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

[Igor Mammedov]

On Wed, 10 Aug 2016 17:19:48 +0200
Paolo Bonzini <address@hidden> wrote:

CCing original author,
here is his varinat how to fix it
https://www.mail-archive.com/address@hidden/msg391790.html


> 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:
> > 
> > address@hidden:~/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:
> > 
> > address@hidden:~/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 <address@hidden>
> > ---
> >  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
>