[PATCH v4 07/15] docs: locking: futex2: Add documentation

André Almeida
2021-06-03T20:00:59Z

Add a new documentation file specifying both userspace API and internal
implementation details of futex2 syscalls.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
---
Documentation/locking/futex2.rst | 198 +++++++++++++++++++++++++++++++
Documentation/locking/index.rst | 1 +
2 files changed, 199 insertions(+)
create mode 100644 Documentation/locking/futex2.rst

diff --git a/Documentation/locking/futex2.rst b/Documentation/locking/futex2.rst
new file mode 100644
index 000000000000..2f74d7c97a55
--- /dev/null
+++ b/Documentation/locking/futex2.rst
@@ -0,0 +1,198 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======
+futex2
+======
+
+:Author: André Almeida <andrealmeid@collabora.com>
+
+futex, or fast user mutex, is a set of syscalls to allow userspace to create
+performant synchronization mechanisms, such as mutexes, semaphores and
+conditional variables in userspace. C standard libraries, like glibc, uses it
+as a means to implement more high level interfaces like pthreads.
+
+The interface
+=============
+
+uAPI functions
+--------------
+
+.. kernel-doc:: kernel/futex2.c
+ :identifiers: sys_futex_wait sys_futex_wake sys_futex_waitv sys_futex_requeue
+
+uAPI structures
+---------------
+
+.. kernel-doc:: include/uapi/linux/futex.h
+
+The ``flag`` argument
+---------------------
+
+The flag is used to specify the size of the futex word
+(FUTEX_[8, 16, 32, 64]). It's mandatory to define one, since there's no
+default size.
+
+By default, the timeout uses a monotonic clock, but can be used as a realtime
+one by using the FUTEX_REALTIME_CLOCK flag.
+
+By default, futexes are of the private type, that means that this user address
+will be accessed by threads that share the same memory region. This allows for
+some internal optimizations, so they are faster. However, if the address needs
+to be shared with different processes (like using ``mmap()`` or ``shm()``), they
+need to be defined as shared and the flag FUTEX_SHARED_FLAG is used to set that.
+
+By default, the operation has no NUMA-awareness, meaning that the user can't
+choose the memory node where the kernel side futex data will be stored. The
+user can choose the node where it wants to operate by setting the
+FUTEX_NUMA_FLAG and using the following structure (where X can be 8, 16, 32 or
+64)::
+
+ struct futexX_numa {
+ __uX value;
+ __sX hint;
+ };
+
+This structure should be passed at the ``void *uaddr`` of futex functions. The
+address of the structure will be used to be waited on/waken on, and the
+``value`` will be compared to ``val`` as usual. The ``hint`` member is used to
+define which node the futex will use. When waiting, the futex will be
+registered on a kernel-side table stored on that node; when waking, the futex
+will be searched for on that given table. That means that there's no redundancy
+between tables, and the wrong ``hint`` value will lead to undesired behavior.
+Userspace is responsible for dealing with node migrations issues that may
+occur. ``hint`` can range from [0, MAX_NUMA_NODES), for specifying a node, or
+-1, to use the same node the current process is using.
+
+When not using FUTEX_NUMA_FLAG on a NUMA system, the futex will be stored on a
+global table on allocated on the first node.
+
+The ``timo`` argument
+---------------------
+
+As per the Y2038 work done in the kernel, new interfaces shouldn't add timeout
+options known to be buggy. Given that, ``timo`` should be a 64-bit timeout at
+all platforms, using an absolute timeout value.
+
+Implementation
+==============
+
+The internal implementation follows a similar design to the original futex.
+Given that we want to replicate the same external behavior of current futex,
+this should be somewhat expected.
+
+Waiting
+-------
+
+For the wait operations, they are all treated as if you want to wait on N
+futexes, so the path for futex_wait and futex_waitv is the basically the same.
+For both syscalls, the first step is to prepare an internal list for the list
+of futexes to wait for (using struct futexv_head). For futex_wait() calls, this
+list will have a single object.
+
+We have a hash table, where waiters register themselves before sleeping. Then
+the wake function checks this table looking for waiters at uaddr. The hash
+bucket to be used is determined by a struct futex_key, that stores information
+to uniquely identify an address from a given process. Given the huge address
+space, there'll be hash collisions, so we store information to be later used on
+collision treatment.
+
+First, for every futex we want to wait on, we check if (``*uaddr == val``).
+This check is done holding the bucket lock, so we are correctly serialized with
+any futex_wake() calls. If any waiter fails the check above, we dequeue all
+futexes. The check (``*uaddr == val``) can fail for two reasons:
+
+- The values are different, and we return -EAGAIN. However, if while
+ dequeueing we found that some futexes were awakened, we prioritize this
+ and return success.
