[PATCH] ARM: spinlock: ensure we have a compiler barrier before sev

[Christoffer Dall]

On Tue, Feb 04, 2014 at 12:28:46PM +0000, Will Deacon wrote:
> When unlocking a spinlock, we require the following, strictly ordered
> sequence of events:
> 
> 	<barrier>	/* dmb */
> 	<unlock>
> 	<barrier>	/* dsb */
> 	<sev>
> 
> Whilst the code does indeed reflect this in terms of the architecture,
> the final <barrier> + <sev> have been contracted into a single inline
> asm without a "memory" clobber, therefore the compiler is at liberty to
> reorder the unlock to the end of the above sequence. In such a case,
> a waiting CPU may be woken up before the lock has been unlocked, leading
> to extremely poor performance.
> 
> This patch reworks the dsb_sev() function to make use of the dsb()
> macro and ensure ordering against the unlock.
> 
> Cc: <stable at vger.kernel.org>
> Reported-by: Mark Rutland <mark.rutland at arm.com>
> Signed-off-by: Will Deacon <will.deacon at arm.com>

FWIW: Reviewed-by: Christoffer Dall <christoffer.dall at linaro.org>