Documentation/locking/robust-futex-ABI.rst

====================
The robust futex ABI
====================

:Author: Started by Paul Jackson <pj@sgi.com>


Robust_futexes provide a mechanism that is used in addition to normal
futexes, for kernel assist of cleanup of held locks on task exit.

The interesting data as to what futexes a thread is holding is kept on a
linked list in user space, where it can be updated efficiently as locks
are taken and dropped, without kernel intervention.  The only additional
kernel intervention required for robust_futexes above and beyond what is
required for futexes is:

 1) a one time call, per thread, to tell the kernel where its list of
    held robust_futexes begins, and
 2) internal kernel code at exit, to handle any listed locks held
    by the exiting thread.

The existing normal futexes already provide a "Fast Userspace Locking"
mechanism, which handles uncontested locking without needing a system
call, and handles contested locking by maintaining a list of waiting
threads in the kernel.  Options on the sys_futex(2) system call support
waiting on a particular futex, and waking up the next waiter on a
particular futex.

For robust_futexes to work, the user code (typically in a library such
as glibc linked with the application) has to manage and place the
necessary list elements exactly as the kernel expects them.  If it fails
to do so, then improperly listed locks will not be cleaned up on exit,
probably causing deadlock or other such failure of the other threads
waiting on the same locks.

A thread that anticipates possibly using robust_futexes should first
issue the system call::

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

The pointer 'head' points to a structure in the threads address space
consisting of three words.  Each word is 32 bits on 32 bit arch's, or 64
bits on 64 bit arch's, and local byte order.  Each thread should have
its own thread private 'head'.

If a thread is running in 32 bit compatibility mode on a 64 native arch
kernel, then it can actually have two such structures - one using 32 bit
words for 32 bit compatibility mode, and one using 64 bit words for 64
bit native mode.  The kernel, if it is a 64 bit kernel supporting 32 bit
compatibility mode, will attempt to process both lists on each task
exit, if the corresponding sys_set_robust_list() call has been made to
setup that list.

  The first word in the memory structure at 'head' contains a
  pointer to a single linked list of 'lock entries', one per lock,
  as described below.  If the list is empty, the pointer will point
  to itself, 'head'.  The last 'lock entry' points back to the 'head'.

  The second word, called 'offset', specifies the offset from the
  address of the associated 'lock entry', plus or minus, of what will
  be called the 'lock word', from that 'lock entry'.  The 'lock word'
  is always a 32 bit word, unlike the other words above.  The 'lock
  word' holds 2 flag bits in the upper 2 bits, and the thread id (TID)
  of the thread holding the lock in the bottom 30 bits.  See further
  below for a description of the flag bits.

  The third word, called 'list_op_pending', contains transient copy of
  the address of the 'lock entry', during list insertion and removal,
  and is needed to correctly resolve races should a thread exit while
  in the middle of a locking or unlocking operation.

Each 'lock entry' on the single linked list starting at 'head' consists
of just a single word, pointing to the next 'lock entry', or back to
'head' if there are no more entries.  In addition, nearby to each 'lock
entry', at an offset from the 'lock entry' specified by the 'offset'
word, is one 'lock word'.

The 'lock word' is always 32 bits, and is intended to be the same 32 bit
lock variable used by the futex mechanism, in conjunction with
robust_futexes.  The kernel will only be able to wakeup the next thread
waiting for a lock on a threads exit if that next thread used the futex
mechanism to register the address of that 'lock word' with the kernel.

For each futex lock currently held by a thread, if it wants this
robust_futex support for exit cleanup of that lock, it should have one
'lock entry' on this list, with its associated 'lock word' at the
specified 'offset'.  Should a thread die while holding any such locks,
the kernel will walk this list, mark any such locks with a bit
indicating their holder died, and wakeup the next thread waiting for
that lock using the futex mechanism.

When a thread has invoked the above system call to indicate it
anticipates using robust_futexes, the kernel stores the passed in 'head'
pointer for that task.  The task may retrieve that value later on by
using the system call::

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

It is anticipated that threads will use robust_futexes embedded in
larger, user level locking structures, one per lock.  The kernel
robust_futex mechanism doesn't care what else is in that structure, so
long as the 'offset' to the 'lock word' is the same for all
robust_futexes used by that thread.  The thread should link those locks
it currently holds using the 'lock entry' pointers.  It may also have
other links between the locks, such as the reverse side of a double
linked list, but that doesn't matter to the kernel.

By keeping its locks linked this way, on a list starting with a 'head'
pointer known to the kernel, the kernel can provide to a thread the
essential service available for robust_futexes, which is to help clean
up locks held at the time of (a perhaps unexpectedly) exit.

