linux-kernel - RE: [PATCH 2/5] efi: Introduce a Runtime Services lock

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Date:	Wed, 16 Oct 2013 23:36:54 +0000
From:	Seiji Aguchi <seiji.aguchi@....com>
To:	Matt Fleming <matt@...sole-pimps.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Tony Luck <tony.luck@...el.com>,
	"x86@...nel.org" <x86@...nel.org>,
	Matt Fleming <matt.fleming@...el.com>,
	Leif Lindholm <leif.lindholm@...aro.org>
Subject: RE: [PATCH 2/5] efi: Introduce a Runtime Services lock

> +#define efi_call_reset_virt(f, args...)					\
> +({									\
> +	unsigned long __flags;						\
> +	bool __nmi = in_nmi();						\
> +									\
> +	if (__nmi)							\
> +		spin_lock_irqsave(&efi_runtime_lock, __flags);		\

If the lock is not held in the nmi context, runtime service may run concurrently
in non-nmi context as follows.
 - In nmi context, cpu0 calls a runtime service (no lock is held.)
 - In non-nmi context, cpu1 call can take the lock and call the runtime service.

To avoid this, using try_lock in nmi context is better..

If(in_nmi())
	try_spin_lock_irqsave();
else
	spin_lock_irqsave();

Please see the commit abd4d5587be911f63592537284dad78766d97d62,
which is introduced to pstore by Don Zickus.

Seiji



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