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Date:   Sun, 05 Jan 2020 14:22:12 -0800 (PST)
From:   David Miller <>
Subject: Re: [Patch v2] drivers/net/b44: Change to non-atomic bit
 operations on pwol_mask

From: "Luck, Tony" <>
Date: Thu, 2 Jan 2020 13:27:06 -0800

> From: Fenghua Yu <>
> Atomic operations that span cache lines are super-expensive on x86
> (not just to the current processor, but also to other processes as all
> memory operations are blocked until the operation completes). Upcoming
> x86 processors have a switch to cause such operations to generate a #AC
> trap. It is expected that some real time systems will enable this mode
> in BIOS.
> In preparation for this, it is necessary to fix code that may execute
> atomic instructions with operands that cross cachelines because the #AC
> trap will crash the kernel.
> Since "pwol_mask" is local and never exposed to concurrency, there is
> no need to set bits in pwol_mask using atomic operations.
> Directly operate on the byte which contains the bit instead of using
> __set_bit() to avoid any big endian concern due to type cast to
> unsigned long in __set_bit().
> Suggested-by: Peter Zijlstra <>
> Signed-off-by: Fenghua Yu <>
> Signed-off-by: Tony Luck <>

Applied, thanks.

I wonder if this is being used in an endian safe way.  Maybe the way
the filter is written into the chip makes it work out, I don't know.

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