Re: [PATCH] alpha: spinlock: don't perform memory access in locked critical section

[Will Deacon <will.deacon@arm.com>]

On Mon, May 06, 2013 at 10:12:38PM +0100, Will Deacon wrote:
> The other (hopefully also wrong) worry that I had was when the manual
> states that:
> 
> `If the virtual and physical addresses for a LDx_L and STx_C sequence are
>  not within the same naturally aligned 16-byte sections of virtual and
>  physical memory, that sequence may always fail, or may succeed despite
>  another processor’s store to the lock range; hence, no useful program
>  should do this'
> 
> This seems like it might have a curious interaction with CoW paging if
> userspace is trying to use these instructions for a lock, since the
> physical address for the conditional store might differ from the one which
> was passed to the load due to CoW triggered by a different thread. Anyway,
> I was still thinking about that one and haven't got as far as TLB
> invalidation yet :)

In case anybody is interested, the software broadcasting of TLB maintenance
solves this problem because the PAL_rti on the ret_to_user path will clear
the lock flag.

Will