diff options
Diffstat (limited to 'libgo/go/runtime/proc.go')
| -rw-r--r-- | libgo/go/runtime/proc.go | 330 |
1 files changed, 330 insertions, 0 deletions
diff --git a/libgo/go/runtime/proc.go b/libgo/go/runtime/proc.go new file mode 100644 index 000000000000..fa90a282866f --- /dev/null +++ b/libgo/go/runtime/proc.go @@ -0,0 +1,330 @@ +// Copyright 2014 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. + +package runtime + +import ( + "runtime/internal/atomic" + "unsafe" +) + +// Functions temporarily called by C code. +//go:linkname newextram runtime.newextram + +// Functions temporarily in C that have not yet been ported. +func allocm(*p, bool, *unsafe.Pointer, *uintptr) *m +func malg(bool, bool, *unsafe.Pointer, *uintptr) *g +func allgadd(*g) + +// C functions for ucontext management. +func setGContext() +func makeGContext(*g, unsafe.Pointer, uintptr) + +// main_init_done is a signal used by cgocallbackg that initialization +// has been completed. It is made before _cgo_notify_runtime_init_done, +// so all cgo calls can rely on it existing. When main_init is complete, +// it is closed, meaning cgocallbackg can reliably receive from it. +var main_init_done chan bool + +// If asked to move to or from a Gscanstatus this will throw. Use the castogscanstatus +// and casfrom_Gscanstatus instead. +// casgstatus will loop if the g->atomicstatus is in a Gscan status until the routine that +// put it in the Gscan state is finished. +//go:nosplit +func casgstatus(gp *g, oldval, newval uint32) { + if (oldval&_Gscan != 0) || (newval&_Gscan != 0) || oldval == newval { + systemstack(func() { + print("runtime: casgstatus: oldval=", hex(oldval), " newval=", hex(newval), "\n") + throw("casgstatus: bad incoming values") + }) + } + + if oldval == _Grunning && gp.gcscanvalid { + // If oldvall == _Grunning, then the actual status must be + // _Grunning or _Grunning|_Gscan; either way, + // we own gp.gcscanvalid, so it's safe to read. + // gp.gcscanvalid must not be true when we are running. + print("runtime: casgstatus ", hex(oldval), "->", hex(newval), " gp.status=", hex(gp.atomicstatus), " gp.gcscanvalid=true\n") + throw("casgstatus") + } + + // See http://golang.org/cl/21503 for justification of the yield delay. + const yieldDelay = 5 * 1000 + var nextYield int64 + + // loop if gp->atomicstatus is in a scan state giving + // GC time to finish and change the state to oldval. + for i := 0; !atomic.Cas(&gp.atomicstatus, oldval, newval); i++ { + if oldval == _Gwaiting && gp.atomicstatus == _Grunnable { + systemstack(func() { + throw("casgstatus: waiting for Gwaiting but is Grunnable") + }) + } + // Help GC if needed. + // if gp.preemptscan && !gp.gcworkdone && (oldval == _Grunning || oldval == _Gsyscall) { + // gp.preemptscan = false + // systemstack(func() { + // gcphasework(gp) + // }) + // } + // But meanwhile just yield. + if i == 0 { + nextYield = nanotime() + yieldDelay + } + if nanotime() < nextYield { + for x := 0; x < 10 && gp.atomicstatus != oldval; x++ { + procyield(1) + } + } else { + osyield() + nextYield = nanotime() + yieldDelay/2 + } + } + if newval == _Grunning && gp.gcscanvalid { + // Run queueRescan on the system stack so it has more space. + systemstack(func() { queueRescan(gp) }) + } +} + +// needm is called when a cgo callback happens on a +// thread without an m (a thread not created by Go). +// In this case, needm is expected to find an m to use +// and return with m, g initialized correctly. +// Since m and g are not set now (likely nil, but see below) +// needm is limited in what routines it can call. In particular +// it can only call nosplit functions (textflag 7) and cannot +// do any scheduling that requires an m. +// +// In order to avoid needing heavy lifting here, we adopt +// the following strategy: there is a stack of available m's +// that can be stolen. Using compare-and-swap +// to pop from the stack has ABA races, so we simulate +// a lock by doing an exchange (via casp) to steal the stack +// head and replace the top pointer with MLOCKED (1). +// This serves as a simple spin lock that we can use even +// without an m. The thread that locks the stack in this way +// unlocks the stack by storing a valid stack head pointer. +// +// In order to make sure that there is always an m structure +// available to be stolen, we maintain the invariant that there +// is always one more than needed. At the beginning of the +// program (if cgo is in use) the list is seeded with a single m. +// If needm finds that it has taken the last m off the list, its job +// is - once it has installed its own m so that it can do things like +// allocate memory - to create a spare m and put it on the list. +// +// Each of these extra m's also has a g0 and a curg that are +// pressed into service as the scheduling stack and current +// goroutine for the duration of the cgo callback. +// +// When the callback is done with the m, it calls dropm to +// put the m back on the list. +//go:nosplit +func needm(x byte) { + if iscgo && !cgoHasExtraM { + // Can happen if C/C++ code calls Go from a global ctor. + // Can not throw, because scheduler is not initialized yet. + write(2, unsafe.Pointer(&earlycgocallback[0]), int32(len(earlycgocallback))) + exit(1) + } + + // Lock extra list, take head, unlock popped list. + // nilokay=false is safe here because of the invariant above, + // that the extra list always contains or will soon contain + // at least one m. + mp := lockextra(false) + + // Set needextram when we've just emptied the list, + // so that the eventual call into cgocallbackg will + // allocate a new m for the extra list. We delay the + // allocation until then so that it can be done + // after exitsyscall makes sure it is okay to be + // running at all (that is, there's no garbage collection + // running right