Re: [PATCH 2/3] block: Introduce blk_rq_completed()

[James Bottomley <jbottomley@parallels.com>]

On Tue, 2014-05-27 at 13:52 +0200, Paolo Bonzini wrote:
> Il 27/05/2014 13:26, James Bottomley ha scritto:
> >> You could use a different mechanism than a softirq to tell the abort
> >> were successful, for example by overriding scsi_done.  But with respect
> >> to the block layer, the mechanics of avoiding the race and double-free
> >> would probably be the same.
> >
> > I think there's some confusion about what the race and double free is:
> > It only occurs with timeouts.  In a timeout situation, the host had
> > decided it's not waiting any longer for the target to respond and
> > proceeds to error recovery.  At any time between the host making this
> > decision up to the point it kicks the target hard enough to clear all
> > in-flight commands, the target may return the command.  If we didn't
> > have some ignore function on command completions while we're handling
> > errors, this would lead to double completion.
> >
> > If we decided to allow arbitrary aborts of running commands, we would
> > send a TMF in during the normal (i.e. un timed out) command period.
> > Because there's no timeout involved, there's no double free problem.
> > The race in this case is whether the abort catches the command or not
> > and to mediate that race we need the normal status return.
> 
> I'm not sure why "no timeout" implies "no double free".  There would 
> still be a race between the interrupt handler and softirq on one side, 
> and the abort handler on the other.

You're assuming the current error handling after timeout methodology.
If we allow arbitrary TMFs, that doesn't hold because we have to wait
for the command completion.

James