Re: [PATCH 2/4] fs: add support for LOOKUP_NONBLOCK

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On Tue, Dec 15, 2020 at 08:29:40AM -0700, Jens Axboe wrote:
> On 12/15/20 5:24 AM, Matthew Wilcox wrote:
> > On Mon, Dec 14, 2020 at 12:13:22PM -0700, Jens Axboe wrote:
> >> +++ b/fs/namei.c
> >> @@ -686,6 +686,8 @@ static bool try_to_unlazy(struct nameidata *nd)
> >>  	BUG_ON(!(nd->flags & LOOKUP_RCU));
> >>  
> >>  	nd->flags &= ~LOOKUP_RCU;
> >> +	if (nd->flags & LOOKUP_NONBLOCK)
> >> +		goto out1;
> > 
> > If we try a walk in a non-blocking context, it fails, then we punt to
> > a thread, do we want to prohibit that thread trying an RCU walk first?
> > I can see arguments both ways -- this may only be a temporary RCU walk
> > failure, or we may never be able to RCU walk this path.
> 
> In my opinion, it's not worth it trying to over complicate matters by
> handling the retry side differently. Better to just keep them the
> same. We'd need a lookup anyway to avoid aliasing.

but by clearing LOOKUP_RCU here, aren't you making the retry handle
things differently?  maybe i got lost.



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