On Sat, Feb 11, 2017 at 07:31:42AM -0500, Jeff Layton wrote:
The basic idea here is that nfsv4 sessions have a "resting state" of 0.
We want to keep them around, but if they go "dead" then we we'll tear
them down if they aren't actively in use at the time. So, we still free
the thing when the refcount goes to zero, but we have an extra condition
before we free it on the put -- that the session is also "dead" (meaning
that the client asked us to destroy it).
Your patch doesn't look like it'll break anything, but I personally find
it harder to follow that way. The freeable reference state will be 1
instead of the normal 0.
Alas, I don't have any examples in mind, but doesn't this pattern happen
all over?
You have objects that live in some data structure. They're freed only
when they're removed from the data structure. You want removal to fail
whenever they're in use.
So it's natural to use an atomic counter to track the number of external
users and some other lock to serialize lookup and destruction.
--b.
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On Sat, Feb 11, 2017 at 8:15 PM, Bruce Fields [off-list ref] wrote:
On Sat, Feb 11, 2017 at 07:31:42AM -0500, Jeff Layton wrote:
quoted
The basic idea here is that nfsv4 sessions have a "resting state" of 0.
We want to keep them around, but if they go "dead" then we we'll tear
them down if they aren't actively in use at the time. So, we still free
the thing when the refcount goes to zero, but we have an extra condition
before we free it on the put -- that the session is also "dead" (meaning
that the client asked us to destroy it).
Your patch doesn't look like it'll break anything, but I personally find
it harder to follow that way. The freeable reference state will be 1
instead of the normal 0.
Alas, I don't have any examples in mind, but doesn't this pattern happen
all over?
The majority of refcounted objects are allocated with refcount=1: the
very fact that they're being allocated means that they already have a
user.
The issue with struct nfsd4_session is that, like struct nfs4_client,
references to it are only taken when the server is actively working on
it. Its default "resting state" is with refcount=0.
I would like to make its default resting state with refcount=1. In
other cases similar to this, we've gotten around it by doing a
semantic +1 to the object's overall refcounting scheme.
Jeff suggested taking an additional reference in init_session(), and
dropping it in is_session_dead(), after determining, in fact, that the
object is DEAD.
You have objects that live in some data structure. They're freed only
when they're removed from the data structure. You want removal to fail
whenever they're in use.
When they're in use, these objects' refcount should be > 0.
So it's natural to use an atomic counter to track the number of external
users and some other lock to serialize lookup and destruction.
When considering refcounted objects, the most "natural" interpretation
of refcount=0 means that the object no longer has any users and can be
freed. Increments on objects with refcount=0 shouldn't be allowed, as
this may indicate a use-after-free condition.
This discussion is difficult because the refcount_t API hasn't yet
been introduced. The purpose of that API is to eliminate
use-after-free bugs.
--b.