mdoc(7) police:
- make SYNOPSIS look better with some troff magic - tidy up the markup
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@ -39,77 +39,87 @@
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.Sh SYNOPSIS
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.In sys/types.h
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.In machine/atomic.h
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.\" XXX
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.ds LB \f[R]\(lB\f[P]
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.ds RB \f[R]\(rB\f[P]
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.ds La \f[R]\(la\f[P]
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.ds Ra \f[R]\(ra\f[P]
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.Ft void
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.Fn atomic_add_{acq_,rel_,}<type> "volatile <type> *p" "<type> v"
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.Fn atomic_add_\*[LB]acq_\*[Ba]rel_\*[RB]\*[La]type\*[Ra] "volatile \*[La]type\*[Ra] *p" "\*[La]type\*[Ra] v"
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.Ft void
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.Fn atomic_clear_{acq_,rel_,}<type> "volatile <type> *p" "<type> v"
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.Fn atomic_clear_\*[LB]acq_\*[Ba]rel_\*[RB]\*[La]type\*[Ra] "volatile \*[La]type\*[Ra] *p" "\*[La]type\*[Ra] v"
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.Ft int
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.Fo atomic_cmpset_{acq_,rel_,}<type>
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.Fa "volatile <type> *dst"
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.Fa "<type> old"
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.Fa "<type> new"
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.Fo atomic_cmpset_\*[LB]acq_\*[Ba]rel_\*[RB]\*[La]type\*[Ra]
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.Fa "volatile \*[La]type\*[Ra] *dst"
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.Fa "\*[La]type\*[Ra] old"
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.Fa "\*[La]type\*[Ra] new"
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.Fc
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.Ft <type>
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.Fn atomic_load_acq_<type> "volatile <type> *p"
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.Ft <type>
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.Fn atomic_readandclear_<type> "volatile <type> *p"
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.Ft \*[La]type\*[Ra]
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.Fn atomic_load_acq_\*[La]type\*[Ra] "volatile \*[La]type\*[Ra] *p"
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.Ft \*[La]type\*[Ra]
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.Fn atomic_readandclear_\*[La]type\*[Ra] "volatile \*[La]type\*[Ra] *p"
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.Ft void
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.Fn atomic_set_{acq_,rel_,}<type> "volatile <type> *p" "<type> v"
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.Fn atomic_set_\*[LB]acq_\*[Ba]rel_\*[RB]\*[La]type\*[Ra] "volatile \*[La]type\*[Ra] *p" "\*[La]type\*[Ra] v"
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.Ft void
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.Fn atomic_subtract_{acq_,rel_,}<type> "volatile <type> *p" "<type> v"
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.Fn atomic_subtract_\*[LB]acq_\*[Ba]rel_\*[RB]\*[La]type\*[Ra] "volatile \*[La]type\*[Ra] *p" "\*[La]type\*[Ra] v"
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.Ft void
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.Fn atomic_store_rel_<type> "volatile <type> *p" "<type> v"
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.Fn atomic_store_rel_\*[La]type\*[Ra] "volatile \*[La]type\*[Ra] *p" "\*[La]type\*[Ra] v"
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.rm LB RB La Ra
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.Sh DESCRIPTION
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Each of the atomic operations is guaranteed to be atomic in the presence of
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interrupts.
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They can be used to implement reference counts or as building blocks for more
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advanced synchronization primitives such as mutexes.
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.Ss Types
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Each atomic operation operates on a specific type.
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Each atomic operation operates on a specific
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.Ar type .
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The type to use is indicated in the function name.
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The available types that can be used are:
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.Bl -tag -offset indent -width short
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.It int
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.Pp
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.Bl -tag -offset indent -width short -compact
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.It Li int
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unsigned integer
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.It long
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.It Li long
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unsigned long integer
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.It ptr
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.It Li ptr
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unsigned integer the size of a pointer
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.It 32
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.It Li 32
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unsigned 32-bit integer
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.It 64
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.It Li 64
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unsigned 64-bit integer
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.El
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.Pp
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For example, the function to atomically add two integers is called
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.Fn atomic_add_int .
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.Pp
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Certain architectures also provide operations for types smaller than int.
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.Bl -tag -offset indent -width short
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.It char
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Certain architectures also provide operations for types smaller than
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.Dq Li int .
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.Pp
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.Bl -tag -offset indent -width short -compact
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.It Li char
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unsigned character
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.It short
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.It Li short
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unsigned short integer
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.It 8
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.It Li 8
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unsigned 8-bit integer
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.It 16
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.It Li 16
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unsigned 16-bit integer
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.El
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.Pp
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These must not be used in MI code because the instructions to implement them
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efficiently may not be available.
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.Ss Memory Barriers
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Memory barriers are used to guarantee the order the order of data accesses in
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Memory barriers are used to guarantee the order of data accesses in
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two ways.
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First, they specify hints to the compiler to not re-order or optimize the
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operations.
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Secondly, on architectures that do not guarantee ordered data accesses,
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Second, on architectures that do not guarantee ordered data accesses,
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special instructions or special variants of instructions are used to indicate
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to the processor that data accesses need to occur in a certain order.
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As a result, most of the atomic operations have three variants in order to
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include optional memory barriers.
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The first form just performs the operation without any explicit barriers.
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The second form uses a read memory barrier, and the final variant uses a write
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The second form uses a read memory barrier, and the third variant uses a write
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memory barrier.
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.Pp
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The second variant of each operation includes a read memory barrier.
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@ -118,9 +128,9 @@ effects of any later data accesses.
