Messages in this thread Patch in this message | | | Date | Fri, 9 Jun 2017 11:24:50 +0200 | From | Peter Zijlstra <> | Subject | [RFC][PATCH]: documentation,atomic: Add a new atomic_t document |
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Since we've vastly expanded the atomic_t interface in recent years the existing documentation is woefully out of date and people seem to get confused a bit.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> ---
--- /dev/null 2017-05-05 13:16:22.636212333 +0200 +++ b/Documentation/atomic_t.txt 2017-06-09 11:05:31.501599153 +0200 @@ -0,0 +1,147 @@ + +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 (it specifically does not order/work/etc. on +IO). + +The 'full' API consists of: + +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,notand}() + atomic_fetch_{and,or,xor,notand}{,_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() + + + +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: + + CPU0 CPU1 + + val = atomic_read(&X) + do { + atomic_set(&X, 0) + new = val + 1; + } while (!atomic_try_cmpxchg(&X, &val, new)); + +Should cause the cmpxchg to *FAIL* (when @val != 0). This is typically true; +on 'normal' platforms; a regular competing STORE will invalidate a LL/SC. + +The obvious case where this is not so is where we need to implement atomic ops +with a spinlock hashtable; 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 Sequentially Consistent; + + - 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 is an ACQUIRE + {}_release: the W of the RmW is a RELEASE + +NOTE: our ACQUIRE/RELEASE are RCpc + + +The barriers: + + smp_mb__{before,after}_atomic() + +only apply to the RmW ops and can be used to augment/upgrade the ordering +inherit 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 SC atomics and these barriers are no-ops. + +So while something like: + + smp_mb__before_atomic(); + val = atomic_dec_return_relaxed(&X); + +is a 'typical' RELEASE pattern (please use atomic_dec_return_release()), the +barrier is strictly stronger than a RELEASE.
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