Can someone explain me the following doubt: 177 #ifdef CONFIG_PREEMPT 178 ENTRY(resume_kernel) 179 cli 180 cmpl $0,TI_preempt_count(%ebp) # non-zero preempt_count ? 181 jnz restore_all 182 need_resched: 183 movl TI_flags(%ebp), %ecx # need_resched set ? 184 testb $_TIF_NEED_RESCHED, %cl 185 jz restore_all 186 testl $IF_MASK,EFLAGS(%esp) # interrupts off (exception path) ? 187 jz restore_all 188 call preempt_schedule_irq 189 jmp need_resched 190 #endif I have observed that while returning from interrupts or exceptions (ret_from intr() assembly code in entry.S), if we are moving back to kernel execution path, we jump to 'resume_kernel', which in turn checks the following things: 1) Can we preempt (means current->preempt_count == 0) 2) if 1st point is true, we check, do we need to reschedule (means current->need_resched is set). 3) if 2nd condition is also true (means we need to reschedule), then we check are the intterupts disabled. If interrupts are disabled, we restore the registers and go back to interrupted kernel execution path, else we call function 'preempt_schedule_irq()', which do the kernel preemption. My question is when we are deliberatly disabling the interrupts at line 179 above, why do we check for interrupts disabled, they will always be disabled, hence preemption will never be done while returning from interrupts or exceptions. I think I am missing something .... please let me know the answer. thanks ... -Gaurav -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