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As a result, the operation is said to have acquire semantics as it acquires a
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pseudo-lock requiring further operations to wait until it has completed.
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To denote this, the suffix
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.Dq _acq
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.Dq Li _acq
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is inserted into the function name immediately prior to the
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.Em _type
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.Dq Li _ Ns Aq Ar type
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suffix.
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For example, to subtract two integers ensuring that any later writes will
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happen after the subtraction is performed, use
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@ -132,9 +142,9 @@ before this operation takes place.
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As a result, the operation is said to have release semantics as it releases
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any pending data accesses to be completed before its operation is performed.
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To denote this, the suffix
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.Dq _rel
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.Dq Li _rel
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is inserted into the function name immediately prior to the
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.Em _type
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.Dq Li _ Ns Aq Ar type
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suffix.
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For example, to add two long integers ensuring that all previous
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writes will happen first, use
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@ -146,7 +156,6 @@ To achieve this, one would use a read barrier when acquiring the lock to
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guarantee that the lock is held before any protected operations are performed.
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Finally, one would use a write barrier when releasing the lock to ensure that
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all of the protected operations are completed before the lock is released.
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.Pp
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.Ss Multiple Processors
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The current set of atomic operations do not necessarily guarantee atomicity
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across multiple processors.
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@ -166,16 +175,16 @@ using a caching policy of either uncached or write back.
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.Ss Semantics
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This section describes the semantics of each operation using a C like notation.
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.Bl -hang
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.It Fn atomic_add "p" "v"
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.Bd -literal
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.It Fn atomic_add p v
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.Bd -literal -compact
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*p += v;
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.Ed
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.It Fn atomic_clear "p" "v"
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.Bd -literal
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.It Fn atomic_clear p v
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.Bd -literal -compact
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*p &= ~v;
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.Ed
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.It Fn atomic_cmpset "dst" "old" "new"
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.Bd -literal
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.It Fn atomic_cmpset dst old new
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.Bd -literal -compact
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if (*dst == old) {
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*dst = new;
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return 1;
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@ -186,10 +195,15 @@ if (*dst == old) {
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.Pp
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The
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.Fn atomic_cmpset
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functions are not implemented for the types char, short, 8, and 16.
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functions are not implemented for the types
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.Dq Li char ,
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.Dq Li short ,
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.Dq Li 8 ,
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and
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.Dq Li 16 .
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.Bl -hang
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.It Fn atomic_load "addr"
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.Bd -literal
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.It Fn atomic_load addr
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.Bd -literal -compact
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return (*addr)
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.Ed
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.El
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@ -198,8 +212,8 @@ The
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.Fn atomic_load
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functions always have acquire semantics.
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.Bl -hang
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.It Fn atomic_readandclear "addr"
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.Bd -literal
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.It Fn atomic_readandclear addr
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.Bd -literal -compact
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temp = *addr;
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*addr = 0;
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return (temp);
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@ -208,19 +222,26 @@ return (temp);
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.Pp
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The
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.Fn atomic_readandclear
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functions are not implemented for the types char, short, ptr, 8, and 16 and do
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functions are not implemented for the types
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.Dq Li char ,
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.Dq Li short ,
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.Dq Li ptr ,
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.Dq Li 8 ,
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and
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.Dq Li 16
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and do
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not have any variants with memory barriers at this time.
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.Bl -hang
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.It Fn atomic_set "p" "v"
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.Bd -literal
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.It Fn atomic_set p v
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.Bd -literal -compact
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*p |= v;
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.Ed
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.It Fn atomic_subtract "p" "v"
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.Bd -literal
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.It Fn atomic_subtract p v
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.Bd -literal -compact
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*p -= v;
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.Ed
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.It Fn atomic_store "p" "v"
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.Bd -literal
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.It Fn atomic_store p v
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.Bd -literal -compact
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*p = v;
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.Ed
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.El
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@ -230,16 +251,20 @@ The
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functions always have release semantics.
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.Pp
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The type
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.Dq 64
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.Dq Li 64
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is currently not implemented for any of the atomic operations on the
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.Tn i386
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architecture.
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.Sh RETURN VALUES
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The
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.Fn atomic_cmpset
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function
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returns the result of the compare operation.
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The
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.Fn atomic_load
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and
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.Fn atomic_readandclear
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functions
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return the value at the specified address.
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.Sh EXAMPLES
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This example uses the
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@ -250,9 +275,9 @@ functions to obtain a sleep mutex and handle recursion.
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Since the
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.Va mtx_lock
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member of a
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.Li struct mtx
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.Vt "struct mtx"
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is a pointer, the
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.Dq ptr
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.Dq Li ptr
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type is used.
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.Bd -literal
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#define _obtain_lock(mp, tid) \\
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@ -279,7 +304,12 @@ and
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.Fn atomic_subtract
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operations were first introduced in
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.Fx 3.0 .
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This first set only supported the types char, short, int, and long.
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This first set only supported the types
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.Dq Li char ,
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.Dq Li short ,
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.Dq Li int ,
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and
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.Dq Li long .
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The
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.Fn atomic_cmpset ,
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.Fn atomic_load ,
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@ -288,7 +318,14 @@ and
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.Fn atomic_store
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operations were added in
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.Fx 5.0 .
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The types 8, 16, 32, 64, and ptr and all of the acquire and release variants
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The types
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.Dq Li 8 ,
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.Dq Li 16 ,
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.Dq Li 32 ,
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.Dq Li 64 ,
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and
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.Dq Li ptr ,
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and all of the acquire and release variants
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were added in
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.Fx 5.0
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as well.
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