linux-kernel - Re: [PATCH 1/4] pci: Allow lockless access path to PCI mmconfig

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Date:   Tue, 14 Mar 2017 14:06:13 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andi Kleen <andi@...stfloor.org>
cc:     bhelgaas@...gle.com, x86@...nel.org, linux-pci@...r.kernel.org,
        eranian@...gle.com, peterz@...radead.org,
        linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/4] pci: Allow lockless access path to PCI mmconfig

On Thu, 2 Mar 2017, Andi Kleen wrote:

> From: Andi Kleen <ak@...ux.intel.com>
> 
> The Intel uncore driver can do a lot of PCI config accesses to read
> performance counters. I had a situation on a 4S system where it
> was spending 40+% of CPU time grabbing the pci_cfg_lock due to that.
> 
> For 64bit x86 with MMCONFIG there isn't really any reason to take
> a lock. The access is directly mapped to an underlying MMIO area,
> which can fully operate lockless.
> 
> Add a new flag that allows the PCI mid layer to skip the lock
> and set it for the 64bit mmconfig code.
> 
> There's a small risk that someone relies on this lock for synchronization,
> but I think that's unlikely because there isn't really any useful
> synchronization at this individual operation level. Any useful
> synchronization would likely need to protect at least a
> read-modify-write or similar.  So I made it unconditional without opt-in.

This part of the changelog is just crap.

The reason why pci_lock exists and is taken for each single read/write
config is that some ops implementations, e.g. the generic ones, must
protect at this granularity level because

	ops->map_bus()
	read/writeX()

needs to be 'atomic'.

MMCONFIG obviously does not require this at all because it's a simple
byte/word/dword read/write which is serialized by itself. So it's obvious
that the serialization with pci_lock is pointless in this case.

It's not that hard to figure it out and write up a proper changelog instead
of handwaving about risk and whatever.

Thanks,

	tglx