When we copy a stack of a goroutine blocked in a channel operation, we
have to be very careful because other goroutines may be writing to
that goroutine's stack. To handle this, stack copying acquires the
locks for the channels a goroutine is waiting on.
One complication is that stack growth may happen while a goroutine
holds these locks, in which case stack copying must *not* acquire
these locks because that would self-deadlock.
Currently, stack growth never acquires these locks because stack
growth only happens when a goroutine is running, which means it's
either not blocking on a channel or it's holding the channel locks
already. Stack shrinking always acquires these locks because shrinking
happens asynchronously, so the goroutine is never running, so there
are either no locks or they've been released by the goroutine.
However, we're about to change when stack shrinking can happen, which
is going to break the current rules. Rather than find a new way to
derive whether to acquire these locks or not, this CL simply adds a
flag to the g struct that indicates that stack copying should acquire
channel locks. This flag is set while the goroutine is blocked on a
channel op.
For #10958, #24543.
Change-Id: Ia2ac8831b1bfda98d39bb30285e144c4f7eaf9ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/172982
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
gp.waiting = mysg
gp.param = nil
c.sendq.enqueue(mysg)
- goparkunlock(&c.lock, waitReasonChanSend, traceEvGoBlockSend, 3)
+ gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanSend, traceEvGoBlockSend, 2)
// Ensure the value being sent is kept alive until the
// receiver copies it out. The sudog has a pointer to the
// stack object, but sudogs aren't considered as roots of the
throw("G waiting list is corrupted")
}
gp.waiting = nil
+ gp.activeStackChans = false
if gp.param == nil {
if c.closed == 0 {
throw("chansend: spurious wakeup")
mysg.c = c
gp.param = nil
c.recvq.enqueue(mysg)
- goparkunlock(&c.lock, waitReasonChanReceive, traceEvGoBlockRecv, 3)
+ gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanReceive, traceEvGoBlockRecv, 2)
// someone woke us up
if mysg != gp.waiting {
throw("G waiting list is corrupted")
}
gp.waiting = nil
+ gp.activeStackChans = false
if mysg.releasetime > 0 {
blockevent(mysg.releasetime-t0, 2)
}
goready(gp, skip+1)
}
+func chanparkcommit(gp *g, chanLock unsafe.Pointer) bool {
+ // There are unlocked sudogs that point into gp's stack. Stack
+ // copying must lock the channels of those sudogs.
+ gp.activeStackChans = true
+ unlock((*mutex)(chanLock))
+ return true
+}
+
// compiler implements
//
// select {
stackguard0 uintptr // offset known to liblink
stackguard1 uintptr // offset known to liblink
- _panic *_panic // innermost panic - offset known to liblink
- _defer *_defer // innermost defer
- m *m // current m; offset known to arm liblink
- sched gobuf
- syscallsp uintptr // if status==Gsyscall, syscallsp = sched.sp to use during gc
- syscallpc uintptr // if status==Gsyscall, syscallpc = sched.pc to use during gc
- stktopsp uintptr // expected sp at top of stack, to check in traceback
- param unsafe.Pointer // passed parameter on wakeup
- atomicstatus uint32
- stackLock uint32 // sigprof/scang lock; TODO: fold in to atomicstatus
- goid int64
- schedlink guintptr
- waitsince int64 // approx time when the g become blocked
- waitreason waitReason // if status==Gwaiting
- preempt bool // preemption signal, duplicates stackguard0 = stackpreempt
- preemptStop bool // transition to _Gpreempted on preemption; otherwise, just deschedule
- paniconfault bool // panic (instead of crash) on unexpected fault address
- gcscandone bool // g has scanned stack; protected by _Gscan bit in status
- throwsplit bool // must not split stack
- raceignore int8 // ignore race detection events
- sysblocktraced bool // StartTrace has emitted EvGoInSyscall about this goroutine
- sysexitticks int64 // cputicks when syscall has returned (for tracing)
- traceseq uint64 // trace event sequencer
- tracelastp puintptr // last P emitted an event for this goroutine
+ _panic *_panic // innermost panic - offset known to liblink
+ _defer *_defer // innermost defer
+ m *m // current m; offset known to arm liblink
+ sched gobuf
+ syscallsp uintptr // if status==Gsyscall, syscallsp = sched.sp to use during gc
+ syscallpc uintptr // if status==Gsyscall, syscallpc = sched.pc to use during gc
+ stktopsp uintptr // expected sp at top of stack, to check in traceback
+ param unsafe.Pointer // passed parameter on wakeup
+ atomicstatus uint32
+ stackLock uint32 // sigprof/scang lock; TODO: fold in to atomicstatus
+ goid int64
+ schedlink guintptr
+ waitsince int64 // approx time when the g become blocked
+ waitreason waitReason // if status==Gwaiting
+ preempt bool // preemption signal, duplicates stackguard0 = stackpreempt
+ preemptStop bool // transition to _Gpreempted on preemption; otherwise, just deschedule
+ paniconfault bool // panic (instead of crash) on unexpected fault address
+ gcscandone bool // g has scanned stack; protected by _Gscan bit in status
+ throwsplit bool // must not split stack
+ // activeStackChans indicates that there are unlocked channels
+ // pointing into this goroutine's stack. If true, stack
+ // copying needs to acquire channel locks to protect these
+ // areas of the stack.
