linux-kernel - Re: alpha: potential race around hae_cache in RESTORE

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Message-ID: <20100927162610.GA18373@jurassic.park.msu.ru>
Date:	Mon, 27 Sep 2010 20:26:10 +0400
From:	Ivan Kokshaysky <ink@...assic.park.msu.ru>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>, rth@...ddle.net,
	linux-kernel@...r.kernel.org, Matt Turner <mattst88@...il.com>
Subject: Re: alpha: potential race around hae_cache in RESTORE_ALL

On Mon, Sep 27, 2010 at 01:46:24PM +0100, Al Viro wrote:
> AFAICS, we have 3 variants:
> 	1) alpha_mv.hae_register == &alpha_mv.hae_cache; all that code
> becomes a no-op.
> 	2) UP boxen with hae_register pointing someplace real; we save
> HAE in SAVE_ALL, restore it in RESTORE_ALL and disable interrupts around
> the updates of hae_cache/*hae_register to keep them in sync.  readl()
> et.al. set HAE, then do memory access and rely on not giving CPU up between
> these moments.  Since alpha doesn't do PREEMPT, we are OK (otherwise we'd
> needed to disable preempt in those places; also not a big deal)
> 	3) SMP t2 boxen; we protect the entire sequence from setting HAE to
> memory access with spinlock and with disabling interrupts.  We don't rely on
> interrupts not modifying the damn thing, but we *do* rely on other CPU not
> messing with HAE on syscall paths outside of spinlock-protected area.  And
> we have RESTORE_ALL hit us on all exits to userland, interrupt, trap and
> syscall alike.
> 
> 	Looks like (3) has always been broken...

Ah, agreed with all of the above.

Looks like we need to drop HAE bits from SAVE_ALL/RESTORE_ALL, which
benefits (1) and automatically fixes (3), and do the entire IO sequences
in (2) with disabled interrupts (if HAE is involved).
The latter includes apecs, lca and jensen.

Ivan.
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