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Subject[RFC][PATCH v3]: documentation,atomic: Add new documents

New version..


---
Subject: documentation,atomic: Add new documents
From: Peter Zijlstra <peterz@infradead.org>
Date: Mon Jun 12 14:50:27 CEST 2017

Since we've vastly expanded the atomic_t interface in recent years the
existing documentation is woefully out of date and people seem to get
confused a bit.

Start a new document to hopefully better explain the current state of
affairs.

The old atomic_ops.txt also covers bitmaps and a few more details so
this is not a full replacement and we'll therefore keep that document
around until such a time that we've managed to write more text to cover
its entire.

Also please, ReST people, go away.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
Documentation/atomic_bitops.txt | 66 ++++++++++++
Documentation/atomic_t.txt | 200 ++++++++++++++++++++++++++++++++++++++
Documentation/memory-barriers.txt | 88 ----------------
3 files changed, 269 insertions(+), 85 deletions(-)

--- /dev/null
+++ b/Documentation/atomic_bitops.txt
@@ -0,0 +1,66 @@
+
+On atomic bitops.
+
+
+While our bitmap_{}() functions are non-atomic, we have a number of operations
+operating on single bits in a bitmap that are atomic.
+
+
+API
+---
+
+The single bit operations are:
+
+Non RmW ops:
+
+ test_bit()
+
+RmW atomic operations without return value:
+
+ {set,clear,change}_bit()
+ clear_bit_unlock()
+
+RmW atomic operations with return value:
+
+ test_and_{set,clear,change}_bit()
+ test_and_set_bit_lock()
+
+Barriers:
+
+ smp_mb__{before,after}_atomic()
+
+
+All RmW atomic operations have a '__' prefixed variant which is non-atomic.
+
+
+SEMANTICS
+---------
+
+Non-atomic ops:
+
+In particular __clear_bit_unlock() suffers the same issue as atomic_set(),
+which is why the generic version maps to clear_bit_unlock(), see atomic_t.txt.
+
+
+RmW ops:
+
+The test_and_{}_bit() operations return the original value of the bit.
+
+
+ORDERING
+--------
+
+Like with atomic_t, the rule of thumb is:
+
+ - non-RmW operations are unordered;
+
+ - RmW operations that have no return value are unordered;
+
+ - RmW operations that have a return value are fully ordered.
+
+Except for test_and_set_bit_lock() which has ACQUIRE semantics and
+clear_bit_unlock() which has RELEASE semantics.
+
+Since a platform only has a single means of achieving atomic operations
+the same barriers as for atomic_t are used, see atomic_t.txt.
+
--- /dev/null
+++ b/Documentation/atomic_t.txt
@@ -0,0 +1,200 @@
+
+On atomic types (atomic_t atomic64_t and atomic_long_t).
+
+The atomic type provides an interface to the architecture's means of atomic
+RmW operations between CPUs (atomic operations on MMIO are not supported and
+can lead to fatal traps on some platforms).
+
+API
+---
+
+The 'full' API consists of (atomic64_ and atomic_long_ prefixes omitted for
+brevity):
+
+Non RmW ops:
+
+ atomic_read(), atomic_set()
+ atomic_read_acquire(), atomic_set_release()
+
+
+RmW atomic operations:
+
+Arithmetic:
+
+ atomic_{add,sub,inc,dec}()
+ atomic_{add,sub,inc,dec}_return{,_relaxed,_acquire,_release}()
+ atomic_fetch_{add,sub,inc,dec}{,_relaxed,_acquire,_release}()
+
+
+Bitwise:
+
+ atomic_{and,or,xor,andnot}()
+ atomic_fetch_{and,or,xor,andnot}{,_relaxed,_acquire,_release}()
+
+
+Swap:
+
+ atomic_xchg{,_relaxed,_acquire,_release}()
+ atomic_cmpxchg{,_relaxed,_acquire,_release}()
+ atomic_try_cmpxchg{,_relaxed,_acquire,_release}()
+
+
+Reference count (but please see refcount_t):
+
+ atomic_add_unless(), atomic_inc_not_zero()
+ atomic_sub_and_test(), atomic_dec_and_test()
+
+
+Misc:
+
+ atomic_inc_and_test(), atomic_add_negative()
+ atomic_dec_unless_positive(), atomic_inc_unless_negative()
+
+
+Barriers:
+
+ smp_mb__{before,after}_atomic()
+
+
+
+SEMANTICS
+---------
+
+Non RmW ops:
+
+The non-RmW ops are (typically) regular LOADs and STOREs and are canonically
