
+ * LONG_MAX minus LOGGER_ENTRY_MAX_LEN.
+ */
+#define DEFINE_LOGGER_DEVICE(VAR, NAME, SIZE) \
+static unsigned char _buf_ ## VAR[SIZE]; \
+static struct logger_log VAR = { \
+	.buffer = _buf_ ## VAR, \
+	.misc = { \
+		.minor = MISC_DYNAMIC_MINOR, \
+		.name = NAME, \
+		.fops = &logger_fops, \
+		.parent = NULL, \
+	}, \
+	.wq = __WAIT_QUEUE_HEAD_INITIALIZER(VAR .wq), \
+	.readers = LIST_HEAD_INIT(VAR .readers), \
+	.mutex = __MUTEX_INITIALIZER(VAR .mutex), \
+	.w_off = 0, \
+	.head = 0, \
+	.size = SIZE, \
+};
+
+DEFINE_LOGGER_DEVICE(log_main, LOGGER_LOG_MAIN, 256*1024)
+DEFINE_LOGGER_DEVICE(log_events, LOGGER_LOG_EVENTS, 256*1024)
+DEFINE_LOGGER_DEVICE(log_radio, LOGGER_LOG_RADIO, 256*1024)
+DEFINE_LOGGER_DEVICE(log_system, LOGGER_LOG_SYSTEM, 256*1024)
+
+static struct logger_log *get_log_from_minor(int minor)
+{
+	if (log_main.misc.minor == minor)
+		return &log_main;
+	if (log_events.misc.minor == minor)
+		return &log_events;
+	if (log_radio.misc.minor == minor)
+		return &log_radio;
+	if (log_system.misc.minor == minor)
+		return &log_system;
+	return NULL;
+}
+
+static int __init init_log(struct logger_log *log)
+{
+	int ret;
+
+	ret = misc_register(&log->misc);
+	if (unlikely(ret)) {
+		printk(KERN_ERR "logger: failed to register misc "
+		       "device for log '%s'!\n", log->misc.name);
+		return ret;
+	}
+
+	printk(KERN_INFO "logger: created %luK log '%s'\n",
+	       (unsigned long) log->size >> 10, log->misc.name);
+
+	return 0;
+}
+
+static int __init logger_init(void)
+{
+	int ret;
+
+	ret = init_log(&log_main);
+	if (unlikely(ret))
+		goto out;
+
+	ret = init_log(&log_events);
+	if (unlikely(ret))
+		goto out;
+
+	ret = init_log(&log_radio);
+	if (unlikely(ret))
+		goto out;
+
+	ret = init_log(&log_system);
+	if (unlikely(ret))
+		goto out;
+
+out:
+	return ret;
+}
+device_initcall(logger_init);
diff --git a/drivers/misc/logger.h b/drivers/misc/logger.h
new file mode 100644
index 0000000..2cb06e9
--- /dev/null
+++ b/drivers/misc/logger.h
@@ -0,0 +1,49 @@
+/* include/linux/logger.h
+ *
+ * Copyright (C) 2007-2008 Google, Inc.
+ * Author: Robert Love <rlove@xxxxxxxxxxx>
+ *
+ * This software is licensed under the terms of the GNU General Public
+ * License version 2, as published by the Free Software Foundation, and
+ * may be copied, distributed, and modified under those terms.
+ *
+ * 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.
+ *
+ */
+
+#ifndef _LINUX_LOGGER_H
+#define _LINUX_LOGGER_H
+
+#include <linux/types.h>
+#include <linux/ioctl.h>
+
+struct logger_entry {
+	__u16		len;	/* length of the payload */
+	__u16		__pad;	/* no matter what, we get 2 bytes of padding */
+	__s32		pid;	/* generating process's pid */
+	__s32		tid;	/* generating process's tid */
+	__s32		sec;	/* seconds since Epoch */
+	__s32		nsec;	/* nanoseconds */
+	char		msg[0];	/* the entry's payload */
+};
+
+#define LOGGER_LOG_RADIO	"log_radio"	/* radio-related messages */
+#define LOGGER_LOG_EVENTS	"log_events"	/* system/hardware events */
+#define LOGGER_LOG_SYSTEM	"log_system"	/* system/framework messages */
+#define LOGGER_LOG_MAIN		"log_main"	/* everything else */
+
+#define LOGGER_ENTRY_MAX_LEN		(4*1024)
+#define LOGGER_ENTRY_MAX_PAYLOAD	\
+	(LOGGER_ENTRY_MAX_LEN - sizeof(struct logger_entry))
+
+#define __LOGGERIO	0xAE
+
+#define LOGGER_GET_LOG_BUF_SIZE		_IO(__LOGGERIO, 1) /* size of log */
+#define LOGGER_GET_LOG_LEN		_IO(__LOGGERIO, 2) /* used log len */
+#define LOGGER_GET_NEXT_ENTRY_LEN	_IO(__LOGGERIO, 3) /* next entry len */
+#define LOGGER_FLUSH_LOG		_IO(__LOGGERIO, 4) /* flush log */
+
+#endif /* _LINUX_LOGGER_H */

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