linux-kernel - Re: [PATCH-v2 1/3] percpu_ida: Make percpu_ida_alloc + callers accept task state bitmask

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Date:	Fri, 24 Jan 2014 22:33:55 -0800
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	kmo@...erainc.com, target-devel <target-devel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH-v2 1/3] percpu_ida: Make percpu_ida_alloc + callers
 accept task state bitmask

On Fri, 2014-01-24 at 16:14 +0100, Peter Zijlstra wrote:
> On Thu, Jan 23, 2014 at 11:38:24AM -0800, Nicholas A. Bellinger wrote:
> > > AFAICT, those changes don't address the original bug that the series was
> > > trying to address, allowing the percpu_ida_alloc() tag stealing slow
> > > path to be interrupted by a signal..
> > > 
> > > Also, keep in mind this change needs to be backported to >= v3.12, which
> > > is why the percpu_ida changes have been kept to a minimum.
> 
> Well, the other option is to revert whatever caused the issue in the
> first place :-)
> 
> I'm not much for making ugly fixes just because its easier to backport.
> 

Me either, but given Kent's comment on subtle issues for larger
improvements in the tag stealing slow path, it's the right approach for
addressing a bug in stable code.

Also, I'd also prefer to avoid introducing new issues for blk-mq in the
first round of stable code, give the amount of testing it's already
undergone with percpu_ida for v3.13.

> > <A little too eager to SEND>
> > 
> > So would you prefer the following addition to the original bugfix
> > instead..?
> 
> I'll make a right old mess out of percpu_ida.c, but yeah.
> 

Kent and I would both like to see your improvements merged in upstream,
just not in a manner that would cause Greg-KH to start cursing loudly
for a stable backport.

That said, unless Jens throws a last minute NACK, I'll be queuing patch
#1 + #3 into target-pending/for-next for sunday night's build.

Jens, please feel free to pickup Patch #2 for a post v3.15 merge at your
earliest convenience.

Thanks,

--nab

> > diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c
> > index a48ce2e..58b6714 100644
> > --- a/lib/percpu_ida.c
> > +++ b/lib/percpu_ida.c
> > @@ -174,7 +174,8 @@ int percpu_ida_alloc(struct percpu_ida *pool, int state)
> >                  *
> >                  * global lock held and irqs disabled, don't need percpu lock
> >                  */
> > -               prepare_to_wait(&pool->wait, &wait, state);
> > +               if (state != TASK_RUNNING)
> > +                       prepare_to_wait(&pool->wait, &wait, state);
> >  
> >                 if (!tags->nr_free)
> >                         alloc_global_tags(pool, tags);
> > @@ -199,8 +200,9 @@ int percpu_ida_alloc(struct percpu_ida *pool, int state)
> >                 local_irq_save(flags);
> >                 tags = this_cpu_ptr(pool->tag_cpu);
> >         }
> > +       if (state != TASK_RUNNING)
> > +               finish_wait(&pool->wait, &wait);
> >  
> > -       finish_wait(&pool->wait, &wait);
> >         return tag;
> >  }
> >  EXPORT_SYMBOL_GPL(percpu_ida_alloc);
> > 
> > 
> --
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