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

[Greg Kurz]

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 ?

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

> > 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.
>

pgpoDZY50DoJr.pgp
Description: OpenPGP digital signature