Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd_install

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sat, Apr 18, 2015 at 12:16:48AM +0200, Mateusz Guzik wrote:

> I would say this makes the use of seq counter impossible. Even if we
> decided to fall back to a lock on retry, we cannot know what to do if
> the slot is reserved - it very well could be that something called
> close, and something else reserved the slot, so putting the file inside
> could be really bad. In fact we would be putting a file for which we
> don't have a reference anymore.
> 
> However, not all hope is lost and I still think we can speed things up.
> 
> A locking primitive which only locks stuff for current cpu and has
> another mode where it locks stuff for all cpus would do the trick just
> fine. I'm not a linux guy, quick search suggests 'lglock' would do what
> I want.
> 
> table reallocation is an extremely rare operation, so this should be
> fine. It would take the lock 'globally' for given table.

It would also mean percpu_alloc() for each descriptor table...