[patch 0/5] lightweight robust futexes: -V1

[Ingo Molnar]

This patchset provides a new (written from scratch) implementation of 
robust futexes, called "lightweight robust futexes". We believe this new 
implementation is faster and simpler than the vma-based robust futex 
solutions presented before, and we'd like this patchset to be adopted in 
the upstream kernel. This is version 1 of the patchset.

Background
----------

what are robust futexes? To answer that, we first need to understand 
what futexes are: normal futexes are special types of locks that in the 
noncontended case can be acquired/released from userspace without having 
to enter the kernel.

A futex is in essence a user-space address, e.g. a 32-bit lock variable 
field. If userspace notices contention (the lock is already owned and 
someone else wants to grab it too) then the lock is marked with a value 
that says "there's a waiter pending", and the sys_futex(FUTEX_WAIT) 
syscall is used to wait for the other guy to release it. The kernel 
creates a 'futex queue' internally, so that it can later on match up the 
waiter with the waker - without them having to know about each other.  
When the owner thread releases the futex, it notices (via the variable 
value) that there were waiter(s) pending, and does the 
sys_futex(FUTEX_WAKE) syscall to wake them up.  Once all waiters have 
taken and released the lock, the futex is again back to 'uncontended' 
state, and there's no in-kernel state associated with it. The kernel 
completely forgets that there ever was a futex at that address. This 
method makes futexes very lightweight and scalable.

"Robustness" is about dealing with crashes while holding a lock: if a 
process exits prematurely while holding a pthread_mutex_t lock that is 
also shared with some other process (e.g. yum segfaults while holding a 
pthread_mutex_t, or yum is kill -9-ed), then waiters for that lock need 
to be notified that the last owner of the lock exited in some irregular 
way.

To solve such types of problems, "robust mutex" userspace APIs were 
created: pthread_mutex_lock() returns an error value if the owner exits 
prematurely - and the new owner can decide whether the data protected by 
the lock can be recovered safely.

There is a big conceptual problem with futex based mutexes though: it is 
the kernel that destroys the owner task (e.g. due to a SEGFAULT), but 
the kernel cannot help with the cleanup: if there is no 'futex queue' 
(and in most cases there is none, futexes being fast lightweight locks) 
then the kernel has no information to clean up after the held lock!  
Userspace has no chance to clean up after the lock either - userspace is 
the one that crashes, so it has no opportunity to clean up. Catch-22.

In practice, when e.g. yum is kill -9-ed (or segfaults), a system reboot 
is needed to release that futex based lock. This is one of the leading 
bugreports against yum.

To solve this problem, 'Robust Futex' patches were created and presented 
on lkml: the one written by Todd Kneisel and David Singleton is the most 
advanced at the moment. These patches all tried to extend the futex 
abstraction by registering futex-based locks in the kernel - and thus 
give the kernel a chance to clean up.

E.g. in David Singleton's robust-futex-6.patch, there are 3 new syscall 
variants to sys_futex(): FUTEX_REGISTER, FUTEX_DEREGISTER and 
FUTEX_RECOVER. The kernel attaches such robust futexes to vmas (via 
vma->vm_file->f_mapping->robust_head), and at do_exit() time, all vmas 
are searched to see whether they have a robust_head set.

Lots of work went into the vma-based robust-futex patch, and recently it 
has improved significantly, but unfortunately it still has two 
fundamental problems left:

 - they have quite complex locking and race scenarios. The vma-based 
   patches had been pending for years, but they are still not completely 
   reliable.

 - they have to scan _every_ vma at sys_exit() time, per thread!

The second disadvantage is a real killer: pthread_exit() takes around 1 
microsecond on Linux, but with thousands (or tens of thousands) of vmas 
every pthread_exit() takes a millisecond or more, also totally 
destroying the CPU's L1 and L2 caches!

This is very much noticeable even for normal process sys_exit_group() 
calls: the kernel has to do the vma scanning unconditionally! (this is 
because the kernel has no knowledge about how many robust futexes there 
are to be cleaned up, because a robust futex might have been registered 
in another task, and the futex variable might have been simply mmap()-ed 
into this process's address space).

This huge overhead forced the creation of CONFIG_FUTEX_ROBUST, but worse 
than that: the overhead makes robust futexes impractical for any type of 
generic Linux distribution.

So it became clear to us, something had to be done. Last week, when 
Thomas Gleixner tried to fix up the vma-based robust futex patch in the 
-rt tree, he found a handful of new races and we were talking about it 
and were analyzing the situation. At that point a fundamentally 
different solution occured to me. This patchset (written in the past 
couple of days) implements that new solution. Be warned though - the 
patchset does things we normally dont do in Linux, so some might find 
the approach disturbing. Parental advice recommended ;-)

New approach to robust futexes
------------------------------

At the heart of this new approach there is a per-thread private list of 
robust locks that userspace is holding (maintained by glibc) - which 
userspace list is registered with the kernel via a new syscall [this 
registration happens at most once per thread lifetime]. At do_exit() 
time, the kernel checks this user-space list: are there any robust futex 
locks to be cleaned up?

