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runtime: fix G passed to schedEnabled and cleanup

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exitsyscall0 contains two G variables: _g_ and gp. _g_ is the active G,
g0, while gp is the G to run (which just exited from a syscall).

It is passing _g_ to schedEnabled, which is incorrect; we are about to
execute gp, so that is what we should be checking the schedulability of.

While this is incorrect and should be fixed, I don't think it has ever
caused a problem in practice:

 * g0 does not have g.startpc set, so schedEnabled simplifies to
   just !sched.disable.user.
 * This is correct provided gp is never a system goroutine.
 * As far as I know, system goroutines never use entersyscall /
   exitsyscall.

As far I can tell, this was a simple copy/paste error from exitsyscall,
where variable _g_ is the G to run.

While we are here, eliminate _g_ entirely, as the one other use is
identical to using gp.

Change-Id: I5df98a34569238b89ab13ff7012cd756fefb10dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/291329
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
This commit is contained in:
Michael Pratt 2021-02-11 10:44:34 -05:00
parent e0ce0af6ef
commit 4b1a24f3cd
1 changed files with 7 additions and 4 deletions
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11
src/runtime/proc.go
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@ -3856,15 +3856,15 @@ func exitsyscallfast_pidle() bool {
// exitsyscall slow path on g0.
// Failed to acquire P, enqueue gp as runnable.
//
// Called via mcall, so gp is the calling g from this M.
//
//go:nowritebarrierrec
func exitsyscall0(gp *g) {
_g_ := getg()
casgstatus(gp, _Gsyscall, _Grunnable)
dropg()
lock(&sched.lock)
var _p_ *p
if schedEnabled(_g_) {
if schedEnabled(gp) {
_p_ = pidleget()
}
if _p_ == nil {
@ -3878,8 +3878,11 @@ func exitsyscall0(gp *g) {
acquirep(_p_)
execute(gp, false) // Never returns.
}
if _g_.m.lockedg != 0 {
if gp.lockedm != 0 {
// Wait until another thread schedules gp and so m again.
//
// N.B. lockedm must be this M, as this g was running on this M
// before entersyscall.
stoplockedm()
execute(gp, false) // Never returns.
}