Re: [ANNOUNCE] 2.6.31-rc4-rt1

[Will Schmidt]

On Thu, 2009-07-30 at 02:48 +0200, Thomas Gleixner wrote:
> We are pleased to announce the next update to our new preempt-rt
> series.
> 
>     - update to 2.6.31-rc4


I'm unable to do a clean build of this on powerpc64.  Related to changes
in the locking code, I'm sure.   I've spent some time poking at this,
but it's not something I think I will be able to figure out short-term.
Suggestions are welcomed. :-)

I can share my .config if requested, but I suspect the problem will be
obvious to someone more familiar with the spinlock changes.  

The build error:
/test/willschm/linux-2.6.31-rtx/kernel/spinlock.c: In function
â_atomic_spin_lockâ:
/test/willschm/linux-2.6.31-rtx/kernel/spinlock.c:103: error: implicit
declaration of function â_raw_atomic_spin_trylockâ
/test/willschm/linux-2.6.31-rtx/kernel/spinlock.c:103: error: implicit
declaration of function â_raw_atomic_spin_relaxâ

I can't find the definitions for â_raw_atomic_spin_trylockâ or
â_raw_atomic_spin_relaxâ with any of my grepping either, for any of the
architectures.   I suspect this ties back into the
CONFIG_GENERIC_LOCKBREAK being defined, or not being defined for the
architecture.  (It's on for Power, not on for x86).

With lots of code trimmed, the relevant code appears to be (in
kernel/spinlock.c) :
#if !defined(CONFIG_GENERIC_LOCKBREAK) || defined(CONFIG_DEBUG_LOCK_ALLOC)
	...
#else
	BUILD_LOCK_OPS(atomic_spin, atomic_spinlock);     /* line 103*/
#fi

As an experiment, I did force the CONFIG_GENERIC_LOCKBREAK option off
and built successfully, but my system turns into a doorstop during very
early boot, down the find_legacy_serial_ports() path, eventually ending
up stuck in _atomic_spin_lock()->debug_smp_processor_id().   This is
before the early console or xmon itself has gone through setup, so not a
lot of debug options.. :-o :-)

