How to add thread-safety in a logging library?

Hi,

I implemented a small logging library with the API like this:

[snip]
logger& log = log_manager::instance().get_logger("my_logger");
log.stream(DEBUG) << "this is a debug message" << std::endl;
log.stream(INFO) << "this is an info message" << std::endl;
[/snip]

Every logger has a unique name, and manages an output stream
(std::ostream). The 'stream(log_level_t)' member function returns an
ostream_proxy object, which is implemented as the following:

[snip]
class ostream_proxy {

public:

  // other member functions...

  template<typename T>
  ostream_proxy& operator<<(T const& t) {
    (*os_) << t;
    return *this;
  }

private:
  std::ostream* os_; // ostream_proxy objects from the same logger
object share
                     // the same output stream.
};
[/snip]

Thus it is possible that two threads retrieve the same logger object
and write messages to the logging stream at the same time.

Now i want to add thread-safety to my logging library, and i realize
that in ostream_proxy, the statement "(*os_) << t;" is not thread-safe
(std::ostream is not thread-safe, right?). So i need to re-implement
the operator << in ostream_proxy like this:

[snip]
class ostream_proxy {

public:

  // other member functions...

  template<typename T>
  ostream_proxy& operator<<(T const& t) {
    {
      boost::mutex::scoped_lock lock(*os_mutex_);
      (*os_) << t;
    }
    return *this;
  }

private:
  std::ostream* os_; // ostream_proxy objects from the same
logger object
  boost::mutex* os_mutex_; // share the same output stream and the
same mutex.
};
[/snip]

In this way, i can guarantee that at any moment, there is at most one
thread that calls "(*os_) << t;".

But since logging may be an action that is frequently performed, the
code above may be too expensive to bear... Surely i can use a policy
to allow user to choose if s/he want thread-safety or not. But in a
multi-threaded application, user still has to pay for logging...

So i would like to know if such implementation is proper, or if there
is a way to make that better.

I would appreciate your advice. Thanks!