Date:	Tue, 9 Dec 2008 18:08:38 +0100
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Clark Williams" <clark.williams@...il.com>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Arnaldo Carvalho de Melo" <acme@...stprotocols.net>,
	"Gregory Haskins" <ghaskins@...ell.com>,
	"Darren Hart" <dvhltc@...ibm.com>
Subject: Re: [ANNOUNCE] The -rt git tree

2008/12/9 Steven Rostedt <rostedt@...dmis.org>:
>
> The -rt branch has been static for some time. We are sorry about that,
> but most of those that maintain it have been working on other parts of the
> kernel. Mostly new mainline work.
>
> People have also been asking about having an -rt git tree. Well, it is
> time to create one.
>
> We have started working on an -rt git tree. Note, it is currently broken
> and not ready for use. An announcement will be made when it is ready. This
> email is only to let people know that we are currently actively working on
> the new -rt tree.
>
> The git tree is located here:
>
>  git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-2.6-rt.git
>
> The branches include:
>
>        linus - the mainline branch that the rest is based on
>        rt-master - all the rt/* branches merged into one
>        master - rt-master + converting of the locks (see below)
>        rt/rt - some rt git scripts
>        rt/convert-scripts - the scripts to convert spinlocks (see below)
>        rt/mainline - clean ups for mainline that we need to push forward
>        rt/not-for-mainline - rt only patches that we never want to push
>                             to mainline
>        rt/fs - filesystem changes for rt
>        rt/ftrace - ftrace updates for rt
>        rt/java - IBM patches for java
>        rt/kmap
>        rt/lockdep
>        rt/pagecache
>        rt/percpu - this may turn into rt/mm
>        rt/rt-locks - the new lock API (see below)
>        rt/sched
>        rt/tasklet
>        rt/threadirqs - full soft and hard irq threading
>        rt/timer
>        rt/workqueue - priority queues
>
> We may add more rt/* branches. Most of those are self explanatory.
>
> The new lock API.  The old -rt patch used macro magic to convert
> spin_locks into mutexes depending on which type they were. A spinlock that
> was declared as spinlock_t would convert into a mutex when PREEMPT_RT was
> configured, where as a spinlock that was declared as raw_spinlock_t would
> stay as a spinlock.
>
> For example:
>
>        spinlock_t      what_am_i;
>        raw_spinlock_t  spinner;
>
>        spin_lock(&what_am_i);
>        spin_lock(&spinner);
>
> The 'what_am_i' would turn into a mutex when PREEMPT_RT was configured and
> would be a spinlock when it was not. 'spinner' would always stay as a
> spinlock. The way this was done was with a builtin gcc option to compare
> the type used to determine what function to call:
>
> #define spin_lock(lock) \
>        if (__builtin_types_compatible(typeof(spinlock_t), lock)) \
>                rt_lock(lock); \
>        else if (__builtin_types_compatible(typeof(raw_spinlock_t), lock)) \
>                raw_spin_lock(lock); \
>        else \
>                __bad_func_type();
>
> If you are looking at this and scratching your head saying "this is not
> acceptable for mainline", we agree with you ;-)  This is the old way of
> doing things, which made porting -rt to mainline much easier than
> converting all locks to a new API. But the time has come to do just that.
>
> The new locks
> -------------
>
> The old way is not acceptable because a developer can not know when seeing
> a 'spin_lock(x)' and knowing if it will stay a spinlock or not on
> PREEMPT_RT. The developer would need to go search for x and see what type
> it is. Then they still can not know if someone might change the type of
> x. This is just ripe for bugs.
>
> The new RT has added a new lock API that will be a mutex when
> PREEMPT_RT is configured and a spinlock when it is not. The type is called
> 'lock_t' and introduces a new API acquire_lock() and release_lock().  This
> also has all the irq variants that spin_locks have (acquire_lock_irqsave,
> acquire_lock_irq, etc).
>
>        lock_t          lock;
>
>        acquire_lock_irqsave(&lock, flags);
>
> Although it has 'irqsave' when PREEMPT_RT is defined, it will not disable
> interrupts. This is fine, because with PREEMPT_RT, interrupts are threads,
> and we do not need to worry about being preempted by an interrupt and have
> that interrupt taking the same lock cause a deadlock.  But now, the
> developer knows that this is a special lock, and that the lock is not a
> spinlock all the time.
>
> Note, you can not do:
>
>        spin_lock(&spinner);
>        acquire_lock(&lock);
>
> This would be the same as doing:
>
>        spin_lock(&spinner);
>        mutex_lock(&mutex);
>
>
> Converting the new locks
> ------------------------
>
> In the git tree, there is a couple of scripts in rt/convert-scripts branch
> that will convert all spinlocks into the lock_t. Then there are scripts to
> convert the necessary spinlocks back to spinlock.  All rt/* branches
> should not need to define the type lock_t except for rt/rt-locks.
>
> This git repo uses similar scripts that tip uses. The rt-master branch is
> a merge of all rt/* branches (except rt/rt). This may define PREEMPT_RT
> but no lock is made into 'lock_t' type yet.
>
> The master branch is a copy of the rt-branch with the scripts to do the
> lock conversion run on the tree.  In other words, the master branch will
> be the new -rt patch.
>
> Note, the current state of this git tree is still very much broken. We are
> rewriting a lot of code to pull this forward to 2.6.28. Unfortunately,
> this means that we are skipping 2.6.27 altogether. I will be making
> another announcement when this is ready for prime time.  Some of the
> branches have not been updated with the changes either.
>
> -- Steve
>


Hi Steve.

That's a good news.

But after converting these spinlocks into lock_t, how will you synchronize with
the mainline on each release? You will have a huge amount of conflicts
at every merges...
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