linux-kernel - Re: [PATCH] fix a race condition in cancelable mcs spinlocks

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Message-ID: <20140603075616.GJ11096@twins.programming.kicks-ass.net>
Date:	Tue, 3 Jun 2014 09:56:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	John David Anglin <dave.anglin@...l.net>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	jejb@...isc-linux.org, deller@....de, linux-parisc@...r.kernel.org,
	linux-kernel@...r.kernel.org, chegu_vinod@...com,
	paulmck@...ux.vnet.ibm.com, Waiman.Long@...com, tglx@...utronix.de,
	riel@...hat.com, akpm@...ux-foundation.org, davidlohr@...com,
	hpa@...or.com, andi@...stfloor.org, aswin@...com,
	scott.norton@...com, Jason Low <jason.low2@...com>
Subject: Re: [PATCH] fix a race condition in cancelable mcs spinlocks

On Mon, Jun 02, 2014 at 12:56:40PM -0700, James Bottomley wrote:
> Architecturally, there is a way we could emulate the atomic exchange
> instructions.  We could have a special section of memory that always
> triggers a page trap.  In the Q state dtlb trap handlers we could
> recognise the "atomic" section of memory and wrap the attempted
> modification in a semaphore.  This would add a bit of overhead, but not
> a huge amount if we do it in the trap handlers like the TMPALIAS
> flushes.  This involves a lot of work for us because we have to decode
> the instructions in software, recognise the operations and manually
> apply the hashed semaphores around them.  If we did it like this, all
> we'd need by way of mainline support is that variables treated as
> atomically exchangeable should be in a separate section (because it's a
> page fault handler effectively, we need them all separated from "normal"
> code).  This would probably require some type of variable marker and if
> we ever saw a xchg or cmpxchg on a variable without the marker, we could
> break the build.

Cute, but I don't think that's entirely feasible given how these things
can be embedded in other structures (some dynamically allocated etc..).



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