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

    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.


    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

    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

    - 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

    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.


    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).

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      Last update: 2006-02-15 16:21    [W:0.034 / U:55.560 seconds]
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