linux-kernel - Re: MMIO and gcc re-ordering issue

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <adamymbvcxi.fsf@cisco.com>
Date:	Tue, 27 May 2008 11:07:05 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Miller <davem@...emloft.net>, linuxppc-dev@...abs.org,
	scottwood@...escale.com, torvalds@...ux-foundation.org,
	tpiepho@...escale.com, alan@...rguk.ukuu.org.uk,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: MMIO and gcc re-ordering issue

 > Um, OK, you've said write twice now ... I was assuming you meant read.
 > Even on an x86, writes are posted, so there's no way a spin lock could
 > serialise a write without an intervening read to flush the posting
 > (that's why only reads have a relaxed version on altix).  Or is there
 > something else I'm missing?

Writes are posted yes, but not reordered arbitrarily.  If I have code like:

	spin_lock(&mmio_lock);
	writel(val1, reg1);
	writel(val2, reg2);
	spin_unlock(&mmio_lock);

then I have a reasonable expectation that if two CPUs run this at the
same time, their writes to reg1/reg2 won't be interleaved with each
other (because the whole section is inside a spinlock).  And Altix
violates that expectation.

 - R.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/