+implemented using READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and
+smp_store_release() respectively.
+
+The one detail to this is that atomic_set{}() should be observable to the RmW
+ops. That is:
+
+ C atomic-set
+
+ {
+ atomic_set(v, 1);
+ }
+
+ P1(atomic_t *v)
+ {
+ atomic_add_unless(v, 1, 0);
+ }
+
+ P2(atomic_t *v)
+ {
+ atomic_set(v, 0);
+ }
+
+ exists
+ (v=2)
+
+In this case we would expect the atomic_set() from CPU1 to either happen
+before the atomic_add_unless(), in which case that latter one would no-op, or
+_after_ in which case we'd overwrite its result. In no case is "2" a valid
+outcome.
+
+This is typically true on 'normal' platforms, where a regular competing STORE
+will invalidate a LL/SC or fail a CMPXCHG.
+
+The obvious case where this is not so is when we need to implement atomic ops
+with a lock:
+
+ CPU0 CPU1
+
+ atomic_add_unless(v, 1, 0);
+ lock();
+ ret = READ_ONCE(v->counter); // == 1
+ atomic_set(v, 0);
+ if (ret != u) WRITE_ONCE(v->counter, 0);
+ WRITE_ONCE(v->counter, ret + 1);
+ unlock();
+
+the typical solution is to then implement atomic_set{}() with atomic_xchg().
+
+
+RmW ops:
+
+These come in various forms:
+
+ - plain operations without return value: atomic_{}()
+
+ - operations which return the modified value: atomic_{}_return()
+
+ these are limited to the arithmetic operations because those are
+ reversible. Bitops are irreversible and therefore the modified value
+ is of dubious utility.
+
+ - operations which return the original value: atomic_fetch_{}()
+
+ - swap operations: xchg(), cmpxchg() and try_cmpxchg()
+
+ - misc; the special purpose operations that are commonly used and would,
+ given the interface, normally be implemented using (try_)cmpxchg loops but
+ are time critical and can, (typically) on LL/SC architectures, be more
+ efficiently implemented.
+
+All these operations are SMP atomic; that is, the operations (for a single
+atomic variable) can be fully ordered and no intermediate state is lost or
+visible.
+
+
+ORDERING (go read memory-barriers.txt first)
+--------
+
+The rule of thumb:
+
+ - non-RmW operations are unordered;
+
+ - RmW operations that have no return value are unordered;
+
+ - RmW operations that have a return value are fully ordered;
+
+ - RmW operations that are conditional are unordered on FAILURE,
+ otherwise the above rules apply.
+
+Except of course when an operation has an explicit ordering like:
+
+ {}_relaxed: unordered
+ {}_acquire: the R of the RmW (or atomic_read) is an ACQUIRE
+ {}_release: the W of the RmW (or atomic_set) is a RELEASE
+
+Where 'unordered' is against other memory locations. Address dependencies are
+not defeated.
+
+Fully ordered primitives are ordered against everything prior and everything
+subsequent. Therefore a fully ordered primitive is like having an smp_mb()
+before and an smp_mb() after the primitive.
+
+
+The barriers:
+
+ smp_mb__{before,after}_atomic()
+
+only apply to the RmW ops and can be used to augment/upgrade the ordering
+inherent to the used atomic op. These barriers provide a full smp_mb().
+
+These helper barriers exist because architectures have varying implicit
+ordering on their SMP atomic primitives. For example our TSO architectures
+provide full ordered atomics and these barriers are no-ops.
+
+Thus:
+
+ atomic_fetch_add();
+
+is equivalent to:
+
+ smp_mb__before_atomic();
+ atomic_fetch_add_relaxed();
+ smp_mb__after_atomic();
+
+However the atomic_fetch_add() might be implemented more efficiently.
+
+Further, while something like:
+
+ smp_mb__before_atomic();
+ atomic_dec(&X);
+
+is a 'typical' RELEASE pattern, the barrier is strictly stronger than
+a RELEASE. Similarly for something like:
+
+
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -498,7 +498,7 @@ VARIETIES OF MEMORY BARRIER
This means that ACQUIRE acts as a minimal "acquire" operation and
RELEASE acts as a minimal "release" operation.

