Re: [Qemu-devel] [RFC 2/2] linux-user: Fix cpu_index generation

[David Gibson]

On Sat, Jul 16, 2016 at 12:11:56AM +0200, Greg Kurz wrote:
> On Thu, 14 Jul 2016 21:59:45 +1000
> David Gibson <address@hidden> wrote:
> 
> > On Thu, Jul 14, 2016 at 03:50:56PM +0530, Bharata B Rao wrote:
> > > On Thu, Jul 14, 2016 at 3:24 PM, Peter Maydell <address@hidden> wrote:  
> > > > On 14 July 2016 at 08:57, David Gibson <address@hidden> wrote:  
> > > >> With CONFIG_USER_ONLY, generation of cpu_index values is done 
> > > >> differently
> > > >> than for full system targets.  This method turns out to be broken, 
> > > >> since
> > > >> it can fairly easily result in duplicate cpu_index values for
> > > >> simultaneously active cpus (i.e. threads in the emulated process).
> > > >>
> > > >> Consider this sequence:
> > > >>     Create thread 1
> > > >>     Create thread 2
> > > >>     Exit thread 1
> > > >>     Create thread 3
> > > >>
> > > >> With the current logic thread 1 will get cpu_index 1, thread 2 will get
> > > >> cpu_index 2 and thread 3 will also get cpu_index 2 (because there are 2
> > > >> threads in the cpus list at the point of its creation).
> > > >>
> > > >> We mostly get away with this because cpu_index values aren't that 
> > > >> important
> > > >> for userspace emulation.  Still, it can't be good, so this patch fixes 
> > > >> it
> > > >> by making CONFIG_USER_ONLY use the same bitmap based allocation that 
> > > >> full
> > > >> system targets already use.
> > > >>
> > > >> Signed-off-by: David Gibson <address@hidden>
> > > >> ---
> > > >>  exec.c | 19 -------------------
> > > >>  1 file changed, 19 deletions(-)
> > > >>
> > > >> diff --git a/exec.c b/exec.c
> > > >> index 011babd..e410dab 100644
> > > >> --- a/exec.c
> > > >> +++ b/exec.c
> > > >> @@ -596,7 +596,6 @@ AddressSpace *cpu_get_address_space(CPUState *cpu, 
> > > >> int asidx)
> > > >>  }
> > > >>  #endif
> > > >>
> > > >> -#ifndef CONFIG_USER_ONLY
> > > >>  static DECLARE_BITMAP(cpu_index_map, MAX_CPUMASK_BITS);
> > > >>
> > > >>  static int cpu_get_free_index(Error **errp)
> > > >> @@ -617,24 +616,6 @@ static void cpu_release_index(CPUState *cpu)
> > > >>  {
> > > >>      bitmap_clear(cpu_index_map, cpu->cpu_index, 1);
> > > >>  }
> > > >> -#else
> > > >> -
> > > >> -static int cpu_get_free_index(Error **errp)
> > > >> -{
> > > >> -    CPUState *some_cpu;
> > > >> -    int cpu_index = 0;
> > > >> -
> > > >> -    CPU_FOREACH(some_cpu) {
> > > >> -        cpu_index++;
> > > >> -    }
> > > >> -    return cpu_index;
> > > >> -}
> > > >> -
> > > >> -static void cpu_release_index(CPUState *cpu)
> > > >> -{
> > > >> -    return;
> > > >> -}
> > > >> -#endif  
> > > >
> > > > Won't this change impose a maximum limit of 256 simultaneous
> > > > threads? That seems a little low for comfort.  
> > > 
> > > This was the reason why the bitmap logic wasn't applied to
> > > CONFIG_USER_ONLY when it was introduced.
> > > 
> > > https://lists.gnu.org/archive/html/qemu-devel/2015-05/msg01980.html  
> > 
> > Ah.. good point.
> > 
> > Hrm, ok, my next idea would be to just (globally) sequentially
> > allocate cpu_index values for CONFIG_USER, and never try to re-use
> > them.  Does that seem reasonable?
> > 
> 
> Isn't it only deferring the problem to later ?

You mean that we could get duplicate indexes after the value wraps
around?

I suppose, but duplicates after spawning 4 billion threads seems like
a substantial improvement over duplicates after spawning 3 in the
wrong order..

> Maybe it is possible to define MAX_CPUMASK_BITS to a much higher
> value fo CONFIG_USER only instead ?

Perhaps.  It does mean carrying around a huge bitmap, though.

Another option is to remove cpu_index entirely for the user only
case.  I have some patches for this, which are very ugly but it's
possible they can be cleaned up to something reasonable (the biggest
chunk is moving a bunch of ARM stuff under #ifndef CONFIG_USER_ONLY
for what I think are registers that aren't accessible in user mode).


> > > But then we didn't have actual removal, but we do now.  
> > 
> > You mean patch 1/2 in this set?  Or something else?
> > 
> > Even so, 256 does seem a bit low for a number of simultaneously active
> > threads - there are some bug hairy multi-threaded programs out there.
> > 
> 



-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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