mirror of https://github.com/ceph/go-ceph
91 lines
3.8 KiB
Go
91 lines
3.8 KiB
Go
package cutil
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import (
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"sync"
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"unsafe"
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)
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// PtrGuard respresents a guarded Go pointer (pointing to memory allocated by Go
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// runtime) stored in C memory (allocated by C)
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type PtrGuard struct {
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// These mutexes will be used as binary semaphores for signalling events from
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// one thread to another, which - in contrast to other languages like C++ - is
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// possible in Go, that is a Mutex can be locked in one thread and unlocked in
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// another.
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stored, release sync.Mutex
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released bool
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}
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// WARNING: using binary semaphores (mutexes) for signalling like this is quite
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// a delicate task in order to avoid deadlocks or panics. Whenever changing the
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// code logic, please review at least three times that there is no unexpected
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// state possible. Usually the natural choice would be to use channels instead,
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// but these can not easily passed to C code because of the pointer-to-pointer
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// cgo rule, and would require the use of a Go object registry.
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// NewPtrGuard writes the goPtr (pointing to Go memory) into C memory at the
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// position cPtr, and returns a PtrGuard object.
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func NewPtrGuard(cPtr CPtr, goPtr unsafe.Pointer) *PtrGuard {
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var v PtrGuard
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// Since the mutexes are used for signalling, they have to be initialized to
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// locked state, so that following lock attempts will block.
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v.release.Lock()
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v.stored.Lock()
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// Start a background go routine that lives until Release is called. This
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// calls a special function that makes sure the garbage collector doesn't touch
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// goPtr, stores it into C memory at position cPtr and then waits until it
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// reveices the "release" signal, after which it nulls out the C memory at
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// cPtr and then exits.
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go func() {
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storeUntilRelease(&v, (*CPtr)(cPtr), uintptr(goPtr))
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}()
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// Wait for the "stored" signal from the go routine when the Go pointer has
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// been stored to the C memory. <--(1)
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v.stored.Lock()
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return &v
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}
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// Release removes the guarded Go pointer from the C memory by overwriting it
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// with NULL.
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func (v *PtrGuard) Release() {
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if !v.released {
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v.released = true
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v.release.Unlock() // Send the "release" signal to the go routine. -->(2)
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v.stored.Lock() // Wait for the second "stored" signal when the C memory
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// has been nulled out. <--(3)
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}
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}
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// The uintptrPtr() helper function below assumes that uintptr has the same size
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// as a pointer, although in theory it could be larger. Therefore we use this
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// constant expression to assert size equality as a safeguard at compile time.
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// How it works: the difference of both sizes is converted into an 8 bit value
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// and left-bit-shifted by 8. This always creates an overflow error at compile
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// time, if the difference of the sizes is not 0.
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const _ = uint8(unsafe.Sizeof(uintptr(0))-PtrSize) << 8 // size assert
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func uintptrPtr(p *CPtr) *uintptr {
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return (*uintptr)(unsafe.Pointer(p))
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}
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//go:uintptrescapes
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// From https://golang.org/src/cmd/compile/internal/gc/lex.go:
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// For the next function declared in the file any uintptr arguments may be
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// pointer values converted to uintptr. This directive ensures that the
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// referenced allocated object, if any, is retained and not moved until the call
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// completes, even though from the types alone it would appear that the object
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// is no longer needed during the call. The conversion to uintptr must appear in
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// the argument list.
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// Also see https://golang.org/cmd/compile/#hdr-Compiler_Directives
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func storeUntilRelease(v *PtrGuard, cPtr *CPtr, goPtr uintptr) {
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uip := uintptrPtr(cPtr)
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*uip = goPtr // store Go pointer in C memory at c_ptr
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v.stored.Unlock() // send "stored" signal to main thread -->(1)
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v.release.Lock() // wait for "release" signal from main thread when
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// Release() has been called. <--(2)
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*uip = 0 // reset C memory to NULL
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v.stored.Unlock() // send second "stored" signal to main thread -->(3)
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}
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