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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// This file implements runtime support for signal handling.
//
// Most synchronization primitives are not available from
// the signal handler (it cannot block, allocate memory, or use locks)
// so the handler communicates with a processing goroutine
// via struct sig, below.
//
// sigsend is called by the signal handler to queue a new signal.
// signal_recv is called by the Go program to receive a newly queued signal.
// Synchronization between sigsend and signal_recv is based on the sig.state
// variable. It can be in 3 states: sigIdle, sigReceiving and sigSending.
// sigReceiving means that signal_recv is blocked on sig.Note and there are no
// new pending signals.
// sigSending means that sig.mask *may* contain new pending signals,
// signal_recv can't be blocked in this state.
// sigIdle means that there are no new pending signals and signal_recv is not blocked.
// Transitions between states are done atomically with CAS.
// When signal_recv is unblocked, it resets sig.Note and rechecks sig.mask.
// If several sigsends and signal_recv execute concurrently, it can lead to
// unnecessary rechecks of sig.mask, but it cannot lead to missed signals
// nor deadlocks.
// +build !plan9
package runtime
import (
"runtime/internal/atomic"
_ "unsafe" // for go:linkname
)
// sig handles communication between the signal handler and os/signal.
// Other than the inuse and recv fields, the fields are accessed atomically.
//
// The wanted and ignored fields are only written by one goroutine at
// a time; access is controlled by the handlers Mutex in os/signal.
// The fields are only read by that one goroutine and by the signal handler.
// We access them atomically to minimize the race between setting them
// in the goroutine calling os/signal and the signal handler,
// which may be running in a different thread. That race is unavoidable,
// as there is no connection between handling a signal and receiving one,
// but atomic instructions should minimize it.
var sig struct {
note note
mask [(_NSIG + 31) / 32]uint32
wanted [(_NSIG + 31) / 32]uint32
ignored [(_NSIG + 31) / 32]uint32
recv [(_NSIG + 31) / 32]uint32
state uint32
delivering uint32
inuse bool
}
const (
sigIdle = iota
sigReceiving
sigSending
)
// sigsend delivers a signal from sighandler to the internal signal delivery queue.
// It reports whether the signal was sent. If not, the caller typically crashes the program.
// It runs from the signal handler, so it's limited in what it can do.
func sigsend(s uint32) bool {
bit := uint32(1) << uint(s&31)
if !sig.inuse || s >= uint32(32*len(sig.wanted)) {
return false
}
atomic.Xadd(&sig.delivering, 1)
// We are running in the signal handler; defer is not available.
if w := atomic.Load(&sig.wanted[s/32]); w&bit == 0 {
atomic.Xadd(&sig.delivering, -1)
return false
}
// Add signal to outgoing queue.
for {
mask := sig.mask[s/32]
if mask&bit != 0 {
atomic.Xadd(&sig.delivering, -1)
return true // signal already in queue
}
if atomic.Cas(&sig.mask[s/32], mask, mask|bit) {
break
}
}
// Notify receiver that queue has new bit.
Send:
for {
switch atomic.Load(&sig.state) {
default:
throw("sigsend: inconsistent state")
case sigIdle:
if atomic.Cas(&sig.state, sigIdle, sigSending) {
break Send
}
case sigSending:
// notification already pending
break Send
case sigReceiving:
if atomic.Cas(&sig.state, sigReceiving, sigIdle) {
notewakeup(&sig.note)
break Send
}
}
}
atomic.Xadd(&sig.delivering, -1)
return true
}
// Called to receive the next queued signal.
// Must only be called from a single goroutine at a time.
//go:linkname signal_recv os/signal.signal_recv
func signal_recv() uint32 {
for {
// Serve any signals from local copy.
for i := uint32(0); i < _NSIG; i++ {
if sig.recv[i/32]&(1<<(i&31)) != 0 {
sig.recv[i/32] &^= 1 << (i & 31)
return i
}
}
// Wait for updates to be available from signal sender.
Receive:
for {
switch atomic.Load(&sig.state) {
default:
throw("signal_recv: inconsistent state")
case sigIdle:
if atomic.Cas(&sig.state, sigIdle, sigReceiving) {
notetsleepg(&sig.note, -1)
noteclear(&sig.note)
break Receive
}
case sigSending:
if atomic.Cas(&sig.state, sigSending, sigIdle) {
break Receive
}
}
}
// Incorporate updates from sender into local copy.
for i := range sig.mask {
sig.recv[i] = atomic.Xchg(&sig.mask[i], 0)
}
}
}
// signalWaitUntilIdle waits until the signal delivery mechanism is idle.
// This is used to ensure that we do not drop a signal notification due
// to a race between disabling a signal and receiving a signal.
// This assumes that signal delivery has already been disabled for
// the signal(s) in question, and here we are just waiting to make sure
// that all the signals have been delivered to the user channels
// by the os/signal package.
//go:linkname signalWaitUntilIdle os/signal.signalWaitUntilIdle
func signalWaitUntilIdle() {
// Although the signals we care about have been removed from
// sig.wanted, it is possible that another thread has received
// a signal, has read from sig.wanted, is now updating sig.mask,
// and has not yet woken up the processor thread. We need to wait
// until all current signal deliveries have completed.
for atomic.Load(&sig.delivering) != 0 {
Gosched()
}
// Although WaitUntilIdle seems like the right name for this
// function, the state we are looking for is sigReceiving, not
// sigIdle. The sigIdle state is really more like sigProcessing.
for atomic.Load(&sig.state) != sigReceiving {
Gosched()
}
}
// Must only be called from a single goroutine at a time.
//go:linkname signal_enable os/signal.signal_enable
func signal_enable(s uint32) {
if !sig.inuse {
// The first call to signal_enable is for us
// to use for initialization. It does not pass
// signal information in m.
sig.inuse = true // enable reception of signals; cannot disable
noteclear(&sig.note)
return
}
if s >= uint32(len(sig.wanted)*32) {
return
}
w := sig.wanted[s/32]
w |= 1 << (s & 31)
atomic.Store(&sig.wanted[s/32], w)
i := sig.ignored[s/32]
i &^= 1 << (s & 31)
atomic.Store(&sig.ignored[s/32], i)
sigenable(s)
}
// Must only be called from a single goroutine at a time.
//go:linkname signal_disable os/signal.signal_disable
func signal_disable(s uint32) {
if s >= uint32(len(sig.wanted)*32) {
return
}
sigdisable(s)
w := sig.wanted[s/32]
w &^= 1 << (s & 31)
atomic.Store(&sig.wanted[s/32], w)
}
// Must only be called from a single goroutine at a time.
//go:linkname signal_ignore os/signal.signal_ignore
func signal_ignore(s uint32) {
if s >= uint32(len(sig.wanted)*32) {
return
}
sigignore(s)
w := sig.wanted[s/32]
w &^= 1 << (s & 31)
atomic.Store(&sig.wanted[s/32], w)
i := sig.ignored[s/32]
i |= 1 << (s & 31)
atomic.Store(&sig.ignored[s/32], i)
}
// sigInitIgnored marks the signal as already ignored. This is called at
// program start by initsig. In a shared library initsig is called by
// libpreinit, so the runtime may not be initialized yet.
//go:nosplit
func sigInitIgnored(s uint32) {
i := sig.ignored[s/32]
i |= 1 << (s & 31)
atomic.Store(&sig.ignored[s/32], i)
}
// Checked by signal handlers.
//go:linkname signal_ignored os/signal.signal_ignored
func signal_ignored(s uint32) bool {
i := atomic.Load(&sig.ignored[s/32])
return i&(1<<(s&31)) != 0
}