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sync: document why copyChecker checks the condition twice

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Fixes #40924
Change-Id: I249a278be1ec3c67088819af4456e6c393431724
This commit is contained in:
qiulaidongfeng 2023-08-12 18:06:08 +08:00
parent ac64a3628b
commit 772c7ae7e1
1 changed files with 4 additions and 0 deletions
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4
src/sync/cond.go
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@ -96,6 +96,10 @@ func (c *Cond) Broadcast() {
type copyChecker uintptr
func (c *copyChecker) check() {
// Check if c has been copied in three steps:
// 1. The first comparison is the fast-path. If c has been initialized and not copied, this will return immediately. Otherwise, c is either not initialized, or has been copied.
// 2. Ensure c is initialized. If the CAS succeeds, we're done. If it fails, c was either initialized concurrently and we simply lost the race, or c has been copied.
// 3. Do step 1 again. Now that c is definitely initialized, if this fails, c was copied.
if uintptr(*c) != uintptr(unsafe.Pointer(c)) &&
!atomic.CompareAndSwapUintptr((*uintptr)(c), 0, uintptr(unsafe.Pointer(c))) &&
uintptr(*c) != uintptr(unsafe.Pointer(c)) {