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command_controller.go
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command_controller.go
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/*
* Copyright 2020, 2021, 2022 Hewlett Packard Enterprise Development LP
* Other additional copyright holders may be indicated within.
*
* The entirety of this work is licensed under the Apache License,
* Version 2.0 (the "License"); you may not use this file except
* in compliance with the License.
*
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package switchtec
import (
"sync"
"go.chromium.org/luci/common/runtime/goroutine"
)
// CommandController - The Command Controller is reponsibile for synchronizing access
// to the managed switchtec devices for commands. This is really just an implementation
// of a re-entrant locking mechanism, but the proud Go developers are enthroned on
// their high horses and think use of a re-entrent lock is some indiciation of program
// deficiency - so refuse to implement it in their language. They're also idiots.
//
// The Switchtec controller is a perfect use case for re-entrant locks - all commands
// consist of a request/response pair that needs to be serialized and non-interruptable,
// and there are certain command sets that must also be serialized and
// non-interruptable.
//
// For example, the collection of commands related to NVMe Admin Passthrough -
// which consists of 3 commands: Start, Transfer Data, and Finish. Taken
// individually, each command request/response cannot be interrupted, and the
// entire sequence of three commands cannot be interrupted. Failure to do so
// will cause the Switchtec device to raise an error that the sequence was
// corrupted.
//
// So, the lesson is to never think your programming language is too good for
// something that has been implemented in every programming language before
// yours.
type commandController struct {
mainLock sync.Mutex // Main lock that represents ownership of the device
innerLock sync.Mutex // Inner lock, used to manage the workings of the lock
cond *sync.Cond // Conditional used to signal when the lock /might/ be available
id goroutine.ID // The is the owning goroutine id
count int // Current number of nested locks owned by the locker
}
func NewCommandController() *commandController {
ctrl := new(commandController)
ctrl.id = goroutine.ID(^uint64(0))
ctrl.count = 0
ctrl.cond = sync.NewCond(&ctrl.innerLock)
return ctrl
}
func (c *commandController) Lock() {
id := goroutine.CurID()
c.innerLock.Lock()
for {
// Check if there is no current owner of the lock and break to
// acquire it.
if c.count == 0 {
c.id = id
break
}
// Check if the current goroutine already has the lock and break to
// acquire it; this is the case of re-entrent locking.
if c.id == id {
break
}
// Wait on the inner lock to become available so we can re-evaluate the lock conditions
// Note the behavior wait - it automatically releases the lock when called, and auto-
// matically acquires the lock on return.
c.cond.Wait()
}
// At this point we've settled on this goroutine acquring the master lock for the whole
// controller. Increment the number of lock calls currently held by the goroutine. For
// very first time it is requested, lock the main mutex.
c.count++
if c.count == 1 {
// Note: Technically this is not needed since the inner-lock controls all the magic;
// but it also acts as a useful tool to validate the main recursive locking behavior.
c.mainLock.Lock()
}
c.innerLock.Unlock()
}
func (c *commandController) Unlock() {
c.innerLock.Lock()
// Decrement the current count of locks held by this controller. When the lock
// count reaches zero, unlock the main mutext and signal to all waiting routines
// the availablity of controller.
c.count--
if c.count == 0 {
c.mainLock.Unlock()
c.cond.Signal()
}
c.innerLock.Unlock()
}