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.Dd July xx, 2016 .Dt LOCALCOUNT 9 .Os .Sh NAME .Nm localcount , .Nm localcount_init , .Nm localcount_fini , .Nm localcount_acquire , .Nm localcount_release , .Nm localcount_drain , .Sh SYNOPSIS n sys/localcount.h .Ft int .Fn localcount_init "struct localcount *lc" .Ft void .Fn localcount_fini "struct localcount *lc" .Ft void .Fn localcount_acquire "struct localcount *lc" .Ft void .Fn localcount_release "struct localcount *lc" "struct kcondvar *cv" \ "struct kmutex *mtx" .Ft void .Fn localcount_drain "struct localcount *lc" "struct kcondvar *cv" \ "struct kmutex *mtx" .Sh DESCRIPTION Localcounts are used in the kernel to implement a medium-weight reference counting mechanism. During normal operations, localcounts do not need the interprocessor synchronization associated with .Xr atomic_ops 3 atomic memory operations, and (unlike .Xr psref 9 ) .Nm references can be held across sleeps and can migrate between CPUs. Draining a .Nm localcount requires more expensive interprocessor synchronization than .Xr atomic_ops 3 (similar to .Xr psref 9 ) . And .Nm references require eight bytes of memory per object per-CPU, significantly more than .Xr atomic_ops 3 and almost always more than .Xr psref 9 .
p As a rough heuristic, .Nm should be used for classes of objects of which there are maybe a few dozen instances but not a few thousand instances (e.g. autoconf devices, but not network flows) and on which there may be a mixture of long-term I/O waits, such as xyzread for a device xyz(4), and short-term fast operations, such as xyzioctl(IOC_READ_A_CPU_REG). .Sh FUNCTIONS l -tag -width abcd t Fn localcount_init "lc"
p Dynamically initialize a localcount for use.
p No other operations can be performed on a localcount until it has been initialized. t Fn localcount_fini "lc"
p Release resources used by a localcount. The caller must have already called .fn localcount_drain . The localcount may not be used after .Fn localcount_fini has been called until it has been re-initialized by .Fn localcount_init . t Fn localcount_acquire "lc"
p Acquire a reference to the localcount. t Fn localcount_release "lc" "cv" "mtx"
p Release a reference to the localcount. Must be called with the mutex .Ar mtx locked. If the localcount is being drained, and the reference count goes to zero, .Fn localcout_release will broadcast availability of the condvar .Ar cv . t Fn localcount_drain "lc" "cv" "mtx"
p Wait for all references to the .Nm to be released. The caller must hold the mutex .Ar mtx ; the mutex will be released during inter-CPU cross-calls (see .Xr xcall 9 ) and while waiting on the condvar .Ar cv . The same .Ar cv and .Ar mtx must be used with .Fn localcount_release .
p The caller must guarantee that no new references can be acquired with localcount_acquire before calling .Fn localcount_drain . For example, any object that may be found in a list and acquired must be removed from the list before calling .Fn localcount_drain . Once the localcount object .Ar lc is passed to .Fn localcount_drain , it must be passed to .Fn localcount_fini before any other re-use. .Sh CODE REFERENCES The core of the localcount implementation is in
a sys/kern/subr_localcount.c .
p The header file
a sys/sys/localcount.h describes the public interface, and interfaces that machine-dependent code must provide to support localcounts. .Sh SEE ALSO .Xr atomic_ops 3 , .Xr condvar 9 , .Xr mutex 9 , and .Xr psref 9 .Sh HISTORY The localcount primitives first appeared in .Nx 8.0 . .Sh AUTHORS This implementation of .Nm was written by .An Taylor R. Campbell .
Indexes created Mon Oct 20 20:10:13 GMT 2025