I understand that the kernel can synchronize processes via the spinlock method. However, when it comes down to one processor how does it do so? How does it use a synchronization object to ensure mutual exclusion?
Is a semaphore at the level of the executive? How does the kernel come into play here?
Are mutexes only implemented at the level of the kernel? They do not give off a signal or message when the resource is free.
You've got several questions here:
I understand that the kernel can synchronize processes via the spinlock method. However, when it comes down to one processor how does it do so? How does it use a synchronization object to ensure mutual exclusion?
On uni-processor machines, acquiring a spinlock simply raises the IRQL to >DISPATCH_LEVEL - a thread at such elevated IRQL cannot be pre-empted, so synchronization is guaranteed.
Is a semaphore at the level of the executive? How does the kernel come into play here?
Semaphores, mutexes, (and most waitable objects, for that matter) are Kernel Dispatch Objects. Such objects are implemented by the kernel, and are made available to user mode applications via various functions exported by KERNEL32.DLL (CreateEvent/Mutex/Semaphore, et.al.). In addition, the "kernel comes into play" by scheduling thread waits, and awakening threads that are waiting on synchronization objects.
Are mutexes only implemented at the level of the kernel?
Mutex objects are indeed kernel dispatch objects (KMUTEX). A mutex object is signalled when it is un-owned. When a thread acquires a mutex, it's state goes to non-signalled, which means that any other thread that attempts to acquire it will be put into a wait state until either the mutex is acquired, or the wait times out.
For more detailed explanations on kernel dispatcher objects, as well as Windows synchronization in general, have a peek at the latest version of "Windows Internals" - every Windows developer should have a copy of this on their desk, IMHO.
'They do not give off a signal or message when the resource is free' - sure they do - they are an inter-thread signaling mechanism! A thread waiting on the mutex is signaled and made ready when the protected resource is released, so acquiring the mutex.
Spinlocks are generally not used on single-core processors - there is no point. TBH, spinlocks need great care on multi-core and clustered systems too if their use is not to be counter-productive.
