Re: [Xen-devel] [PATCH 05 of 10] arm: More SMP bringup

[Tim Deegan <tim@xxxxxxx>]

At 19:16 +0000 on 23 Feb (1330024560), Ian Campbell wrote:
> On Thu, 2012-02-23 at 17:40 +0000, Tim Deegan wrote:
> [...]
> > +   /* Signal the next non-boot CPU to come and join us here */
> > +   ldr   r0, =boot_gate         /* VA of gate */
> > +   add   r0, r0, r10            /* PA of gate */
> > +   mov   r1, #0                 /* (0 == unlocked) */
> > +   str   r1, [r0]
> > +   dsb
> > +   isb
> > +   sev
> 
> Here we have released the next CPU from the holding pen...
> 
> [...]
> > +   /* Non-boot CPUs report that they've got this far */
> > +   ldr   r0, =ready_cpus
> > +   ldr   r1, [r0]               /* Read count of ready CPUs */
> > +   add   r1, r1, #1             /* ++ */
> > +   str   r1, [r0]               /* Writeback */
> > +   dsb
> 
> ... and here we do a non-atomic update of a shared variable.
> 
> What prevents the following CPU from catching us up and conflicting
> here?
> 
> Would we be better signalling the next CPU after the increment instead?

Yes, we would.  I'll fix that.

Tim.

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