Re: [Xen-devel] [PATCH] xen: arm: clear the exclusive monitor on exception return

[Tim Deegan <tim@xxxxxxx>]

At 09:31 +0100 on 18 Jul (1374139884), Ian Campbell wrote:
> On Wed, 2013-07-17 at 20:48 +0100, Tim Deegan wrote:
> > At 12:18 +0100 on 17 Jul (1374063531), Ian Campbell wrote:
> > > Otherwise context switching between two vcpus which are contending the 
> > > same
> > > lock can result in a spurious success.
> > 
> > Shouldn't this go in ctxt_swicth_to(), then?  I think any use that Xen
> > itself makes of the monitor should end with it cleared.
> 
> Our atomic read/set which we stole^Winherited from Linux relies on this
> behaviour (interlocking with atomic_add_and_blah stuff).
>         /*
>          * On ARM, ordinary assignment (str instruction) doesn't clear the 
> local
>          * strex/ldrex monitor on some implementations. The reason we can use 
> it for
>          * atomic_set() is the clrex or dummy strex done on every exception 
> return.
>          */

Ah, OK.  So, Ack, but if it's not already committed can you edit the
commit message to refer to this?

> As well as atomics our spin unlock (also inherited) is just a plain
> store.

Well, we already avoid having locks shared between interrupt/exception
handlers and plain code, but I guess it can't hurt.

Tim.

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