error: ‘SPIN_LOCK_UNLOCKED’ undeclared here (not in a function)

While writing a kernel module using the spin locks in linux, we might hit the error

error: ‘SPIN_LOCK_UNLOCKED’ undeclared here (not in a function)

This might happen if we are trying to compile the module for kernel version below 3.0 Because before kernel version 3.0 SPIN_LOCK_UNLOCKED was defined in spinlock.h

30 #define SPIN_LOCK_UNLOCKED (spinlock_t) { 0, 0 }

But this definition is no longer available, and is replaced by DEFINE_SPINLOCK in spinlock_types.h

#define DEFINE_SPINLOCK(x) spinlock_t x = __SPIN_LOCK_UNLOCKED(x)

Thus in the module remove the initialization with SPIN_LOCK_UNLOCKED and add the macro DEFINE_SPINLOCK to get around the error.