What spin_lock_irqsave() does, when to use it ?

The spin_lock_irqsave() is a synchronization mechanism function in Linux is used to acquire a spinlock with interrupts disabled. This function is typically used in device driver code to protect critical sections of code that access shared hardware resources. 

 

Below is the step-by-step explanation of what spin_lock_irqsave() does:

 

1. Disables interrupts: The first step of spin_lock_irqsave() is to disable interrupts on the CPU where it is executing. This is done to prevent any interrupt handlers from preempting the current code and modifying the shared hardware resources.  
2. Acquires the spinlock: The next step is to acquire the spinlock (as we know that a spinlock is a kind of locking mechanism that is held by the CPU executing the critical section of code). If another CPU attempts to acquire the same lock, it will keep spin in a loop, continuously checking to see if the lock has been released or not. When the lock is finally released, the waiting CPU can acquire it and execute its own critical section of code.
3. Saves the interrupt state: Once the spinlock has been acquired, spin_lock_irqsave() saves the current interrupt state in a variable. This is done so that interrupts can be re-enabled later without losing the original state.
4. Enables interrupts: After saving the interrupt state, spin_lock_irqsave() re-enables interrupts on the CPU. This allows interrupt handlers to run again, but they will be unable to preempt the current critical section of code because it holds the spinlock.
5. Returns the interrupt state: Finally, spin_lock_irqsave() returns the saved interrupt state to the caller. This allows the caller to restore the original interrupt state when it releases the spinlock later.

In summary, spin_lock_irqsave() provides a way to protect critical sections of code that access shared hardware resources while ensuring that interrupt handling can still occur. By disabling interrupts and acquiring a spinlock, the function ensures that only one CPU can access the shared resources at a time, preventing race conditions and other synchronization problems.