In the common case, at do_exit() time, there is no list registered, so 
the cost of robust futexes is just a simple current->robust_list != NULL 
comparison. If the thread has registered a list, then normally the list 
is empty. If the thread/process crashed or terminated in some incorrect 
way then the list might be non-empty: in this case the kernel carefully 
walks the list [not trusting it], and marks all locks that are owned by 
this thread with the FUTEX_OWNER_DEAD bit, and wakes up one waiter (if 
any).

The list is guaranteed to be private and per-thread, so it's lockless. 
There is one race possible though: since adding to and removing from the 
list is done after the futex is acquired by glibc, there is a few 
instructions window for the thread (or process) to die there, leaving 
the futex hung. To protect against this possibility, userspace (glibc) 
also maintains a simple per-thread 'list_op_pending' field, to allow the 
kernel to clean up if the thread dies after acquiring the lock, but just 
before it could have added itself to the list. Glibc sets this 
list_op_pending field before it tries to acquire the futex, and clears 
it after the list-add (or list-remove) has finished.

That's all that is needed - all the rest of robust-futex cleanup is done 
in userspace [just like with the previous patches].

Ulrich Drepper has implemented the necessary glibc support for this new 
mechanism, which fully enables robust mutexes. (Ulrich plans to commit 
these changes to glibc-HEAD later today.)

Key differences of this userspace-list based approach, compared to the 
vma based method:

 - it's much, much faster: at thread exit time, there's no need to loop 
   over every vma (!), which the VM-based method has to do. Only a very 
   simple 'is the list empty' op is done.

 - no VM changes are needed - 'struct address_space' is left alone.

 - no registration of individual locks is needed: robust mutexes dont 
   need any extra per-lock syscalls. Robust mutexes thus become a very 
   lightweight primitive - so they dont force the application designer 
   to do a hard choice between performance and robustness - robust 
   mutexes are just as fast.

 - no per-lock kernel allocation happens.

 - no resource limits are needed.

 - no kernel-space recovery call (FUTEX_RECOVER) is needed.

 - the implementation and the locking is "obvious", and there are no 
   interactions with the VM.

Performance
-----------

I have benchmarked the time needed for the kernel to process a list of 1 
million (!) held locks, using the new method [on a 2GHz CPU]:

 - with FUTEX_WAIT set [contended mutex]: 130 msecs
 - without FUTEX_WAIT set [uncontended mutex]: 30 msecs

I have also measured an approach where glibc does the lock notification 
[which it currently does for !pshared robust mutexes], and that took 256 
msecs - clearly slower, due to the 1 million FUTEX_WAKE syscalls 
userspace had to do.

(1 million held locks are unheard of - we expect at most a handful of 
locks to be held at a time. Nevertheless it's nice to know that this 
approach scales nicely.)

Implementation details
----------------------

The patch adds two new syscalls: one to register the userspace list, and 
one to query the registered list pointer:

 asmlinkage long
 sys_set_robust_list(struct robust_list_head __user *head,
                     size_t len);

 asmlinkage long
 sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr,
                     size_t __user *len_ptr);

List registration is very fast: the pointer is simply stored in 
current->robust_list. [Note that in the future, if robust futexes become 
widespread, we could extend sys_clone() to register a robust-list head 
for new threads, without the need of another syscall.]

So there is virtually zero overhead for tasks not using robust futexes, 
and even for robust futex users, there is only one extra syscall per 
thread lifetime, and the cleanup operation, if it happens, is fast and 
straightforward. The kernel doesnt have any internal distinction between 
robust and normal futexes.

If a futex is found to be held at exit time, the kernel sets the highest 
bit of the futex word:

	#define FUTEX_OWNER_DIED        0x40000000

and wakes up the next futex waiter (if any). User-space does the rest of 
the cleanup.

Otherwise, robust futexes are acquired by glibc by putting the TID into 
the futex field atomically. Waiters set the FUTEX_WAITERS bit:

	#define FUTEX_WAITERS           0x80000000

and the remaining bits are for the TID.

Testing, architecture support
-----------------------------

i've tested the new syscalls on x86 and x86_64, and have made sure the 
parsing of the userspace list is robust [ ;-) ] even if the list is 
deliberately corrupted.

i386 and x86_64 syscalls are wired up at the moment, and Ulrich has 
tested the new glibc code (on x86_64 and i386), and it works for his 
robust-mutex testcases.

All other architectures should build just fine too - but they wont have 
the new syscalls yet.

Architectures need to implement the new futex_atomic_cmpxchg_inuser() 
inline function before writing up the syscalls (that function returns 
-ENOSYS right now).

	Ingo
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