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

[Hirokazu Honda]

Hi Laurent and Umang,


On Fri, Nov 12, 2021 at 7:53 AM Laurent Pinchart
<laurent.pinchart at ideasonboard.com> wrote:
>
> Hello,
>
> 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


> > > 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.

> > > +{
> > > +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.

-Hiro

> > > +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