Re: [Xen-devel] [PATCH v4 1/3] libxc: do some retries in xc_cpupool_removecpu() for EBUSY case

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Apr 12, 2016 at 03:45:06PM +0200, Juergen Gross wrote:
> On 12/04/16 15:02, Olaf Hering wrote:
> > On Thu, Mar 10, Juergen Gross wrote:
> > 
> >> +#define NUM_RMCPU_BUSY_RETRIES 5
> >> +
> >>  int xc_cpupool_removecpu(xc_interface *xch,
> >>                           uint32_t poolid,
> >>                           int cpu)
> >>  {
> >> +    unsigned retries;
> >> +    int err;
> >>      DECLARE_SYSCTL;
> >>  
> >>      sysctl.cmd = XEN_SYSCTL_cpupool_op;
> >>      sysctl.u.cpupool_op.op = XEN_SYSCTL_CPUPOOL_OP_RMCPU;
> >>      sysctl.u.cpupool_op.cpupool_id = poolid;
> >>      sysctl.u.cpupool_op.cpu = (cpu < 0) ? XEN_SYSCTL_CPUPOOL_PAR_ANY : 
> >> cpu;
> >> -    return do_sysctl_save(xch, &sysctl);
> >> +    for ( retries = 0; retries < NUM_RMCPU_BUSY_RETRIES; retries++ ) {
> >> +        err = do_sysctl_save(xch, &sysctl);
> >> +        if ( err < 0 && errno == EBUSY )
> >> +            sleep(1);
> >> +        else
> >> +            break;
> >> +    }
> >> +    return err;
> > 
> > This may fail with gcc-4.8, at least with -Og in 13.1:
> > 
> > [  105s] xc_cpupool.c: In function 'xc_cpupool_removecpu':
> > [  105s] xc_cpupool.c:168:5: error: 'err' may be used uninitialized in this 
> > function [-Werror=maybe-uninitialized]
> > [  105s]      return err;
> > [  105s]      ^
> 
> IMO this is a compiler bug. The compiler could detect easily that err
> can't be uninitialized at the return statement (e.g. via loop
> unrolling).
> 
> I can do a patch, of course. The question is whether I should. :-)
> 

You should -- and document that it is to make buggy compiler happy.
We've done this before.

Wei.

> 
> Juergen

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