On Mon, 11 Feb 2002, John Baldwin wrote:
>
> On 12-Feb-02 Julian Elischer wrote:
> > The proclock is needed to get the reference,
> > guarding against other threads, and giant is needed fo rnot to free it
> > because if it reaches a refcount of 0 it needs to call free(). (which john
> > assures me needs Giant at this time).
> > We could avoid the proclock with judicious use of an atomic refcount
> > incrementing method.
>
> _No_! The proc lock is protecting p_ucred, it can't go away! What _can_ go
> away is the per-ucred mutex to protect the refcount if we ever revive the
> refcount API.
hmm ok Alfred, here's the way I see it..
You are never permitted to "CHANGE" a cred.
You ALWAYS allocate another and switch them.
When you switch them you decrement the refcount of the old one.
If someone else takes a reference on it at the same moment that you
drop it, then the order is important down to the bus-cycle
as to whether it gets freed or not. We already know that dereferencing it
from the proc structure is not important, because a "stale" ucred pointer
is only preventable from the userland.
All that is important is that the pointer is a REAL pointer to a cred
and that it is not allowed to go to 0 references in its way to 1
reference.
An atomic reference count increment that checks
against 0 would be part of it but might not be enough.
I think we also need to have a lock on something because
it might get freed and used for something else that happens to place a "1"
in that location inbetween the time that p->p_ucred is read
and the refcount is decremented.
As an asside:
I think that in NT they may have refcounts in the same location in all
structures as I think they derived all their structures from a bas class
that has ref counts. In that case it WOULD have a "1" in that location if
it were re-used. (It's been a long time since I saw NT code so I may be
wrong)
>
> > When Giant goes away it won't be so bad but it will STILL be quicker to
> > not drop it across userland.
>
> Yes. Actually, calling free() can still be rather expensive even when Giant is
> gone.
>
> --
>
> John Baldwin <[EMAIL PROTECTED]> <>< http://www.FreeBSD.org/~jhb/
> "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/
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