now). + mp.needextram = mp.schedlink == 0 + unlockextra(mp.schedlink.ptr()) + + // Save and block signals before installing g. + // Once g is installed, any incoming signals will try to execute, + // but we won't have the sigaltstack settings and other data + // set up appropriately until the end of minit, which will + // unblock the signals. This is the same dance as when + // starting a new m to run Go code via newosproc. + msigsave(mp) + sigblock() + + // Install g (= m->curg). + setg(mp.curg) + atomic.Store(&mp.curg.atomicstatus, _Gsyscall) + setGContext() + + // Initialize this thread to use the m. + minit() +} + +var earlycgocallback = []byte("fatal error: cgo callback before cgo call\n") + +// newextram allocates m's and puts them on the extra list. +// It is called with a working local m, so that it can do things +// like call schedlock and allocate. +func newextram() { + c := atomic.Xchg(&extraMWaiters, 0) + if c > 0 { + for i := uint32(0); i < c; i++ { + oneNewExtraM() + } + } else { + // Make sure there is at least one extra M. + mp := lockextra(true) + unlockextra(mp) + if mp == nil { + oneNewExtraM() + } + } +} + +// oneNewExtraM allocates an m and puts it on the extra list. +func oneNewExtraM() { + // Create extra goroutine locked to extra m. + // The goroutine is the context in which the cgo callback will run. + // The sched.pc will never be returned to, but setting it to + // goexit makes clear to the traceback routines where + // the goroutine stack ends. + var g0SP unsafe.Pointer + var g0SPSize uintptr + mp := allocm(nil, true, &g0SP, &g0SPSize) + gp := malg(true, false, nil, nil) + gp.gcscanvalid = true // fresh G, so no dequeueRescan necessary + gp.gcRescan = -1 + + // malg returns status as Gidle, change to Gdead before adding to allg + // where GC will see it. + // gccgo uses Gdead here, not Gsyscall, because the split + // stack context is not initialized. + casgstatus(gp, _Gidle, _Gdead) + gp.m = mp + mp.curg = gp + mp.locked = _LockInternal + mp.lockedg = gp + gp.lockedm = mp + gp.goid = int64(atomic.Xadd64(&sched.goidgen, 1)) + if raceenabled { + gp.racectx = racegostart(funcPC(newextram)) + } + // put on allg for garbage collector + allgadd(gp) + + // The context for gp will be set up in needm. + // Here we need to set the context for g0. + makeGContext(mp.g0, g0SP, g0SPSize) + + // Add m to the extra list. + mnext := lockextra(true) + mp.schedlink.set(mnext) + unlockextra(mp) +} + +// dropm is called when a cgo callback has called needm but is now +// done with the callback and returning back into the non-Go thread. +// It puts the current m back onto the extra list. +// +// The main expense here is the call to signalstack to release the +// m's signal stack, and then the call to needm on the next callback +// from this thread. It is tempting to try to save the m for next time, +// which would eliminate both these costs, but there might not be +// a next time: the current thread (which Go does not control) might exit. +// If we saved the m for that thread, there would be an m leak each time +// such a thread exited. Instead, we acquire and release an m on each +// call. These should typically not be scheduling operations, just a few +// atomics, so the cost should be small. +// +// TODO(rsc): An alternative would be to allocate a dummy pthread per-thread +// variable using pthread_key_create. Unlike the pthread keys we already use +// on OS X, this dummy key would never be read by Go code. It would exist +// only so that we could register at thread-exit-time destructor. +// That destructor would put the m back onto the extra list. +// This is purely a performance optimization. The current version, +// in which dropm happens on each cgo call, is still correct too. +// We may have to keep the current version on systems with cgo +// but without pthreads, like Windows. +func dropm() { + // Clear m and g, and return m to the extra list. + // After the call to setg we can only call nosplit functions + // with no pointer manipulation. + mp := getg().m + + // Block signals before unminit. + // Unminit unregisters the signal handling stack (but needs g on some systems). + // Setg(nil) clears g, which is the signal handler's cue not to run Go handlers. + // It's important not to try to handle a signal between those two steps. + sigmask := mp.sigmask + sigblock() + unminit() + + // gccgo sets the stack to Gdead here, because the splitstack + // context is not initialized. + mp.curg.atomicstatus = _Gdead + mp.curg.gcstack = nil + mp.curg.gcnextsp = nil + + mnext := lockextra(true) + mp.schedlink.set(mnext) + + setg(nil) + + // Commit the release of mp. + unlockextra(mp) + + msigrestore(sigmask) +} + +// A helper function for EnsureDropM. +func getm() uintptr { + return uintptr(unsafe.Pointer(getg().m)) +} + +var extram uintptr +var extraMWaiters uint32 + +// lockextra locks the extra list and returns the list head. +// The caller must unlock the list by storing a new list head +// to extram. If nilokay is true, then lockextra will +// return a nil list head if that's what it finds. If nilokay is false, +// lockextra will keep waiting until the list head is no longer nil. +//go:nosplit +func lockextra(nilokay bool) *m { + const locked = 1 + + incr := false + for { + old := atomic.Loaduintptr(&extram) + if old == locked { + yield := osyield + yield() + continue + } + if old == 0 && !nilokay { + if !incr { + // Add 1 to the number of threads + // waiting for an M. + // This is cleared by newextram. + atomic.Xadd(&extraMWaiters, 1) + incr = true + } + usleep(1) + continue + } + if atomic.Casuintptr(&extram, old, locked) { + return (*m)(unsafe.Pointer(old)) + } + yield := osyield + yield() + continue + } +} + +//go:nosplit +func unlockextra(mp *m) { + atomic.Storeuintptr(&extram, uintptr(unsafe.Pointer(mp))) +} |