[libcamera-devel] [RFC PATCH 2/6] libcamera: base: Add mutex classes with thread safety annotations

[Laurent Pinchart]

Hi Hiro,

On Fri, Nov 12, 2021 at 04:20:01PM +0900, Hirokazu Honda wrote:
> On Fri, Nov 12, 2021 at 7:53 AM Laurent Pinchart wrote:
> > On Thu, Nov 11, 2021 at 10:08:35PM +0530, Umang Jain wrote:
> > > On 10/29/21 9:44 AM, Hirokazu Honda wrote:
> > > > Mutex2 and MutexLocker2 are annotated by clang thread safety
> > >
> > > Not sure I like the naming here. but I don't have any better suggestion
> > > as of now, still contemplating on that.. maybe AMutex and AMutexLocker
> > > to denote they are annotated?
> >
> > I'm also concerned by this. Can't we replace the current Mutex and
> > MutexLocker with annotated versions, without having two implementations
> > ?
> 
> The problem is condition_variable. It takes MutexLocker
> (std::unique_lock) on wait.
> However, the new MutexLocker is not std::unique_lock. I avoid this
> problem by MutexLocker2::get().
> I found [1] MutexLocker derives std::unique_lock, but not sure if it is ok.
> [1] https://stackoverflow.com/questions/40468897/clang-thread-safety-with-stdcondition-variable

I understand that, but can't we then drop the

using Mutex = std::mutex;
using MutexLocker = std::unique_lock<std::mutex>;

and rename Mutex2 and MutexLocker2 to Mutex and MutexLocker ? Having two
options is really awful.

> > > > annotations. So we can add annotation to code where the classes are used.
> > > > In the future, they will replace Mutex and MutexLocker.
> > >
> > > Ok so I see a patch following, that does this replacement. I went into
> > > the territory of thinking if this annotations can be used full libcamera
> > > codebase. Can it be done (not as part of the series) but as a separate
> > > series. I am not sure how much useful it would be since annotation is
> > > clang-only. Let's see.
> > >
> > > So, the series really:
> > >
> > >          In the future, they will replace Mutex and MutexLocker in the
> > > android HAL layer.
> > >
> > > > Signed-off-by: Hirokazu Honda <hiroh at chromium.org>
> > > > ---
> > > >   include/libcamera/base/meson.build |  1 +
> > > >   include/libcamera/base/mutex.h     | 66 ++++++++++++++++++++++++++++++
> > >
> > > Please refer to my comment on 1/6 about putting this in
> > > include/libcamera/internal, maybe that's a better place?
> >
> > I think base makes sense, as we already define Mutex and MutexLocker
> > there.
> >
> > > >   include/libcamera/base/thread.h    |  5 +--
> > > >   3 files changed, 68 insertions(+), 4 deletions(-)
> > > >   create mode 100644 include/libcamera/base/mutex.h
> > > >
> > > > diff --git a/include/libcamera/base/meson.build b/include/libcamera/base/meson.build
> > > > index 1a71ce5a..37c4435a 100644
> > > > --- a/include/libcamera/base/meson.build
> > > > +++ b/include/libcamera/base/meson.build
> > > > @@ -13,6 +13,7 @@ libcamera_base_headers = files([
> > > >       'flags.h',
> > > >       'log.h',
> > > >       'message.h',
> > > > +    'mutex.h',
> > > >       'object.h',
> > > >       'private.h',
> > > >       'semaphore.h',
> > > > diff --git a/include/libcamera/base/mutex.h b/include/libcamera/base/mutex.h
> > > > new file mode 100644
> > > > index 00000000..d130988e
> > > > --- /dev/null
> > > > +++ b/include/libcamera/base/mutex.h
> > > > @@ -0,0 +1,66 @@
> > > > +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> > > > +/*
> > > > + * Copyright (C) 2021, Google Inc.
> > > > + *
> > > > + * thread.h - Thread support
> > >
> > >
> > > oops it should be mutex.h
> > >
> > >
> > > Patch itself looks good to me.
> > >
> > > > + */
> > > > +#ifndef __LIBCAMERA_BASE_MUTEX_H__
> > > > +#define __LIBCAMERA_BASE_MUTEX_H__
> > > > +
> > > > +#include <mutex>
> > > > +
> > > > +#include <libcamera/base/thread_annotations.h>
> > > > +
> > > > +namespace libcamera {
> > > > +
> > > > +using Mutex = std::mutex;
> > > > +using MutexLocker = std::unique_lock<std::mutex>;
> > > > +
> > > > +class CAPABILITY("mutex") Mutex2 final
> >
> > Documentation is missing.
> >
> > Thinking further about this class, given that libc++ annotates
> > std::mutex, do we a custom annotated class ? It would only be used when
> > compiling with clang and libstd++, which seems a bit of a cornercase.
> 
> Yeah, once libc++ annotats std::unique_lock, I would let Mutex be
> std::unique_lock if libc++ and otherwise this custom class.

