CPU_SWITCHTO(9) | Kernel Developer's Manual | CPU_SWITCHTO(9)

# NAME

`cpu_switchto` — machine-dependent LWP context switching interface

# SYNOPSIS

`#include <sys/cpu.h>`

lwp_t *
`cpu_switchto`(lwp_t *oldlwp, lwp_t *newlwp, bool returning);

# DESCRIPTION

The `cpu_switchto`() function saves the context of the LWP which is currently running on the processor, and restores the context of the LWP specified by newlwp.

Remarks:

1. `cpu_switchto`() does not switch address spaces.
2. `cpu_switchto`() sets curlwp(9) to newlwp. If the architecture does non-interlocked adaptive mutex release, `cpu_switchto`() does an equivalent of membar_producer(3), before and after the modification of curlwp(9).
3. `cpu_switchto`() should be called at `IPL_SCHED`. When the function returns, the caller should lower the priority level as soon as possible.
4. `cpu_switchto`() might be called with spin mutexes held.

The function takes the following arguments.

oldlwp
  Specify the LWP from which the switch is going to be made, i.e., the calling LWP. If it was `NULL`, the context of the LWP currently running on this processor is not saved.
newlwp
  Specify the LWP to which to switch. It must not be `NULL`.
returning
  Only meaningful if the architecture implements fast software interrupts. If true, it indicates that oldlwp is a soft interrupt LWP that is blocking. It is a good indication that any kind of address space or user activity can be completely ignored. For example: ras_lookup(9), cache flushes, TLB wirings, adjusting lazy FPU state. All that is required is to restore the register state and stack, and return to the interrupted LWP.

# RETURN VALUES

The `cpu_switchto`() function does not return until another LWP calls `cpu_switchto`(). It returns the oldlwp argument of the `cpu_switchto`() which is called to switch back to our LWP. It is either a LWP which called `cpu_switchto`() to switch to us or `NULL` in case the LWP was exiting.

# SEE ALSO

membar_producer(3), swapcontext(3), intro(9), mutex(9), spl(9)

June 2, 2011 | NetBSD 11.0