linux/Documentation/locking/robust-futexes.rst

========================================
A description of what robust futexes are
========================================

:Started by: Ingo Molnar <mingo@redhat.com>

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, the traditional approach was to extend the vma
(virtual memory area descriptor) concept to have a notion of 'pending
robust futexes attached to this area'. This approach requires 3 new
syscall variants to sys_futex(): FUTEX_REGISTER, FUTEX_DEREGISTER and
FUTEX_RECOVER. At do_exit() time, all vmas are searched to see whether
they have a robust_head set. This approach has two fundamental problems
left:

 - it has quite complex locking and race scenarios. The vma-based
   approach 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 so that
normal kernels can turn it off, but worse than that: the overhead makes
robust futexes impractical for any type of generic Linux distribution.

So something had to be done.

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_DIED bit, and wakes up one waiter (if
any).

The list is guaranteed to be private and per-thread at do_exit() time,
so it can be accessed by the kernel in a lockless way.

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.

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 don't
   need any extra per-lock syscalls. Robust mutexes thus become a very
   lightweight primitive - so they don't 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 doesn't have any internal distinction between
robust and normal futexes.

If a futex is found to be held at exit time, the kernel sets the
following 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 won't have
the new syscalls yet.

Architectures need to implement the new futex_atomic_cmpxchg_inatomic()
inline function before writing up the syscalls.