diff options
author | Russ Cox <rsc@golang.org> | 2022-01-26 16:53:50 -0500 |
---|---|---|
committer | Gopher Robot <gobot@golang.org> | 2022-06-06 20:48:03 +0000 |
commit | a71ca3dfbd32faf351ff68bcc26a4d5abd9b06d7 (patch) | |
tree | 0523f7aeacea7089b127ea3a8e5e58dded85fa0e /src/sync | |
parent | 3651a6117e9a88576615c29c4faf7eeec55d7691 (diff) | |
download | go-git-a71ca3dfbd32faf351ff68bcc26a4d5abd9b06d7.tar.gz |
runtime, sync, sync/atomic: document happens-before guarantees
A few of these are copied from the memory model doc.
Many are entirely new, following discussion on #47141.
See https://research.swtch.com/gomm for background.
The rule we are establishing is that each type that is meant
to help synchronize a Go program should document its
happens-before guarantees.
For #50859.
Change-Id: I947c40639b263abe67499fa74f68711a97873a39
Reviewed-on: https://go-review.googlesource.com/c/go/+/381316
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Diffstat (limited to 'src/sync')
-rw-r--r-- | src/sync/atomic/doc.go | 8 | ||||
-rw-r--r-- | src/sync/cond.go | 8 | ||||
-rw-r--r-- | src/sync/map.go | 7 | ||||
-rw-r--r-- | src/sync/mutex.go | 7 | ||||
-rw-r--r-- | src/sync/once.go | 4 | ||||
-rw-r--r-- | src/sync/pool.go | 5 | ||||
-rw-r--r-- | src/sync/rwmutex.go | 8 | ||||
-rw-r--r-- | src/sync/waitgroup.go | 3 |
8 files changed, 49 insertions, 1 deletions
diff --git a/src/sync/atomic/doc.go b/src/sync/atomic/doc.go index bb3b8f673e..4d426826da 100644 --- a/src/sync/atomic/doc.go +++ b/src/sync/atomic/doc.go @@ -36,6 +36,14 @@ // The load and store operations, implemented by the LoadT and StoreT // functions, are the atomic equivalents of "return *addr" and // "*addr = val". +// +// In the terminology of the Go memory model, if the effect of +// an atomic operation A is observed by atomic operation B, +// then A “synchronizes before” B. +// Additionally, all the atomic operations executed in a program +// behave as though executed in some sequentially consistent order. +// This definition provides the same semantics as +// C++'s sequentially consistent atomics and Java's volatile variables. package atomic import ( diff --git a/src/sync/cond.go b/src/sync/cond.go index 841be96896..19f986e478 100644 --- a/src/sync/cond.go +++ b/src/sync/cond.go @@ -18,6 +18,10 @@ import ( // when calling the Wait method. // // A Cond must not be copied after first use. +// +// In the terminology of the Go memory model, Cond arranges that +// a call to Broadcast or Signal “synchronizes before” any Wait call +// that it unblocks. type Cond struct { noCopy noCopy @@ -85,11 +89,13 @@ func (c *copyChecker) check() { } } -// noCopy may be embedded into structs which must not be copied +// noCopy may be added to structs which must not be copied // after the first use. // // See https://golang.org/issues/8005#issuecomment-190753527 // for details. +// +// Note that it must not be embedded, due to the Lock and Unlock methods. type noCopy struct{} // Lock is a no-op used by -copylocks checker from `go vet`. diff --git a/src/sync/map.go b/src/sync/map.go index 2fa3253429..ec529e056b 100644 --- a/src/sync/map.go +++ b/src/sync/map.go @@ -24,6 +24,13 @@ import ( // contention compared to a Go map paired with a separate Mutex or RWMutex. // // The zero Map is empty and ready for use. A Map must not be copied after first use. +// +// In the terminology of the Go memory model, Map arranges that a write operation +// “synchronizes before” any read operation that observes the effect of the write, where +// read and write operations are defined as follows. +// Load, LoadAndDelete, LoadOrStore are read operations; +// Delete, LoadAndDelete, and Store are write operations; +// and LoadOrStore is a write operation when it returns loaded set to false. type Map struct { mu Mutex diff --git a/src/sync/mutex.go b/src/sync/mutex.go index 80bb827054..2ea024e585 100644 --- a/src/sync/mutex.go +++ b/src/sync/mutex.go @@ -24,6 +24,13 @@ func fatal(string) // The zero value for a Mutex is an unlocked mutex. // // A Mutex must not be copied after first use. +// +// In the terminology of the Go memory model, +// the n'th call to Unlock “synchronizes before” the m'th call to Lock +// for any n < m. +// A successful call to TryLock is equivalent to a call to Lock. +// A failed call to TryLock does not establish any “synchronizes before” +// relation at all. type Mutex struct { state int32 sema uint32 diff --git a/src/sync/once.go b/src/sync/once.go index 38373160b9..b6399cfc3d 100644 --- a/src/sync/once.go +++ b/src/sync/once.go @@ -11,6 +11,10 @@ import ( // Once is an object that will perform exactly one action. // // A Once must not be copied after first use. +// +// In the terminology of the Go memory model, +// the return from f “synchronizes before” +// the return from any call of once.Do(f). type Once struct { // done indicates whether the action has been performed. // It is first in the struct because it is used in the hot path. diff --git a/src/sync/pool.go b/src/sync/pool.go index ea142bb181..cf01e2e189 100644 --- a/src/sync/pool.go +++ b/src/sync/pool.go @@ -41,6 +41,11 @@ import ( // free list. // // A Pool must not be copied after first use. +// +// In the terminology of the Go memory model, a call to Put(x) “synchronizes before” +// a call to Get returning that same value x. +// Similarly, a call to New returning x “synchronizes before” +// a call to Get returning that same value x. type Pool struct { noCopy noCopy diff --git a/src/sync/rwmutex.go b/src/sync/rwmutex.go index 7b10808ec4..e914f3eba0 100644 --- a/src/sync/rwmutex.go +++ b/src/sync/rwmutex.go @@ -25,6 +25,14 @@ import ( // recursive read locking. This is to ensure that the lock eventually becomes // available; a blocked Lock call excludes new readers from acquiring the // lock. +// +// In the terminology of the Go memory model, +// the n'th call to Unlock “synchronizes before” the m'th call to Lock +// for any n < m, just as for Mutex. +// For any call to RLock, there exists an n such that +// the n'th call to Unlock “synchronizes before” that call to RLock, +// and the corresponding call to RUnlock “synchronizes before” +// the n+1'th call to Lock. type RWMutex struct { w Mutex // held if there are pending writers writerSem uint32 // semaphore for writers to wait for completing readers diff --git a/src/sync/waitgroup.go b/src/sync/waitgroup.go index 9c6662d04b..9f26ae106c 100644 --- a/src/sync/waitgroup.go +++ b/src/sync/waitgroup.go @@ -17,6 +17,9 @@ import ( // Wait can be used to block until all goroutines have finished. // // A WaitGroup must not be copied after first use. +// +// In the terminology of the Go memory model, a call to Done +// “synchronizes before” the return of any Wait call that it unblocks. type WaitGroup struct { noCopy noCopy |