+
+- When trying to access the user address, we do so with page faults
+ disabled because we are holding a bucket's spin lock (and can't sleep
+ while holding a spin lock). If there's an error, it might be a page
+ fault, or an invalid address. We release the lock, dequeue everyone
+ (because it's illegal to sleep while there are futexes enqueued, we
+ could lose wakeups) and try again with page fault enabled. If we
+ succeed, this means that the address is valid, but we need to do
+ all the work again. For serialization reasons, we need to have the
+ spin lock when getting the user value. Additionally, for shared
+ futexes, we also need to recalculate the hash, since the underlying
+ mapping mechanisms could have changed when dealing with page fault.
+ If, even with page fault enabled, we can't access the address, it
+ means it's an invalid user address, and we return -EFAULT. For this
+ case, we prioritize the error, even if some futexes were awaken.
+
+If the check is OK, they are enqueued on a linked list in our bucket, and
+proceed to the next one. If all waiters succeed, we put the thread to sleep
+until a futex_wake() call, timeout expires or we get a signal. After waking up,
+we dequeue everyone, and check if some futex was awakened. This dequeue is done
+by iteratively walking at each element of struct futex_head list.
+
+All enqueuing/dequeuing operations requires to hold the bucket lock, to avoid
+racing while modifying the list.
+
+Waking
+------
+
+We get the bucket that's storing the waiters at uaddr, and wake the required
+number of waiters, checking for hash collision.
+
+There's an optimization that makes futex_wake() not take the bucket lock if
+there's no one to be woken on that bucket. It checks an atomic counter that each
+bucket has, if it says 0, then the syscall exits. In order for this to work, the
+waiter thread increases it before taking the lock, so the wake thread will
+correctly see that there's someone waiting and will continue the path to take
+the bucket lock. To get the correct serialization, the waiter issues a memory
+barrier after increasing the bucket counter and the waker issues a memory
+barrier before checking it.
+
+Requeuing
+---------
+
+The requeue path first checks for each struct futex_requeue and their flags.
+Then, it will compare the expected value with the one at uaddr1::uaddr.
+Following the same serialization explained at Waking_, we increase the atomic
+counter for the bucket of uaddr2 before taking the lock. We need to have both
+buckets locks at same time so we don't race with other futex operation. To
+ensure the locks are taken in the same order for all threads (and thus avoiding
+deadlocks), every requeue operation takes the "smaller" bucket first, when
+comparing both addresses.
+
+If the compare with user value succeeds, we proceed by waking ``nr_wake``
+futexes, and then requeuing ``nr_requeue`` from bucket of uaddr1 to the uaddr2.
+This consists in a simple list deletion/addition and replacing the old futex key
+with the new one.
+
+Futex keys
+----------
+
+There are two types of futexes: private and shared ones. The private are futexes
+meant to be used by threads that share the same memory space, are easier to be
+uniquely identified and thus can have some performance optimization. The
+elements for identifying one are: the start address of the page where the
+address is, the address offset within the page and the current->mm pointer.
+
+Now, for uniquely identifying a shared futex:
+
+- If the page containing the user address is an anonymous page, we can
+ just use the same data used for private futexes (the start address of
+ the page, the address offset within the page and the current->mm
+ pointer); that will be enough for uniquely identifying such futex. We
+ also set one bit at the key to differentiate if a private futex is
+ used on the same address (mixing shared and private calls does not
+ work).
+
+- If the page is file-backed, current->mm maybe isn't the same one for
+ every user of this futex, so we need to use other data: the
+ page->index, a UUID for the struct inode and the offset within the
+ page.
+
+Note that members of futex_key don't have any particular meaning after they
+are part of the struct - they are just bytes to identify a futex. Given that,
+we don't need to use a particular name or type that matches the original data,
+we only need to care about the bitsize of each component and make both private
+and shared fit in the same memory space.
+
+Source code documentation
+=========================
+
+.. kernel-doc:: kernel/futex2.c
+ :no-identifiers: sys_futex_wait sys_futex_wake sys_futex_waitv sys_futex_requeue
diff --git a/Documentation/locking/index.rst b/Documentation/locking/index.rst
index 7003bd5aeff4..9bf03c7fa1ec 100644
--- a/Documentation/locking/index.rst
+++ b/Documentation/locking/index.rst
@@ -24,6 +24,7 @@ locking
percpu-rw-semaphore
robust-futexes
robust-futex-ABI
+ futex2

.. only:: subproject and html

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