Re: [Qemu-devel] [PATCH] correctly handle resize of empty hash tree

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/08/2016 03:05, Brent W. Baccala wrote:
> Hi -
> 
> This is a bug report that includes a patch.  My understanding of your
> submission guidelines in that it should go to qemu-devel and not to the bug
> tracker.  Sorry if I didn't understand your procedures.
> 
> I'm running qemu-system-i386 built from the master branch from git://
> git.qemu-project.org/qemu.git.
> 
> The problem shows up with a Debian/Hurd guest using remote GDB on the Mach
> kernel.  Setting a breakpoint in the kernel with GDB and then hitting the
> breakpoint triggers a SEGV in qemu, i.e:
> 
> baccala@baccala:~/src/qemu$ gdb i386-softmmu/qemu-system-i386
> (gdb) run -enable-kvm -m 512 -nographic -drive cache=writeback,format=raw,
> file=/home/baccala/hurd/debian-hurd.img -net nic -net
> user,hostfwd=tcp::3333-:22 -s
> 
> After Hurd boots, in a different window, I attach GDB and set a kernel
> breakpoint:
> 
> baccala@baccala:~/hurd$ gdb gnumach-17-486-dbg
> (gdb) target remote localhost:1234
> Remote debugging using localhost:1234
> machine_idle (cpu=0) at ../i386/i386at/model_dep.c:207
> 207    ../i386/i386at/model_dep.c: No such file or directory.
> (gdb) where
> #0  machine_idle (cpu=0) at ../i386/i386at/model_dep.c:207
> #1  0x810293fa in idle_thread_continue () at ../kern/sched_prim.c:1674
> #2  0x00000000 in ?? ()
> (gdb) break elf_db_search_symbol
> Breakpoint 1 at 0x8100d080: file ../ddb/db_elf.c, line 159.
> (gdb) cont
> Continuing.
> 
> Now, I trigger the kernel breakpoint (by hitting Cntl-Alt-d to enter the
> Mach debugger), and qemu segfaults.  Here's what GDB on qemu itself shows:
> 
> Thread 1 "qemu-system-i38" received signal SIGSEGV, Segmentation fault.
> 0x0000555555b9d0c3 in qht_reset_size (ht=0x55555617f698 <tcg_ctx+216>,
> n_elems=32768) at util/qht.c:422
> 422        if (n_buckets != map->n_buckets) {
> (gdb) where
> #0  0x0000555555b9d0c3 in qht_reset_size (ht=0x55555617f698 <tcg_ctx+216>,
> n_elems=32768) at util/qht.c:422
> #1  0x000055555573f9d1 in tb_flush (cpu=0x0) at /home/baccala/src/qemu/
> translate-all.c:855
> #2  0x000055555577ac96 in gdb_vm_state_change (opaque=0x0, running=0,
> state=RUN_STATE_DEBUG)
>     at /home/baccala/src/qemu/gdbstub.c:1276
> #3  0x000055555589d167 in vm_state_notify (running=0,
> state=RUN_STATE_DEBUG) at vl.c:1585
> #4  0x000055555576bd9c in do_vm_stop (state=RUN_STATE_DEBUG) at
> /home/baccala/src/qemu/cpus.c:743
> #5  0x000055555576d550 in vm_stop (state=RUN_STATE_DEBUG) at
> /home/baccala/src/qemu/cpus.c:1476
> #6  0x000055555589d7c4 in main_loop_should_exit () at vl.c:1856
> #7  0x000055555589d933 in main_loop () at vl.c:1912
> #8  0x00005555558a4ee7 in main (argc=12, argv=0x7fffffffddc8,
> envp=0x7fffffffde30) at vl.c:4603
> (gdb)
> 
> "map" is NULL at this point.  I'm not sure what should happen when
> ght_reset_size() gets called on a hash tree with a NULL map.  I applied the
> following patch and everything seems to work.  I can now hit kernel
> breakpoints and step/next through the Mach kernel code without segfaults
> from qemu.  I don't know qemu, so it's just an educated guess.
> 
>     agape
>     brent
> 
> Signed-off-by: Brent Baccala <cosine@freesoft.org>
> ---
>  util/qht.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/util/qht.c b/util/qht.c
> index 16a8d79..9af21e3 100644
> --- a/util/qht.c
> +++ b/util/qht.c
> @@ -419,6 +419,14 @@ bool qht_reset_size(struct qht *ht, size_t n_elems)
> 
>      qemu_mutex_lock(&ht->lock);
>      map = ht->map;
> +
> +    if (!map) {
> +        new = qht_map_create(n_buckets);
> +        atomic_rcu_set(&ht->map, new);
> +        qemu_mutex_unlock(&ht->lock);
> +        return true;
> +    }
> +
>      if (n_buckets != map->n_buckets) {
>          new = qht_map_create(n_buckets);
>          resize = true;
> 

The patch makes sense, but I think we don't need to call qht_reset_size
at all.

tb_flush should not do anything if using KVM.  There are several ways to
do this:

- put the tb_flush call under "if (tcg_enabled())"

- add an "if (!tcg_enabled()) return;" in tb_flush

- add an "if (!tcg_ctx.tb_ctx.nb_tbs) return;" in tb_flush

Thanks,

Paolo