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

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Message-ID: <20130506221152.GA24639@mudshark.cambridge.arm.com>
Date:	Mon, 6 May 2013 23:11:52 +0100
From:	Will Deacon <will.deacon@....com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Matt Turner <mattst88@...il.com>,
	"linux-alpha@...r.kernel.org" <linux-alpha@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Richard Henderson <rth@...ddle.net>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>
Subject: Re: [PATCH] alpha: spinlock: don't perform memory access in locked
 critical section

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
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