lkml.org 
[lkml]   [2012]   [Sep]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 5/5] virtio-scsi: introduce multiqueue support
Il 04/09/2012 15:35, Michael S. Tsirkin ha scritto:
> I see. I guess you can rewrite this as:
> atomic_inc
> if (atomic_read() == 1)
> which is a bit cheaper, and make the fact
> that you do not need increment and return to be atomic,
> explicit.

It seems more complicated to me for hardly any reason. (Besides, is it
cheaper? It has one less memory barrier on some architectures I frankly
do not care much about---not on x86---but it also has two memory
accesses instead of one on all architectures).

> Another simple idea: store last processor id in target,
> if it is unchanged no need to play with req_vq
> and take spinlock.

Not so sure, consider the previous example with last_processor_id equal
to 1.

queuecommand on CPU #0 queuecommand #2 on CPU #1
--------------------------------------------------------------
atomic_inc_return(...) == 1
atomic_inc_return(...) == 2
virtscsi_queuecommand to queue #1
last_processor_id == 0? no
spin_lock
tgt->req_vq = queue #0
spin_unlock
virtscsi_queuecommand to queue #0

This is not a network driver, there are still a lot of locks around.
This micro-optimization doesn't pay enough for the pain.

> Also - some kind of comment explaining why a similar race can not happen
> with this lock in place would be nice: I see why this specific race can
> not trigger but since lock is dropped later before you submit command, I
> have hard time convincing myself what exactly gurantees that vq is never
> switched before or even while command is submitted.

Because tgt->reqs will never become zero (which is a necessary condition
for tgt->req_vq to change), as long as one request is executing
virtscsi_queuecommand.

Paolo


\
 
 \ /
  Last update: 2012-09-04 16:22    [W:0.062 / U:0.144 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site