coro.go

     1  // Copyright 2023 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package runtime
     6  
     7  import "unsafe"
     8  
     9  // A coro represents extra concurrency without extra parallelism,
    10  // as would be needed for a coroutine implementation.
    11  // The coro does not represent a specific coroutine, only the ability
    12  // to do coroutine-style control transfers.
    13  // It can be thought of as like a special channel that always has
    14  // a goroutine blocked on it. If another goroutine calls coroswitch(c),
    15  // the caller becomes the goroutine blocked in c, and the goroutine
    16  // formerly blocked in c starts running.
    17  // These switches continue until a call to coroexit(c),
    18  // which ends the use of the coro by releasing the blocked
    19  // goroutine in c and exiting the current goroutine.
    20  //
    21  // Coros are heap allocated and garbage collected, so that user code
    22  // can hold a pointer to a coro without causing potential dangling
    23  // pointer errors.
    24  type coro struct {
    25  	gp guintptr
    26  	f  func(*coro)
    27  }
    28  
    29  //go:linkname newcoro
    30  
    31  // newcoro creates a new coro containing a
    32  // goroutine blocked waiting to run f
    33  // and returns that coro.
    34  func newcoro(f func(*coro)) *coro {
    35  	c := new(coro)
    36  	c.f = f
    37  	pc := getcallerpc()
    38  	gp := getg()
    39  	systemstack(func() {
    40  		start := corostart
    41  		startfv := *(**funcval)(unsafe.Pointer(&start))
    42  		gp = newproc1(startfv, gp, pc)
    43  	})
    44  	gp.coroarg = c
    45  	gp.waitreason = waitReasonCoroutine
    46  	casgstatus(gp, _Grunnable, _Gwaiting)
    47  	c.gp.set(gp)
    48  	return c
    49  }
    50  
    51  //go:linkname corostart
    52  
    53  // corostart is the entry func for a new coroutine.
    54  // It runs the coroutine user function f passed to corostart
    55  // and then calls coroexit to remove the extra concurrency.
    56  func corostart() {
    57  	gp := getg()
    58  	c := gp.coroarg
    59  	gp.coroarg = nil
    60  
    61  	c.f(c)
    62  	coroexit(c)
    63  }
    64  
    65  // coroexit is like coroswitch but closes the coro
    66  // and exits the current goroutine
    67  func coroexit(c *coro) {
    68  	gp := getg()
    69  	gp.coroarg = c
    70  	gp.coroexit = true
    71  	mcall(coroswitch_m)
    72  }
    73  
    74  //go:linkname coroswitch
    75  
    76  // coroswitch switches to the goroutine blocked on c
    77  // and then blocks the current goroutine on c.
    78  func coroswitch(c *coro) {
    79  	gp := getg()
    80  	gp.coroarg = c
    81  	mcall(coroswitch_m)
    82  }
    83  
    84  // coroswitch_m is the implementation of coroswitch
    85  // that runs on the m stack.
    86  //
    87  // Note: Coroutine switches are expected to happen at
    88  // an order of magnitude (or more) higher frequency
    89  // than regular goroutine switches, so this path is heavily
    90  // optimized to remove unnecessary work.
    91  // The fast path here is three CAS: the one at the top on gp.atomicstatus,
    92  // the one in the middle to choose the next g,
    93  // and the one at the bottom on gnext.atomicstatus.
    94  // It is important not to add more atomic operations or other
    95  // expensive operations to the fast path.
    96  func coroswitch_m(gp *g) {
    97  	// TODO(rsc,mknyszek): add tracing support in a lightweight manner.
    98  	// Probably the tracer will need a global bool (set and cleared during STW)
    99  	// that this code can check to decide whether to use trace.gen.Load();
   100  	// we do not want to do the atomic load all the time, especially when
   101  	// tracer use is relatively rare.
   102  	c := gp.coroarg
   103  	gp.coroarg = nil
   104  	exit := gp.coroexit
   105  	gp.coroexit = false
   106  	mp := gp.m
   107  
   108  	if exit {
   109  		gdestroy(gp)
   110  		gp = nil
   111  	} else {
   112  		// If we can CAS ourselves directly from running to waiting, so do,
   113  		// keeping the control transfer as lightweight as possible.
   114  		gp.waitreason = waitReasonCoroutine
   115  		if !gp.atomicstatus.CompareAndSwap(_Grunning, _Gwaiting) {
   116  			// The CAS failed: use casgstatus, which will take care of
   117  			// coordinating with the garbage collector about the state change.
   118  			casgstatus(gp, _Grunning, _Gwaiting)
   119  		}
   120  
   121  		// Clear gp.m.
   122  		setMNoWB(&gp.m, nil)
   123  	}
   124  
   125  	// The goroutine stored in c is the one to run next.
   126  	// Swap it with ourselves.
   127  	var gnext *g
   128  	for {
   129  		// Note: this is a racy load, but it will eventually
   130  		// get the right value, and if it gets the wrong value,
   131  		// the c.gp.cas will fail, so no harm done other than
   132  		// a wasted loop iteration.
   133  		// The cas will also sync c.gp's
   134  		// memory enough that the next iteration of the racy load
   135  		// should see the correct value.
   136  		// We are avoiding the atomic load to keep this path
   137  		// as lightweight as absolutely possible.
   138  		// (The atomic load is free on x86 but not free elsewhere.)
   139  		next := c.gp
   140  		if next.ptr() == nil {
   141  			throw("coroswitch on exited coro")
   142  		}
   143  		var self guintptr
   144  		self.set(gp)
   145  		if c.gp.cas(next, self) {
   146  			gnext = next.ptr()
   147  			break
   148  		}
   149  	}
   150  
   151  	// Start running next, without heavy scheduling machinery.
   152  	// Set mp.curg and gnext.m and then update scheduling state
   153  	// directly if possible.
   154  	setGNoWB(&mp.curg, gnext)
   155  	setMNoWB(&gnext.m, mp)
   156  	if !gnext.atomicstatus.CompareAndSwap(_Gwaiting, _Grunning) {
   157  		// The CAS failed: use casgstatus, which will take care of
   158  		// coordinating with the garbage collector about the state change.
   159  		casgstatus(gnext, _Gwaiting, _Grunnable)
   160  		casgstatus(gnext, _Grunnable, _Grunning)
   161  	}
   162  
   163  	// Switch to gnext. Does not return.
   164  	gogo(&gnext.sched)
   165  }
   166
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