Actual locking and unlocking, during normal operations, is handled
entirely by user level code in the contending threads, and by the
existing futex mechanism to wait for, and wakeup, locks.  The kernels
only essential involvement in robust_futexes is to remember where the
list 'head' is, and to walk the list on thread exit, handling locks
still held by the departing thread, as described below.

There may exist thousands of futex lock structures in a threads shared
memory, on various data structures, at a given point in time. Only those
lock structures for locks currently held by that thread should be on
that thread's robust_futex linked lock list a given time.

A given futex lock structure in a user shared memory region may be held
at different times by any of the threads with access to that region. The
thread currently holding such a lock, if any, is marked with the threads
TID in the lower 30 bits of the 'lock word'.

When adding or removing a lock from its list of held locks, in order for
the kernel to correctly handle lock cleanup regardless of when the task
exits (perhaps it gets an unexpected signal 9 in the middle of
manipulating this list), the user code must observe the following
protocol on 'lock entry' insertion and removal:

On insertion:

 1) set the 'list_op_pending' word to the address of the 'lock entry'
    to be inserted,
 2) acquire the futex lock,
 3) add the lock entry, with its thread id (TID) in the bottom 30 bits
    of the 'lock word', to the linked list starting at 'head', and
 4) clear the 'list_op_pending' word.

On removal:

 1) set the 'list_op_pending' word to the address of the 'lock entry'
    to be removed,
 2) remove the lock entry for this lock from the 'head' list,
 3) release the futex lock, and
 4) clear the 'lock_op_pending' word.

Please note that the removal of a robust futex purely in userspace is
racy. Refer to the next chapter to learn more and how to avoid this.

On exit, the kernel will consider the address stored in
'list_op_pending' and the address of each 'lock word' found by walking
the list starting at 'head'.  For each such address, if the bottom 30
bits of the 'lock word' at offset 'offset' from that address equals the
exiting threads TID, then the kernel will do two things:

 1) if bit 31 (0x80000000) is set in that word, then attempt a futex
    wakeup on that address, which will waken the next thread that has
    used to the futex mechanism to wait on that address, and
 2) atomically set  bit 30 (0x40000000) in the 'lock word'.

In the above, bit 31 was set by futex waiters on that lock to indicate
they were waiting, and bit 30 is set by the kernel to indicate that the
lock owner died holding the lock.

The kernel exit code will silently stop scanning the list further if at
any point:

 1) the 'head' pointer or an subsequent linked list pointer
    is not a valid address of a user space word
 2) the calculated location of the 'lock word' (address plus
    'offset') is not the valid address of a 32 bit user space
    word
 3) if the list contains more than 1 million (subject to
    future kernel configuration changes) elements.

When the kernel sees a list entry whose 'lock word' doesn't have the
current threads TID in the lower 30 bits, it does nothing with that
entry, and goes on to the next entry.

Robust release is racy
----------------------

The removal of a robust futex from the list is racy when doing it solely in
userspace. Quoting Thomas Gleixner for the explanation:

  The robust futex unlock mechanism is racy in respect to the clearing of the
  robust_list_head::list_op_pending pointer because unlock and clearing the
  pointer are not atomic. The race window is between the unlock and clearing
  the pending op pointer. If the task is forced to exit in this window, exit
  will access a potentially invalid pending op pointer when cleaning up the
  robust list. That happens if another task manages to unmap the object
  containing the lock before the cleanup, which results in an UAF. In the
  worst case this UAF can lead to memory corruption when unrelated content
  has been mapped to the same address by the time the access happens.

A full in-depth analysis can be read at
https://lore.kernel.org/lkml/20260316162316.356674433@kernel.org/

To overcome that, the kernel needs to participate in the lock release operation.
This ensures that the release happens "atomically" with regard to releasing
the lock and removing the address from ``list_op_pending``. If the release is
interrupted by a signal, the kernel will also verify if it interrupted the
release operation.

For the contended unlock case, where other threads are waiting for the lock
release, there's the ``FUTEX_ROBUST_UNLOCK`` operation feature flag for the
``futex()`` system call, which must be used with one of the following
operations: ``FUTEX_WAKE``, ``FUTEX_WAKE_BITSET`` or ``FUTEX_UNLOCK_PI``.
The kernel will release the lock (set the futex word to zero), clean the
``list_op_pending`` field. Then, it will proceed with the normal wake path.

For the non-contended path, there's still a race between checking the futex word
and clearing the ``list_op_pending`` field. To solve this without the need of a
complete system call, userspace should call the virtual syscall
``__vdso_futex_robust_listXX_try_unlock()`` (where XX is either 32 or 64,
depending on the size of the pointer). If the vDSO call succeeds, it means that
it released the lock and cleared ``list_op_pending``. If it fails, that means
that there are waiters for this lock and a call to ``futex()`` syscall with
``FUTEX_ROBUST_UNLOCK`` is needed.