spin_lock_irqsave vs spin_lock_irq

Question

On an SMP machine we must use spin_lock_irqsave and not spin_lock_irq from interrupt context.

Why would we want to save the flags (which contain the IF)?

Is there another interrupt routine that could interrupt us?

Answer 1

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:

1. Lets say interrupt x was disabled before spin lock was acquired
2. spin_lock_irq will disable the interrupt x and take the the lock
3. spin_unlock_irq will enable the interrupt x.

So in the 3rd step above after releasing the lock we will have interrupt x enabled which was earlier disabled before the lock was acquired.

So only when you are sure that interrupts are not disabled only then you should spin_lock_irq otherwise you should always use spin_lock_irqsave.

Answer 2

If interrupts are already disabled before your code starts locking, when you call spin_unlock_irq you will forcibly re-enable interrupts in a potentially unwanted manner. If instead you also save the current interrupt enable state in flags through spin_lock_irqsave, attempting to re-enable interrupts with the same flags after releasing the lock, the function will just restore the previous state (thus not necessarily enabling interrupts).

Example with spin_lock_irqsave:

spinlock_t mLock = SPIN_LOCK_UNLOCK; unsigned long flags;  spin_lock_irqsave(&mLock, flags); // Save the state of interrupt enable in flags and then disable interrupts // Critical section spin_unlock_irqrestore(&mLock, flags); // Return to the previous state saved in flags 

Example with spin_lock_irq( without irqsave ):

spinlock_t mLock = SPIN_LOCK_UNLOCK; unsigned long flags;  spin_lock_irq(&mLock); // Does not know if interrupts are already disabled // Critical section spin_unlock_irq(&mLock); // Could result in an unwanted interrupt re-enable...