// 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) } // Checked by signal handlers. func signal_ignored(s uint32) bool { i := atomic.Load(&sig.ignored[s/32]) return i&(1<<(s&31)) != 0 }