netdev - Re: Using a waiting MDIO does not go well with a spinlocked bridge

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Message-ID: <CAE4R7bC34kpgfd0CcewrJ+JmKOxv4py4xL1_OZubcGEL=O28ug@mail.gmail.com>
Date:	Mon, 23 Mar 2015 11:37:32 -0700
From:	Scott Feldman <sfeldma@...il.com>
To:	Jonas Johansson <jonasj76@...il.com>
Cc:	Netdev <netdev@...r.kernel.org>,
	"stephen@...workplumber.org" <stephen@...workplumber.org>,
	Florian Fainelli <f.fainelli@...il.com>,
	Jiří Pírko <jiri@...nulli.us>
Subject: Re: Using a waiting MDIO does not go well with a spinlocked bridge

On Mon, Mar 23, 2015 at 8:42 AM, Jonas Johansson <jonasj76@...il.com> wrote:
>
>
> On Fri, 20 Mar 2015, Scott Feldman wrote:
>
>> On Fri, Mar 20, 2015 at 5:22 AM, Jonas Johansson <jonasj76@...il.com>
>> wrote:
[cut]
>> I think we need to hook switchdev in fdb_delete(), then it'll get
>> called from flush and ageing out operations, rather than adding a new
>> switch_flush().  But, that's an aside for your main issue that the
>> bridge will hold a spinlock for most (all?) FDB delete operations.  I
>> don't see a way around relaxing that, on the bridge side, since it's
>> doing things like walking lists while deleting list elements.  So that
>> means the call into switchdev will be spinlocked, so switchdev driver
>> needs to deal with that.  Scheduling to work queue is one option, as
>> you mention, if FDB delete can't be done under the spinlock.
>>
>>
> Thanks for the input.
> My idea of using a switch_flush() was to take advantage of the HW to flush
> all FDB entries in one single operation. If I'm not mistaken, using
> fdb_delete() will result in a call for each FDB entry, which will result in
> a lot of overhead.

Let's get the most general case working (delete of a single FDB entry)
and then add optimizations later, if needed.  I suspect, for drivers
doing deletes atomically, the overhead is negligible, and for drivers
doing deferred deletes, again the overhead would be negligible.

-scott
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