> Date: Sun, 3 Apr 2016 00:39:14 -0700
> From: Philip Guenther <[email protected]>
>
> On Sat, 2 Apr 2016, Philip Guenther wrote:
> > I ask as I see some "this can't work" logic between the kernel and gdb for
> > at least i386 and possibly others and I'd like to hear what *is* working
> > that I should verify I don't break when making changes...
>
> Here's a quick fix for the initial bug that caught my eye. On i386,
> savectx() sets PCB_SAVECTX in pcb_flags. Okay, that's nice, but
>
> a) savectx() is called from cpu_fork(), which results in the flag being
> set on both parent and child, and
>
> b) nothing clears it
>
> Result is that it's set on *all* threads, which makes gdb's "kvm proc"
> never show the frame of mi_switch(), as it thinks this pcb is always from
> savectx().
>
> Diff below fixes that by eliminating the savectx() call in cpu_fork().
> Turns out that call is completely unnecessary: savectx() only updates
> pcb_esp and pcb_ebp (and the PCB_SAVECTX flag), but cpu_fork() overwrites
> both those values in the child's pcb, while the values in the parent are
> invalid as soon as it returns.
>
> before:
> (gdb) bt
> #0 0xd03bf814 in sleep_finish (sls=0x0, do_sleep=1)
> at ../../../../kern/kern_synch.c:266
> #1 0xd03bfddf in tsleep (ident=0xd0b8ae44, priority=288,
> wmesg=0xd09e9690 "nanosleep", timo=0) at
> ../../../../kern/kern_synch.c:148
> #2 0xd03c1faa in sys_nanosleep (p=0xd7203cdc, v=0xf5d5cf5c,
> retval=0xf5d5cf7c)
> at ../../../../kern/kern_time.c:289
>
> after:
> (gdb) bt
> #0 mi_switch () at cpu.h:208
> #1 0xd03bf814 in sleep_finish (sls=0xf5d84e7c, do_sleep=1)
> at ../../../../kern/kern_synch.c:266
> #2 0xd03bfddf in tsleep (ident=0xd0b8ae44, priority=288,
> wmesg=0xd09e9690 "nanosleep", timo=600001)
> at ../../../../kern/kern_synch.c:148
> #3 0xd03c1faa in sys_nanosleep (p=0xd7204b70, v=0xf5d84f5c,
> retval=0xf5d84f7c)
> at ../../../../kern/kern_time.c:289
>
> Note the bogus sls=0x0 in before case.
>
> ok?
ok kettenis@
> Index: vm_machdep.c
> ===================================================================
> RCS file: /data/src/openbsd/src/sys/arch/i386/i386/vm_machdep.c,v
> retrieving revision 1.63
> diff -u -p -r1.63 vm_machdep.c
> --- vm_machdep.c 5 May 2015 02:13:46 -0000 1.63
> +++ vm_machdep.c 3 Apr 2016 07:19:50 -0000
> @@ -85,13 +85,8 @@ cpu_fork(struct proc *p1, struct proc *p
>
> p2->p_md.md_flags = p1->p_md.md_flags;
>
> - /* Copy pcb from proc p1 to p2. */
> - if (p1 == curproc) {
> - /* Sync the PCB before we copy it. */
> - savectx(curpcb);
> - }
> #ifdef DIAGNOSTIC
> - else if (p1 != &proc0)
> + if (p1 != curproc && p1 != &proc0)
> panic("cpu_fork: curproc");
> #endif
> *pcb = p1->p_addr->u_pcb;
>
>
