First, remember the goal of these ‘synchronizing objects’ : These objects were designed to provide an efficient and coherent use of ‘shared data’ between more than 1 thread among 1 process or from different processes. These objects can be ‘acquired’ or ‘released’. That is it!!! End of story!!! Now, if it helps to you, let … Read more
spin_lock_irqsave is basically used to save the interrupt state before taking the spin lock, this is because spin lock disables the interrupt, when the lock is taken in interrupt context, and re-enables it when while unlocking. The interrupt state is saved so that it should reinstate the interrupts again. Example: Lets say interrupt x was … Read more
Looks fine to me. Btw, here is the textbook implementation that is more efficient even in the contended case. void lock(volatile int *exclusion) { while (__sync_lock_test_and_set(exclusion, 1)) while (*exclusion) ; }
It’s the same (and probably compiles to) as the x86 opcode PAUSE and equivalent the Win32 macro YieldProcessor, GCC’s __mm_pause() and the C# method Thread.SpinWait It’s a very weakened form of yielding: it tells your CPU that you are in a loop that may burn many CPU-cycles waiting for something to happen (busy-waiting). This way, … Read more
It depends on what you’re doing. In general application code, you’ll want to avoid spinlocks. In low-level stuff where you’ll only hold the lock for a couple of instructions, and latency is important, a spinlock mat be a better solution than a lock. But those cases are rare, especially in the kind of applications where … Read more
PAUSE notifies the CPU that this is a spinlock wait loop so memory and cache accesses may be optimized. See also pause instruction in x86 for some more details about avoiding the memory-order mis-speculation when leaving the spin-loop. PAUSE may actually stop CPU for some time to save power. Older CPUs decode it as REP … Read more
When you use regular locks (mutexes, critical sections etc), operating system puts your thread in the WAIT state and preempts it by scheduling other threads on the same core. This has a performance penalty if the wait time is really short, because your thread now has to wait for a preemption to receive CPU time … Read more
The Theory In theory, when a thread tries to lock a mutex and it does not succeed, because the mutex is already locked, it will go to sleep, immediately allowing another thread to run. It will continue to sleep until being woken up, which will be the case once the mutex is being unlocked by … Read more