On Thu, 03 Jun 2021, Andr� Almeida wrote:
Add a new documentation file specifying both userspace API and internal implementation details of futex2 syscalls.
I think equally important would be to provide a manpage for each new syscall you are introducing, and keep mkt in the loop as in the past he extensively documented and improved futex manpages, and overall has a lot of experience with dealing with kernel interfaces.
Thanks, Davidlohr
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
-- 2.31.1