Re: [Lse-tech] SCSI io_request_lock patch

Jens Axboe
2001-11-13T08:23:54Z

On Mon, Nov 12 2001, Jonathan Lahr wrote:
>
> This is a request for comments on the patch described below which
> implements a revised approach to reducing io_request_lock contention
> in 2.4.
>
> This new version of the io_request_lock patch (siorl-v0) is available
> at http://sourceforge.net/projects/lse/. It employs the same
> concurrent request queueing scheme as the iorlv0 patch but isolates
> code changes to the SCSI subsystem and engages the new locking scheme
> only for SCSI drivers which explicitly request it. I took this more
> restricted approach after additional development based on comments from
> Jens and others indicated that iorlv0 impacted the IDE subsystem and
> was unnecessarily broad in general.
>
> The siorl-v0 patch allows drivers to enable concurrent queueing through
> the concurrent_queue field in the Scsi_Host_Template which is copied to
> the request queue. It creates SCSI-specific versions of generic block
> i/o functions used by the SCSI subsystem and modifies them to conditionally
> engage the new locking scheme based on this field. It allows control over
> which drivers use concurrent queueing and preserves original block i/o
> behavior by default.

Sorry Jonathan, but this is even more broken than the last patch. In
different ways. In no particular order:

o You are duplicating way too much code and exporting block internals
o You are breaking SCSI merge completely, why on earth are you suddenly
using ll_*_merge functions for SCSI?!
o scsi_make_request need not worry about head active
o scsi_make_request can safe the q->*_merge indirect
o scsi_dispatch_cmd() io_request_lock removal looks racy

At least you are not breaking anything other than SCSI this time...

--
Jens Axboe