New CAM locking preview

[Steven Hartland]

----- Original Message ----- 
From: "Hans Petter Selasky" <hps at bitfrost.no>
To: "Alexander Motin" <mav at FreeBSD.org>
Cc: "Scott Long" <scottl at FreeBSD.org>; "Jeff Roberson" <jeff at freebsd.org>; "ken" <ken at freebsd.org>; <freebsd-hackers at FreeBSD.org>; 
"FreeBSD SCSI" <freebsd-scsi at freebsd.org>; "Steven Hartland" <smh at FreeBSD.org>; "Justin T. Gibbs" <gibbs at FreeBSD.org>
Sent: Friday, August 16, 2013 1:38 AM
Subject: Re: New CAM locking preview


> On 08/15/13 23:40, Alexander Motin wrote:
>> Hi.
>>
>> Last weeks I've made substantial progress on my CAM locking work. In
>> fact, at this moment I think I've tied all loose ends good enough to
>> consider the new design viable and implementation worth further testing
>> and bug fixing. So I would like to ask for review of my work from
>> everybody who interested in CAM internals.
>>
>> In short, my idea was to split single per-SIM lock, that creates huge
>> congestion under high IOPS, into several smaller ones. So design I've
>> finally chosen includes such locks:
>>   1) New per-device (per-LUN) locks to protect state of the devices and
>> respective periphs. In most cases peripheral drivers just use that lock
>> instead of SIM lock used before, so code modification is minimal and
>> straightforward.
>>   2) New per-target lock to protect list of LUNs fetched from the device.
>>   3) Old single per-SIM lock to protect SIM driver internals, but only
>> that. No parts of CAM itself use that lock. Keeping it for SIMs allows
>> to keep API and hopefully ABI compatibility. Reducing its scope allows
>> to reduce congestion.
>>   4) New per-SIM lock to protect SIM and device command queues. That
>> allows execute queued commands from any context unrelated to other
>> locks. Also this lock serializes accesses to sim_action() method for the
>> most of commands, this allows to mostly avoid busy spilling on SIM lock
>> collision.
>>   5) New per-bus locks to protect target, device and periphs reference
>> counters. It allows to create and destroy paths unrelated to other locks
>> in any possible context.
>>
>
> Sounds very good! I assume you have tested USB mass storage device unplug during various file system operations?

It does indeed sound like some very good work, thanks Alexander!

@Hans if USB mass storage device unplug is something important to you
then might I suggest its a good idea to grab the patches, run your own
tests and report any issues you might find as I'm sure this would be
most appreciated :)

    Regards
    Steve 


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