Re: [PATCH 4/6] UBI: Fastmap: Fix races in ubi_wl_get_peb()

[Tanya Brokhman]

On 12/5/2014 11:08 PM, Richard Weinberger wrote:

> > > > >             spin_unlock(&ubi->wl_lock);
> > > > > +        if (retried) {
> > > > > +            ubi_err(ubi, "Unable to get a free PEB from user WL pool");
> > > > > +            ret = -ENOSPC;
> > > > > +            goto out;
> > > > > +        }
> > > > > +        retried = 1;
> > > >
> > > > Why did you decide to retry in this function? and why only 1 retry attempt? I'm not against it, trying to understand the logic.
> > >
> > > Because failing immediately with -ENOSPC is not nice.
> >
> > Why not? this is what was done before....
>
> The behavior from before was not good.
> If we return here a -ENOSPC it is not because we ran out of free PEBs, it is because the pool contains
> no free PEBs and needs refilling.
> As between refilling the pool and requesting a fresh PEB from it another thread could "steal" all PEBs
> we retry.
>
> > I think what I really bothers me in this case is that you don't sleep, you branch immediately to retry again, so the chances that there will be context switch and free pebs appear
> > aren't that high.
> > I'm used to functions using some sort of "retry" logic to sleep before retrying. Of course sleeping isn't a good idea here. That's why the "retry" bugs me a bit.
>
> You mean a cond_resched()?
> This retry-logic is common pattern in UBI. For exmaple see ubi_wl_put_peb().

you're right. didn't pay much attention to ubi_wl_put_peb() before. 
don't like it there either :)
perhaps we can rethink this later for both cases.

> Thanks,
> //richard


Thanks,
Tanya Brokhman
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