Re: deferring __fput()

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

On Mon, Jun 25, 2012 at 02:03:25PM +0200, Peter Zijlstra wrote:
> On Sat, 2012-06-23 at 21:57 +0100, Al Viro wrote:
> > 3) at that point task_work is equal in size (and layout, BTW) to rcu_head.  So we can add it
> > into the same union in struct file where we already have list_head and rcu_head.  No space
> > eaten up.  fput() would, once the counter reaches 0, remove the file from list (the only
> > place walking that list skips the ones with zero refcount anyway) and, if we are in a normal
> > process, use task_work_add() to have __fput() done to it.  If we are in kernel thread or
> > atomic context, just move the sucker to global list and use schedule_work() to have said
> > list emptied and everything in it fed to __fput(). 
> 
> So we're now Ok with doing fput() async?
> 
> Last time I remember this coming up people thought this wasn't such a
> hot idea.

You mean, doing that from RCU callbacks?  Still a bad idea, IMO; you will end up with a context
switch and unpleasantness with delayed user-visible effects of syscalls.  With aio we did have
a delayed execution of fput() anyway; all that has changed there is that we use generic
mechanism instead of home-grown analog thereof.

I'll need to reread that thread to comment on the specifics (had been too long ago; I don't
remember the details), but...  See Linus' objections to full-async fput() circa this April
or March.  There's a reason why this patchset uses task_work_add() whenever possible.