-A subset of the atomic operations described in core-api/atomic_ops.rst have
+A subset of the atomic operations described in atomic_t.txt have ACQUIRE
ACQUIRE and RELEASE variants in addition to fully-ordered and relaxed (no
barrier semantics) definitions. For compound atomics performing both a load
and a store, ACQUIRE semantics apply only to the load and RELEASE semantics
@@ -1876,8 +1876,7 @@ compiler and the CPU from reordering the
This makes sure that the death mark on the object is perceived to be set
*before* the reference counter is decremented.

- See Documentation/core-api/atomic_ops.rst for more information. See the
- "Atomic operations" subsection for information on where to use these.
+ See Documentation/atomic_t.txt for more information.


(*) lockless_dereference();
@@ -2503,88 +2502,7 @@ operations are noted specially as some o
some don't, but they're very heavily relied on as a group throughout the
kernel.

-Any atomic operation that modifies some state in memory and returns information
-about the state (old or new) implies an SMP-conditional general memory barrier
-(smp_mb()) on each side of the actual operation (with the exception of
-explicit lock operations, described later). These include:
-
- xchg();
- atomic_xchg(); atomic_long_xchg();
- atomic_inc_return(); atomic_long_inc_return();
- atomic_dec_return(); atomic_long_dec_return();
- atomic_add_return(); atomic_long_add_return();
- atomic_sub_return(); atomic_long_sub_return();
- atomic_inc_and_test(); atomic_long_inc_and_test();
- atomic_dec_and_test(); atomic_long_dec_and_test();
- atomic_sub_and_test(); atomic_long_sub_and_test();
- atomic_add_negative(); atomic_long_add_negative();
- test_and_set_bit();
- test_and_clear_bit();
- test_and_change_bit();
-
- /* when succeeds */
- cmpxchg();
- atomic_cmpxchg(); atomic_long_cmpxchg();
- atomic_add_unless(); atomic_long_add_unless();
-
-These are used for such things as implementing ACQUIRE-class and RELEASE-class
-operations and adjusting reference counters towards object destruction, and as
-such the implicit memory barrier effects are necessary.
-
-
-The following operations are potential problems as they do _not_ imply memory
-barriers, but might be used for implementing such things as RELEASE-class
-operations:
-
- atomic_set();
- set_bit();
- clear_bit();
- change_bit();
-
-With these the appropriate explicit memory barrier should be used if necessary
-(smp_mb__before_atomic() for instance).
-
-
-The following also do _not_ imply memory barriers, and so may require explicit
-memory barriers under some circumstances (smp_mb__before_atomic() for
-instance):
-
- atomic_add();
- atomic_sub();
- atomic_inc();
- atomic_dec();
-
-If they're used for statistics generation, then they probably don't need memory
-barriers, unless there's a coupling between statistical data.
-
-If they're used for reference counting on an object to control its lifetime,
-they probably don't need memory barriers because either the reference count
-will be adjusted inside a locked section, or the caller will already hold
-sufficient references to make the lock, and thus a memory barrier unnecessary.
-
-If they're used for constructing a lock of some description, then they probably
-do need memory barriers as a lock primitive generally has to do things in a
-specific order.
-
-Basically, each usage case has to be carefully considered as to whether memory
-barriers are needed or not.
-
-The following operations are special locking primitives:
-
- test_and_set_bit_lock();
- clear_bit_unlock();
- __clear_bit_unlock();
-
-These implement ACQUIRE-class and RELEASE-class operations. These should be
-used in preference to other operations when implementing locking primitives,
-because their implementations can be optimised on many architectures.
-
-[!] Note that special memory barrier primitives are available for these
-situations because on some CPUs the atomic instructions used imply full memory
-barriers, and so barrier instructions are superfluous in conjunction with them,
-and in such cases the special barrier primitives will be no-ops.
-
-See Documentation/core-api/atomic_ops.rst for more information.
+See Documentation/atomic_t.txt for more information.


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