Thanks, 
-Will



> - rework of the locking infrastructure
>    
>    Up to now the -rt patches changed the raw_spinlock_t to
>    __raw_spinlock_t and added another two levels of underscores to
>    many of the locking primitives. A compiler trick was used to chose
>    the implementation for RT=y and RT=n compiles depending on the lock
>    type in the lock definition.
> 
>    This is nasty as there is no destinction in the source code which
>    kind of lock we are dealing with except if one looks up the lock
>    definition/declaration. It definitely was a clever move in the
>    first place to get things going, but aside of the underscore
>    conflicts which were introduced by lockdep it was not longer
>    acceptable to hide the fact that we are treating a lock
>    differently. Same applies for the changes to (rw_)semaphores which
>    used the compat_ trick for those ownerless anonymous semaphores
>    which are taken in one context and released in another.
> 
>    The annotation of the code which uses those special treated locks
>    has been long discussed and one of the proposed solutions was to
>    change all spinlocks which are converted by -rt to sleeping
>    spinlocks from spinlock_t to lock_t and have another set of
>    lock/unlock/trylock functions for those. That is definitely the
>    _preferred_ solution, but it's a massive and horribly intrusive
>    change. Steven was working on it for some time, but it simply does
>    not scale IMNSHO.
> 
>    I went the other way round. In -RT we have identified the locks
>    which can _not_ be converted to sleeping locks and so I went there
>    and converted them to atomic_spinlock_t and created a set of
>    functions for them. I converted the already known locks to that
>    type and fixed up all the functions (s/spin_*/atomic_spin_*/) which
>    annotates the code and makes it clear what we are dealing with.
> 
>    [ I admit "atomic_spinlock_t" is a horrible name, but it's the best
>      I came up with so far. If you have a better idea please feel free
>      to add it to 
> 
>          http://rt.wiki.kernel.org/index.php/Atomic_Spinlock
> 
>      instead of starting a bikeshed painting thread on the mailing
>      lists about that name. Once we have something better it's just a
>      sed script to fix it. ]
> 
>    For !RT the spin_* functions are mapped to atomic_spin_* via inline
>    functions which do the type conversion. That has another nice side
>    effect: some places in the kernel (mostly scheduler) use
>    _raw_spin_* functions on locks to avoid the lockdep invocation in
>    some places. With the type conversion a lock needs to be defined
>    atomic_spinlock_t (or raw_spinlock_t) to have access to that
>    _raw_spin_* functions. Using e.g. _raw_spin_lock() on a lock
>    defined with spinlock_t/DEFINE_SPINLOCK will cause a compiler
>    warning. I think that's a Good Thing.
> 
>    On RT the spin_* functions are mapped to the corresponding rt_lock
>    functions with inlines as well. Very simple and much more
>    understandable than the nifty PICK_OP magic with the underscore
>    convolution. :)
> 
>    I did the same conversion for all (rw_)semaphores which are known
>    from -rt to be ownerless anonymous semaphores; i.e. taken in one
>    context and released in another. We renamed them to
>    compat_(rw_)semaphores up to now and let the compiler pick the
>    right function. Again here I went down the road and annotated the
>    code for those with newly created anon_* and [read|write]_anon_*
>    functions. In !RT the non annotated ones map to the anon_ functions
>    and on RT we map them to the corresponging rt_* ones. This
>    annotation should also be helpful to cover at least the non
>    anonymous (rw_)semaphores via lockdep.
> 
>    Part of that semaphore rework is the RFC patch series I posted
>    recently to get rid of the init_MUTEX[_LOCKED] irritation (minus
>    the ones which turned out to be wrong)
> 
>    The spinlock and semaphore annotation work is separate now and can
>    be found in the rt/atomic-lock and rt/semaphore branches of the
>    -tip git repository, which leads me to the next important point:
> 
> - start of gitification
> 
>    While reworking all of the above I went through the quilt queue and
>    sorted out patches into different rt/ branches. If you clone the
>    -tip git tree you'll find a bunch of branches starting with rt/.
>    They contain various independent changes which are all part of the
>    -rt patch. The combination of those branches can be found in the
>    rt/base branch.
> 
>    I still have a leftover of ~140 patches (roughly 40% of the -rt
>    queue) which I committed into the rt/rt-2.6.31-rc4 branch just as
>    is simply because I ran out of time. My annual summer vacation
>    (helping my wife to run the kitchen in the church community kids
>    summer camp) is starting on friday.
> 
>    While the other rt/ branches are mostly bisectable the final one is
>    not yet there. I restructured the patch queue in a logical way, but
>    there is more work to be done to clean it up. So expect it to be
>    replaced.
> 
> Further plans:
> 
>    1) We seriously want to tackle the elimination of the PREEMPT_RT
>    annoyance #1, aka BKL. The Big Kernel Lock is still used in ~330
>    files all across the kernel. A lot of work has been done already to
>    push down the lock into the code which still thinks it needs to be
>    protected by it. Some work has been done already in the (a little
>    bit stale) kill-the-BKL and core/kill-the-BKL branches of the -tip
>    git tree. If you want to help, please check those branches whether
>    the code has been tackled already or not to avoid redundant
>    work. If you decide to take care of one please note it on:
> 
>        http://rt.wiki.kernel.org/index.php/Big_Kernel_Lock
> 
>    2) I'm going on vacation for 10 days. Please send patches and
>    bugreports^Wsuccess stories to the mailinglist as usual. There are
>    folks looking out.
> 
> Enough said. Get the code and have fun!
> 
> Download locations:
> 
>     http://rt.et.redhat.com/download/
>     http://www.kernel.org/pub/linux/kernel/projects/rt/
> 
> Git:
> 
>     git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip.git rt/rt-2.6.31-rc4-rt1
> 
> Gitweb:
>     http://git.kernel.org/?p=linux/kernel/git/tip/linux-2.6-tip.git;a=shortlog;h=rt/rt-2.6.31-rc4
> 
> Information on the RT patch can be found at:
> 
>     http://rt.wiki.kernel.org/index.php/Main_Page
> 
> to build the 2.6.31-rc4-rt1 tree, the following patches should be
> applied:
> 
>     http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.30.tar.bz2
>     http://kernel.org/pub/linux/kernel/v2.6/testing/patch-2.6.31-rc4.bz2
>     http://www.kernel.org/pub/linux/kernel/projects/rt/patch-2.6.31-rc4-rt1.bz2
> 
> Thanks to Carsten Emde, Clark Williams and John Kacur who were testing
> my various steps to get the code into the shape where it is now.
> 
> Enjoy !
> 
>        tglx
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