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From: Ingo Molnar <mingo <at> elte.hu>
Subject: [patch 00/10] PI-futex: -V1
Newsgroups: gmane.linux.kernel
Date: Saturday 25th March 2006 18:45:28 UTC (over 10 years ago)
We are pleased to announce "lightweight userspace priority inheritance" 
(PI) support for futexes. The following patchset and glibc patch 
implements it, ontop of the robust-futexes patchset which is included in 

We are calling it lightweight for 3 reasons:

 - in the user-space fastpath a PI-enabled futex involves no kernel work 
   (or any other PI complexity) at all. No registration, no extra kernel
   calls - just pure fast atomic ops in userspace.

 - in the slowpath (in the lock-contention case), the system call and 
   scheduling pattern is in fact better than that of normal futexes,
   due to the 'integrated' nature of FUTEX_LOCK_PI. [more about that 
   further down]

 - the in-kernel PI implementation is streamlined around the mutex
   abstraction, with strict rules that keep the implementation
   relatively simple: only a single owner may own a lock (i.e. no
   read-write lock support), only the owner may unlock a lock, no
   recursive locking, etc.

Priority Inheritance - why, oh why???

Many of you heard the horror stories about the evil PI code circling 
Linux for years, which makes no real sense at all and is only used by 
buggy applications and which has horrible overhead. Some of you have 
dreaded this very moment, when someone actually submits working PI code 

So why would we like to see PI support for futexes?

We'd like to see it done purely for technological reasons. We dont think 
it's a buggy concept, we think it's useful functionality to offer to 
applications, which functionality cannot be achieved in other ways. We 
also think it's the right thing to do, and we think we've got the right 
arguments and the right numbers to prove that. We also believe that we 
can address all the counter-arguments as well. For these reasons (and 
the reasons outlined below) we are submitting this patch-set for 
upstream kernel inclusion.

What are the benefits of PI?

The short reply:

User-space PI helps achieving/improving determinism for user-space 
applications. In the best-case, it can help achieve determinism and 
well-bound latencies. Even in the worst-case, PI will improve the 
statistical distribution of locking related application delays.

The longer reply:

Firstly, sharing locks between multiple tasks is a common programming 
technique that often cannot be replaced with lockless algorithms. As we 
can see it in the kernel [which is a quite complex program in itself], 
lockless structures are rather the exception than the norm - the current 
ratio of lockless vs. locky code for shared data structures is somewhere 
between 1:10 and 1:100. Lockless is hard, and the complexity of lockless 
algorithms often endangers to ability to do robust reviews of said code.  
I.e. critical RT apps often choose lock structures to protect critical 
data structures, instead of lockless algorithms. Furthermore, there are 
cases (like shared hardware, or other resource limits) where lockless 
access is mathematically impossible.

Media players (such as Jack) are an example of reasonable application 
design with multiple tasks (with multiple priority levels) sharing 
short-held locks: for example, a highprio audio playback thread is 
combined with medium-prio construct-audio-data threads and low-prio 
display-colory-stuff threads. Add video and decoding to the mix and 
we've got even more priority levels.

So once we accept that synchronization objects (locks) are an 
unavoidable fact of life, and once we accept that multi-task userspace 
apps have a very fair expectation of being able to use locks, we've got 
to think about how to offer the option of a deterministic locking 
implementation to user-space.

Most of the technical counter-arguments against doing priority 
inheritance only apply to kernel-space locks. But user-space locks are 
different, there we cannot disable interrupts or make the task 
non-preemptible in a critical section, so the 'use spinlocks' argument 
does not apply (user-space spinlocks have the same priority inversion 
problems as other user-space locking constructs). Fact is, pretty much 
the only technique that currently enables good determinism for userspace 
locks (such as futex-based pthread mutexes) is priority inheritance:

Currently (without PI), if a high-prio and a low-prio task shares a lock 
[this is a quite common scenario for most non-trivial RT applications], 
even if all critical sections are coded carefully to be deterministic 
(i.e. all critical sections are short in duration and only execute a 
limited number of instructions), the kernel cannot guarantee any 
deterministic execution of the high-prio task: any medium-priority task 
could preempt the low-prio task while it holds the shared lock and 
executes the critical section, and could delay it indefinitely.


As mentioned before, the userspace fastpath of PI-enabled pthread 
mutexes involves no kernel work at all - they behave quite similarly to 
normal futex-based locks: a 0 value means unlocked, and a value==TID 
means locked. (This is the same method as used by list-based robust 
futexes.) Userspace uses atomic ops to lock/unlock these mutexes without 
entering the kernel.

