Current

• this pointer denotes the current running task on a CPU.

• it is accessible only in process context and not interrupts.

• it is defined in <arch/x86/include/asm/current.h> for x86 processors

static __always_inline struct task_struct *get_current(void)
{
	if (IS_ENABLED(CONFIG_USE_X86_SEG_SUPPORT))
		return this_cpu_read_const(const_pcpu_hot.current_task);

	return this_cpu_read_stable(pcpu_hot.current_task);
}

• returns task_struct for the current task running on the CPU

this_cpu_read_const and this_cpu_read_stable are used to obtain per-cpu variables. More information is given at <arch/x86/include/asm/percpu.h>

this_cpu_read() makes gcc load the percpu variable every time it is
accessed while this_cpu_read_stable() allows the value to be cached.
this_cpu_read_stable() is more efficient and can be used if its value
is guaranteed to be valid across cpus.  The current users include
pcpu_hot.current_task and pcpu_hot.top_of_stack, both of which are
actually per-thread variables implemented as per-CPU variables and
thus stable for the duration of the respective task.

The source code here is taken from linux-6.8.1