diff mbox series

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

Message ID 20210603195924.361327-8-andrealmeid@collabora.com
State New
Headers show
Series Add futex2 syscalls | expand

Commit Message

André Almeida June 3, 2021, 7:59 p.m. UTC
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

Comments

Davidlohr Bueso June 6, 2021, 7:23 p.m. UTC | #1
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

>
diff mbox series

Patch

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