To handle the slowpath, we have added two new futex ops:


If the lock-acquire fastpath fails, [i.e. an atomic transition from 0 to 
TID fails], then FUTEX_LOCK_PI is called. The kernel does all the 
remaining work: if there is no futex-queue attached to the futex address 
yet then the code looks up the task that owns the futex [it has put its 
own TID into the futex value], and attaches a 'PI state' structure to 
the futex-queue. The pi_state includes an rt-mutex, which is a PI-aware, 
kernel-based synchronization object. The 'other' task is made the owner 
of the rt-mutex, and the FUTEX_WAITERS bit is atomically set in the 
futex value. Then this task tries to lock the rt-mutex, on which it 
blocks. Once it returns, it has the mutex acquired, and it sets the 
futex value to its own TID and returns. Userspace has no other work to 
perform - it now owns the lock, and futex value contains 

If the unlock side fastpath succeeds, [i.e. userspace manages to do a 
TID -> 0 atomic transition of the futex value], then no kernel work is 

If the unlock fastpath fails (because the FUTEX_WAITERS bit is set), 
then FUTEX_UNLOCK_PI is called, and the kernel unlocks the futex on the 
behalf of userspace - and it also unlocks the attached 
pi_state->rt_mutex and thus wakes up any potential waiters.

Note that under this approach, contrary to other PI-futex approaches, 
there is no prior 'registration' of a PI-futex. [which is not quite 
possible anyway, due to existing ABI properties of pthread mutexes.]

Also, under this scheme, 'robustness' and 'PI' are two orthogonal 
properties of futexes, and all four combinations are possible: futex, 
robust-futex, PI-futex, robust+PI-futex.

glibc support:

Ulrich Drepper and Jakub Jelinek have written glibc support for 
PI-futexes (and robust futexes), enabling robust and PI 
be added later on too, no additional kernel changes are needed for 
that). [NOTE: The glibc patch is obviously inofficial and unsupported 
without matching upstream kernel functionality.]

the patch-queue and the glibc patch can also be downloaded from:


a diffstat is attached below. The patch-queue is against 2.6.16-mm1, 
plus the following small updates to -mm1:


all have been sent to Andrew and are independent of PI-futexes.

many thanks go to the people who helped us create this kernel feature: 
Steven Rostedt, Esben Nielsen, Benedikt Spranger, Daniel Walker, John 
Cooper, Arjan van de Ven, Oleg Nesterov and others. Credits for related 
prior projects goes to Dirk Grambow, Inaky Perez-Gonzalez, Bill Huey and 
many others.

	Ingo Molnar, Thomas Gleixner


 Documentation/rtmutex.txt                    |   60 +
 arch/i386/mm/pageattr.c                      |    4 
 include/linux/futex.h                        |   11 
 include/linux/init_task.h                    |    2 
 include/linux/mm.h                           |   11 
 include/linux/plist.h                        |  226 ++++++
 include/linux/rtmutex.h                      |  119 +++
 include/linux/rtmutex_internal.h             |  187 +++++
 include/linux/sched.h                        |   34 
 include/linux/syscalls.h                     |    4 
 init/Kconfig                                 |    5 
 kernel/Makefile                              |    3 
 kernel/exit.c                                |    9 
 kernel/fork.c                                |    6 
 kernel/futex.c                               |  929
 kernel/futex_compat.c                        |   11 
 kernel/rtmutex-debug.c                       |  511 +++++++++++++
 kernel/rtmutex-debug.h                       |   32 
 kernel/rtmutex-tester.c                      |  436 +++++++++++
 kernel/rtmutex.c                             |  997
 kernel/rtmutex.h                             |   28 
 kernel/sched.c                               |  136 +++
 lib/Kconfig                                  |    6 
 lib/Kconfig.debug                            |   20 
 lib/Makefile                                 |    1 
 lib/plist.c                                  |   72 +
 mm/page_alloc.c                              |    4 
 mm/slab.c                                    |    3 
 scripts/rt-tester/check-all.sh               |   21 
 scripts/rt-tester/rt-tester.py               |  222 ++++++
 scripts/rt-tester/t2-l1-2rt-sameprio.tst     |  101 ++
 scripts/rt-tester/t2-l1-pi.tst               |   84 ++
 scripts/rt-tester/t2-l1-signal.tst           |   79 ++
 scripts/rt-tester/t2-l2-2rt-deadlock.tst     |   91 ++
 scripts/rt-tester/t3-l1-pi-1rt.tst           |   95 ++
 scripts/rt-tester/t3-l1-pi-2rt.tst           |   96 ++
 scripts/rt-tester/t3-l1-pi-3rt.tst           |   95 ++
 scripts/rt-tester/t3-l1-pi-signal.tst        |   98 ++
 scripts/rt-tester/t3-l1-pi-steal.tst         |   99 ++
 scripts/rt-tester/t3-l2-pi.tst               |   95 ++
 scripts/rt-tester/t4-l2-pi-deboost.tst       |  127 +++
 scripts/rt-tester/t5-l4-pi-boost-deboost.tst |  148 ++++
 42 files changed, 5138 insertions(+), 180 deletions(-)
CD: 5ms