On 2/1/06, Mulyadi Santosa <mulyadi.santosa@xxxxxxxxx> wrote: > Hello > > > > Why dont we consider preemption while returning from interrupts or > > > exceptions ?? > > > > i think we are considering this case, AFAIK you can register an > > interrupt handler and specify whether this interrupt handler runs > > with interrupts enabled or disabled. so we are just considering > > preemption of the interrupt context just if the interrupts where > > enabled while executing the interrupt handler > > Gaurav, maybe you can check how exception handler is called and see > whether IF is cleared or set there before pushed into kernel stack. I > am not sure why exception needs a condition where interrupt is masked > (or disabled?). Hi Mulyadi, As far as I know, exceptions (CPU on occurence of exception) do not disable the interrupts and nor even the exception handlers. I dont know why the check for interrupts being disabled (IF flag pushed on to stack) is done here. I am not able to figure out any case where this check is useful. May be somebody else on this mailing list be able to help us in understanding this. If anybody knows, please put some light on this. > > Why kernel preemption is disabled during exception? IMHO, because I dont think, kernel preemption is disabled while exception handling. Here is one of the exception code and there is no disabling of preemption. 403 ENTRY(coprocessor_error) 404 pushl $0 405 pushl $do_coprocessor_error 406 jmp error_code 379 error_code: 380 pushl %ds 381 pushl %eax 382 xorl %eax, %eax 383 pushl %ebp 384 pushl %edi 385 pushl %esi 386 pushl %edx 387 decl %eax # eax = -1 388 pushl %ecx 389 pushl %ebx 390 cld 391 movl %es, %ecx 392 movl ES(%esp), %edi # get the function address 393 movl ORIG_EAX(%esp), %edx # get the error code 394 movl %eax, ORIG_EAX(%esp) 395 movl %ecx, ES(%esp) 396 movl $(__USER_DS), %ecx 397 movl %ecx, %ds 398 movl %ecx, %es 399 movl %esp,%eax # pt_regs pointer 400 call *%edi 401 jmp ret_from_exception > whenever exception arises, that means something critical happens and it > needs to be handled ASAP. If interrupt is enabled to during this > handler thus opening the possibility to be interleaved by other > interrupt handler, there is a chance it introduces another exception > that might lead to system crash I also dont think that interrupts are disabled while handling exceptions. I think an interrupt can occur while we are in exception handler. Correct me if I am wrong. -Guarav > > Just my 2 cents idea... > > regards, > > Mulyadi > > > -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
