Why is interrupt disabled between spin_lock and spin_unlock in Linux?

I was reading the implementation of Linux semaphores. Due to atomicity, signal and wait (up and down in the source code) use spin locks. Then I saw Linux disabled interrupt in spin_lock_irqsave and reenabled interrupt in spin_unlock. This confused me. In my opinion, there is really no point disabling interrupt within a critical section.

For example, proc A (currently active) acquired the lock, proc B (blocked) is waiting for the lock and proc C is doing some unrelated stuff. It makes perfect sense to context switch to C within the critical section between A and B. Even if C also tries to acquire the lock, since the lock is already locked by A, the result would be C being blocked and A resuming execution.

Therefore, I don’t know why Linux decided to disable interrupt within critical sections guarded by spin locks. It probably won’t cause any problems but seems like a redundant operation to me.

Answer

Allow me to start off with a disclaimer that I am not a Linux expert, so my answer may not be the most accurate. Please point out any flaws and problems that you may find.

Imagine if some shared data is used by various parts of the kernel, including operations such as interrupt handlers that need to be fast and cannot block. Let’s say system call foo is currently active and has acquired a lock to use/access shared data bar, and interrupts are not disabled when/before acquiring said lock.

Now a (hardware) interrupt handler, e.g. the keyboard, kicks in and also needs access to bar (hardware interrupts have higher priority than system calls). Since bar is currently being locked by syscall foo, the interrupt handler cannot do anything. Interrupt handlers do need to be fast & not be blocked though, so they just keep spinning while trying to acquire the lock, which would cause a deadlock (i.e. system freeze) since syscall foo never gets a chance to finish and release its lock.

If you disable interrupts before trying to acquire the lock in foo, though, then foo will be able to finish up whatever it’s doing and ultimately release the lock (and restore interrupts). Any interrupts trying to come in while foo holds the spinlock will be left on the queue, and will be able to start when the lock is released. This way, you won’t run into the problem described above. However, care must also be taken to ensure that the lock for bar is held for as short as possible, so that other higher priority operations can take over whenever required.