Re: deferring __fput()

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

On Sun, Jun 24, 2012 at 05:16:52AM +0100, Al Viro wrote:
> We could, in principle, add a "ok_late" argument, allowing to add after
> PF_EXITING has been set only if it's true and run the list twice, but
> that's really more convoluted than I would like...
> 
> Comments?

OK...  What I had in mind is (modulo really dire need of saner commit messages
and probably a different ordering/splitup of the first 3 commits) is
in vfs.git#untested.  WARNING: the branch name is no joke; it builds, but
I hadn't even tried to boot the resulting kernel yet.

Comments would be very welcome.  It should get us the situation when
	* __fput() is always called with no locks held by caller and
can take any locks whatsoever.
	* fput() is legal to call from any contexts
	* fput() done by a syscall will be completed before the process
returns to userland or terminates
	* no extra context switches, unless we have the final fput() done
from interrupt (instant death on the current kernel) or from the
kernel thread.
	* SCM_RIGHTS datagram destruction should be no worse than it is now;
probably a bit kinder on stack, even...  Again, no extra context switches.
	* Neither struct file nor struct task_struct changed size.
	* task_work and rcu_head are identical at that point; I'd appreciate
a better name (I ended up calling that sucker callback_head, defined in
types.h, with #define rcu_head callback_head next to it, to avoid global
rename from hell).  I can live with two identical structs (and a union
of those two in a few places), but I really see no point in going that way.