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runtime: don't do a direct G handoff in semrelease on systemstack

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semrelease is safe to call on the system stack (since it just readies
goroutines) except for the fact that it might perform a direct G
handoff and call into the scheduler. If handoff is set to false this is
exceptionally rare, but could happen, and has happened for the trace
reader goroutine which releases a trace.doneSema.

Fixes .

Change-Id: I37ece678bc4721bbb6e5879d74daac762b7d742a
Reviewed-on: https://go-review.googlesource.com/c/go/+/680315
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
This commit is contained in:
Michael Anthony Knyszek
2025-06-09 21:45:33 +00:00
committed by Gopher Robot
parent f18d046568
commit a189516d3a
1 changed files with 8 additions and 3 deletions

11
src/runtime/sema.go

@ -261,11 +261,13 @@ func semrelease1(addr *uint32, handoff bool, skipframes int) {
s.ticket = 1
}
readyWithTime(s, 5+skipframes)
if s.ticket == 1 && getg().m.locks == 0 {
if s.ticket == 1 && getg().m.locks == 0 && getg() != getg().m.g0 {
// Direct G handoff
//
// readyWithTime has added the waiter G as runnext in the
// current P; we now call the scheduler so that we start running
// the waiter G immediately.
//
// Note that waiter inherits our time slice: this is desirable
// to avoid having a highly contended semaphore hog the P
// indefinitely. goyield is like Gosched, but it emits a
@ -275,9 +277,12 @@ func semrelease1(addr *uint32, handoff bool, skipframes int) {
// the non-starving case it is possible for a different waiter
// to acquire the semaphore while we are yielding/scheduling,
// and this would be wasteful. We wait instead to enter starving
// regime, and then we start to do direct handoffs of ticket and
// P.
// regime, and then we start to do direct handoffs of ticket and P.
//
// See issue 33747 for discussion.
//
// We don't handoff directly if we're holding locks or on the
// system stack, since it's not safe to enter the scheduler.
goyield()
}
}