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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 10:05:51 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 10:05:51 +0000 |
commit | 5d1646d90e1f2cceb9f0828f4b28318cd0ec7744 (patch) | |
tree | a94efe259b9009378be6d90eb30d2b019d95c194 /Documentation/core-api/refcount-vs-atomic.rst | |
parent | Initial commit. (diff) | |
download | linux-5d1646d90e1f2cceb9f0828f4b28318cd0ec7744.tar.xz linux-5d1646d90e1f2cceb9f0828f4b28318cd0ec7744.zip |
Adding upstream version 5.10.209.upstream/5.10.209upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'Documentation/core-api/refcount-vs-atomic.rst')
-rw-r--r-- | Documentation/core-api/refcount-vs-atomic.rst | 168 |
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diff --git a/Documentation/core-api/refcount-vs-atomic.rst b/Documentation/core-api/refcount-vs-atomic.rst new file mode 100644 index 000000000..79a009ce1 --- /dev/null +++ b/Documentation/core-api/refcount-vs-atomic.rst @@ -0,0 +1,168 @@ +=================================== +refcount_t API compared to atomic_t +=================================== + +.. contents:: :local: + +Introduction +============ + +The goal of refcount_t API is to provide a minimal API for implementing +an object's reference counters. While a generic architecture-independent +implementation from lib/refcount.c uses atomic operations underneath, +there are a number of differences between some of the ``refcount_*()`` and +``atomic_*()`` functions with regards to the memory ordering guarantees. +This document outlines the differences and provides respective examples +in order to help maintainers validate their code against the change in +these memory ordering guarantees. + +The terms used through this document try to follow the formal LKMM defined in +tools/memory-model/Documentation/explanation.txt. + +memory-barriers.txt and atomic_t.txt provide more background to the +memory ordering in general and for atomic operations specifically. + +Relevant types of memory ordering +================================= + +.. note:: The following section only covers some of the memory + ordering types that are relevant for the atomics and reference + counters and used through this document. For a much broader picture + please consult memory-barriers.txt document. + +In the absence of any memory ordering guarantees (i.e. fully unordered) +atomics & refcounters only provide atomicity and +program order (po) relation (on the same CPU). It guarantees that +each ``atomic_*()`` and ``refcount_*()`` operation is atomic and instructions +are executed in program order on a single CPU. +This is implemented using READ_ONCE()/WRITE_ONCE() and +compare-and-swap primitives. + +A strong (full) memory ordering guarantees that all prior loads and +stores (all po-earlier instructions) on the same CPU are completed +before any po-later instruction is executed on the same CPU. +It also guarantees that all po-earlier stores on the same CPU +and all propagated stores from other CPUs must propagate to all +other CPUs before any po-later instruction is executed on the original +CPU (A-cumulative property). This is implemented using smp_mb(). + +A RELEASE memory ordering guarantees that all prior loads and +stores (all po-earlier instructions) on the same CPU are completed +before the operation. It also guarantees that all po-earlier +stores on the same CPU and all propagated stores from other CPUs +must propagate to all other CPUs before the release operation +(A-cumulative property). This is implemented using +smp_store_release(). + +An ACQUIRE memory ordering guarantees that all post loads and +stores (all po-later instructions) on the same CPU are +completed after the acquire operation. It also guarantees that all +po-later stores on the same CPU must propagate to all other CPUs +after the acquire operation executes. This is implemented using +smp_acquire__after_ctrl_dep(). + +A control dependency (on success) for refcounters guarantees that +if a reference for an object was successfully obtained (reference +counter increment or addition happened, function returned true), +then further stores are ordered against this operation. +Control dependency on stores are not implemented using any explicit +barriers, but rely on CPU not to speculate on stores. This is only +a single CPU relation and provides no guarantees for other CPUs. + + +Comparison of functions +======================= + +case 1) - non-"Read/Modify/Write" (RMW) ops +------------------------------------------- + +Function changes: + + * atomic_set() --> refcount_set() + * atomic_read() --> refcount_read() + +Memory ordering guarantee changes: + + * none (both fully unordered) + + +case 2) - increment-based ops that return no value +-------------------------------------------------- + +Function changes: + + * atomic_inc() --> refcount_inc() + * atomic_add() --> refcount_add() + +Memory ordering guarantee changes: + + * none (both fully unordered) + +case 3) - decrement-based RMW ops that return no value +------------------------------------------------------ + +Function changes: + + * atomic_dec() --> refcount_dec() + +Memory ordering guarantee changes: + + * fully unordered --> RELEASE ordering + + +case 4) - increment-based RMW ops that return a value +----------------------------------------------------- + +Function changes: + + * atomic_inc_not_zero() --> refcount_inc_not_zero() + * no atomic counterpart --> refcount_add_not_zero() + +Memory ordering guarantees changes: + + * fully ordered --> control dependency on success for stores + +.. note:: We really assume here that necessary ordering is provided as a + result of obtaining pointer to the object! + + +case 5) - generic dec/sub decrement-based RMW ops that return a value +--------------------------------------------------------------------- + +Function changes: + + * atomic_dec_and_test() --> refcount_dec_and_test() + * atomic_sub_and_test() --> refcount_sub_and_test() + +Memory ordering guarantees changes: + + * fully ordered --> RELEASE ordering + ACQUIRE ordering on success + + +case 6) other decrement-based RMW ops that return a value +--------------------------------------------------------- + +Function changes: + + * no atomic counterpart --> refcount_dec_if_one() + * ``atomic_add_unless(&var, -1, 1)`` --> ``refcount_dec_not_one(&var)`` + +Memory ordering guarantees changes: + + * fully ordered --> RELEASE ordering + control dependency + +.. note:: atomic_add_unless() only provides full order on success. + + +case 7) - lock-based RMW +------------------------ + +Function changes: + + * atomic_dec_and_lock() --> refcount_dec_and_lock() + * atomic_dec_and_mutex_lock() --> refcount_dec_and_mutex_lock() + +Memory ordering guarantees changes: + + * fully ordered --> RELEASE ordering + control dependency + hold + spin_lock() on success |