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Re: [libvirt] [PATCH 8/8] secret: Handle object list removal and deletion properly

On 07/11/2017 11:52 AM, Michal Privoznik wrote:
> On 06/03/2017 03:27 PM, John Ferlan wrote:
>> Rather than rely on virSecretObjEndAPI to make the final virObjectUnref
>> after the call virSecretObjListRemove, be more explicit by calling
>> virObjectUnref and setting @obj to NULL for secretUndefine and in
>> the error path of secretDefineXML.
>> This also fixes a leak during virSecretLoad if the virSecretLoadValue
>> fails the code jumps to cleanup without setting @ret = obj, thus calling
>> virSecretObjListRemove which only accounts for the object reference
>> related to adding the object to the list during virSecretObjListAdd,
>> but does not account for the reference to the object itself as the
>> return of @ret would be NULL so the caller wouldn't call virSecretObjEndAPI
>> on the object recently added thus reducing the refcnt to zero. Thus
>> cleaning up the virSecretLoadValue error path to make it clearer what
>> needs to be done on failure.
> I think the real reason is that we cannot call virSecretObjEndAPI()
> because that *unlocks* the secret object. However, the object is already
> unlocked at that point by virSecretObjListRemove() and thus we would
> unlock twice while locking just once. Honestly, I'd rather see that
> explanation in the commit message. But it's your call.

Partially true - although calling Unlock on something already Unlocked
by the same thread IIRC doesn't do much other than cause an error, but
we don't fail on that error.

It's ironic this relates to something Erik and I discussed during the
virNodeDev* changes w/r/t the "owner" (driver) after the Add/Append now
has a reference on the object and would always need to Unref it even
after removing it from the List.

This just happened to be where I had that oh sh*t moment and realized
that when calling Remove we were essentially unlocking and thus yes
calling EndAPI would unlock and unlocked object.  I'll add something in
about that..


As an aside, this is exactly why I started down the path of common
objects. Consider how the callers to virDomainObjListAdd go through
great lengths to managed the returned object some quite differently
based on what they know about how many refs are returned and whether the
calling function calls virDomainObjEndAPI.  If it does, there's always a
virObjectRef done on the object after the Add returns. It's a subtle
thing, but confusing nonetheless.

The thing is the *Remove API will call virHashRemoveEntry which
decrements a refcnt for each table; however, the *Add API only
increments the refcnt once for adding into each table.  Callers are very
careful to understand and manage that.

I'd rather see that mgmt go away. It should be "safe" to call EndAPI obj
the @obj regardless if Add or Lookup was used.  The callers shouldn't
have to know they need to either use *EndAPI or ObjUnlock.  In any case,
I digress and that's a different issue for another day

>> Signed-off-by: John Ferlan <jferlan redhat com>
>> ---
>>  src/conf/virsecretobj.c    | 14 ++++++--------
>>  src/secret/secret_driver.c |  9 +++++++--
>>  2 files changed, 13 insertions(+), 10 deletions(-)
> Michal

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