Hi Will, On 15/10/2019 21:07, Will Deacon wrote: > Patch looks good apart from one thing... > > On Tue, Oct 15, 2019 at 06:25:44PM +0100, James Morse wrote: >> diff --git a/include/linux/sched.h b/include/linux/sched.h >> index 2c2e56bd8913..67a1d86981a9 100644 >> --- a/include/linux/sched.h >> +++ b/include/linux/sched.h >> @@ -223,6 +223,7 @@ extern long schedule_timeout_uninterruptible(long >> timeout); >> extern long schedule_timeout_idle(long timeout); >> asmlinkage void schedule(void); >> extern void schedule_preempt_disabled(void); >> +asmlinkage void preempt_schedule_irq(void); > > I don't understand the need for this hunk, since we're only calling the > function from C now. Please could you explain?
(A prototype is needed to make the thing build[0], but) you mean the asmlinkage? The definition in kernel/sched/core.c has asmlinkage. It does nothing on arm64, but if another architecture does add a C call, and uses asmlinkage to tinker with the calling convention, it would need to be here so callers use the correct convention. e.g. for X86_32 defines asmlinkage in arch/x86/include/asm/linkage.h: | #define asmlinkage CPP_ASMLINKAGE __attribute__((regparm(0))) This forces all arguments out of registers and onto the stack [1]. Without this annotation, asm->preempt_schedule_irq() callers would put arguments on the stack, but C->preempt_schedule_irq() callers would use whatever the C->C calling convention is, which might not match. schedule() further up the hunk does the same. I agree it doesn't matter today, but omitting it would be a bug for the next user to debug! Thanks, James [0] Without that hunk, ../arch/arm64/kernel/process.c: In function ‘arm64_preempt_schedule_irq’: ../arch/arm64/kernel/process.c:650:3: error: implicit declaration of function ‘preempt_schedule_irq’; did you mean ‘preempt_schedule’? [-Werror=implicit-function-declaration] preempt_schedule_irq(); ^~~~~~~~~~~~~~~~~~~~ preempt_schedule cc1: some warnings being treated as errors make[3]: *** [../scripts/Makefile.build:266: arch/arm64/kernel/process.o] Error 1 make[3]: *** Waiting for unfinished jobs.... make[2]: *** [../scripts/Makefile.build:509: arch/arm64/kernel] Error 2 make[2]: *** Waiting for unfinished jobs.... make[1]: *** [/home/morse/kernel/linux/Makefile:1649: arch/arm64] Error 2 make[1]: *** Waiting for unfinished jobs.... make: *** [../Makefile:179: sub-make] Error 2 [1] https://gcc.gnu.org/onlinedocs/gcc/x86-Function-Attributes.html