+ activeStackChans bool
+
+ raceignore int8 // ignore race detection events
+ sysblocktraced bool // StartTrace has emitted EvGoInSyscall about this goroutine
+ sysexitticks int64 // cputicks when syscall has returned (for tracing)
+ traceseq uint64 // trace event sequencer
+ tracelastp puintptr // last P emitted an event for this goroutine
lockedm muintptr
sig uint32
writebuf []byte
}
func selparkcommit(gp *g, _ unsafe.Pointer) bool {
+ // There are unlocked sudogs that point into gp's stack. Stack
+ // copying must lock the channels of those sudogs.
+ gp.activeStackChans = true
// This must not access gp's stack (see gopark). In
// particular, it must not access the *hselect. That's okay,
// because by the time this is called, gp.waiting has all
// wait for someone to wake us up
gp.param = nil
gopark(selparkcommit, nil, waitReasonSelect, traceEvGoBlockSelect, 1)
+ gp.activeStackChans = false
sellock(scases, lockorder)
_32bit uintptr // size on 32bit platforms
_64bit uintptr // size on 64bit platforms
}{
- {runtime.G{}, 212, 368}, // g, but exported for testing
+ {runtime.G{}, 216, 376}, // g, but exported for testing
}
for _, tt := range tests {
}
// Lock channels to prevent concurrent send/receive.
- // It's important that we *only* do this for async
- // copystack; otherwise, gp may be in the middle of
- // putting itself on wait queues and this would
- // self-deadlock.
var lastc *hchan
for sg := gp.waiting; sg != nil; sg = sg.waitlink {
if sg.c != lastc {
// Copies gp's stack to a new stack of a different size.
// Caller must have changed gp status to Gcopystack.
-//
-// If sync is true, this is a self-triggered stack growth and, in
-// particular, no other G may be writing to gp's stack (e.g., via a
-// channel operation). If sync is false, copystack protects against
-// concurrent channel operations.
-func copystack(gp *g, newsize uintptr, sync bool) {
+func copystack(gp *g, newsize uintptr) {
if gp.syscallsp != 0 {
throw("stack growth not allowed in system call")
}
// Adjust sudogs, synchronizing with channel ops if necessary.
ncopy := used
- if sync {
+ if !gp.activeStackChans {
adjustsudogs(gp, &adjinfo)
} else {
- // sudogs can point in to the stack. During concurrent
- // shrinking, these areas may be written to. Find the
- // highest such pointer so we can handle everything
- // there and below carefully. (This shouldn't be far
- // from the bottom of the stack, so there's little
- // cost in handling everything below it carefully.)
+ // sudogs may be pointing in to the stack and gp has
+ // released channel locks, so other goroutines could
+ // be writing to gp's stack. Find the highest such
+ // pointer so we can handle everything there and below
+ // carefully. (This shouldn't be far from the bottom
+ // of the stack, so there's little cost in handling
+ // everything below it carefully.)
adjinfo.sghi = findsghi(gp, old)
// Synchronize with channel ops and copy the part of
// The concurrent GC will not scan the stack while we are doing the copy since
// the gp is in a Gcopystack status.
- copystack(gp, newsize, true)
+ copystack(gp, newsize)
if stackDebug >= 1 {
print("stack grow done\n")
}
print("shrinking stack ", oldsize, "->", newsize, "\n")
}
- copystack(gp, newsize, false)
+ copystack(gp, newsize)
}
// freeStackSpans frees unused stack spans at the end of GC.