I know we need to annotate std::unique_lock, so we need to wrap it in
MutexLocker, but with libc++ do we need a custom-annotated Mutex class,
or could we simply use

using Mutex = std::mutex;

then ?

> > > > +{
> > > > +public:
> >
> > The std::mutex constructor is constexpr, I think we should do the same
> > here to replicate the same API.
> >
> > > > +   void lock() ACQUIRE()
> > > > +   {
> > > > +           mutex_.lock();
> > > > +   }
> > > > +
> > > > +   void unlock() RELEASE()
> > > > +   {
> > > > +           mutex_.unlock();
> > > > +   }
> >
> > We also need try_lock().
> >
> > > > +
> > > > +   std::mutex &get() { return mutex_; }
> >
> > Missing blank line.
> >
> > This function is only used by MutexLocker2. I think a friend declaration
> > would be better here, to avoid exposing get().
> >
> > > > +private:
> > > > +   std::mutex mutex_;
> > > > +};
> > > > +
> > > > +class SCOPED_CAPABILITY MutexLocker2 final
> > > > +{
> > > > +public:
> > > > +   MutexLocker2(Mutex2 &mutex) ACQUIRE(mutex)
> > > > +           : lock_(mutex.get())
> > > > +   {
> > > > +   }
> > > > +
> > > > +   ~MutexLocker2() RELEASE()
> > > > +   {
> > > > +   }
> > > > +
> > > > +   void lock() ACQUIRE()
> > > > +   {
> > > > +           lock_.lock();
> > > > +   }
> > > > +
> > > > +   void unlock() RELEASE()
> > > > +   {
> > > > +           lock_.unlock();
> > > > +   }
> >
> > There are more functions in the std::unique_lock API (including more
> > constructors), should we implement them too ?
> >
> > > > +
> > > > +   std::unique_lock<std::mutex> &get() { return lock_; }
> >
> > Missing blank line.
> >
> > This function is only used to support std::condition_variable::wait(),
> > wouldn't it be better to implement a ConditionVariable class that would
> > take a MutexLocker argument in wait() ?
> 
> I wondered about it. If we don't like MutexLocker derives
> ConditionVariable, then I would try it.

It's the get() that bothers me a bit.

> > > > +private:
> > > > +   std::unique_lock<std::mutex> lock_;
> > > > +};
> > > > +
> > > > +} /* namespace libcamera */
> > > > +
> > > > +#endif /* __LIBCAMERA_BASE_MUTEX_H__ */
> > > > diff --git a/include/libcamera/base/thread.h b/include/libcamera/base/thread.h
> > > > index e0ca0aea..ae630563 100644
> > > > --- a/include/libcamera/base/thread.h
> > > > +++ b/include/libcamera/base/thread.h
> > > > @@ -8,13 +8,13 @@
> > > >   #define __LIBCAMERA_BASE_THREAD_H__
> > > >
> > > >   #include <memory>
> > > > -#include <mutex>
> > > >   #include <sys/types.h>
> > > >   #include <thread>
> > > >
> > > >   #include <libcamera/base/private.h>
> > > >
> > > >   #include <libcamera/base/message.h>
> > > > +#include <libcamera/base/mutex.h>
> > > >   #include <libcamera/base/signal.h>
> > > >   #include <libcamera/base/utils.h>
> > > >
> > > > @@ -26,9 +26,6 @@ class Object;
> > > >   class ThreadData;
> > > >   class ThreadMain;
> > > >
> > > > -using Mutex = std::mutex;
> > > > -using MutexLocker = std::unique_lock<std::mutex>;
> > > > -
> > > >   class Thread
> > > >   {
> > > >   public:

-- 
Regards,

Laurent Pinchart