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..3ab49f0e741c --- /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]). 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, or +32):: + + 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
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