How to do proper locking

[Max Laier]

On Thursday 04 August 2005 13:40, Hans Petter Selasky wrote:
> This is a copy and paste from the kernel sources:
>
> struct ucred *
> crhold(struct ucred *cr)
> {
> The problem is, what happens if the kernel switches thread right here, and
> then the other thread calls "crfree()" on this structure, that will
> "free()" memory pointed to by "cr". Then the first line of code below will
> access freed memory, and then we are going for a segment fault or worse.
>
>         mtx_lock(cr->cr_mtxp);
>         cr->cr_ref++;
>         mtx_unlock(cr->cr_mtxp);
>         return (cr);
> }

It seems that you are misunderstanding the concept of reference counting.  The 
idea is to have a reference as long as you are handing around the object.  In 
order to call crhold() you must already hold a reference to the credential.  
This will prevent the credential structure to vanish and allow you to pass 
the credential to another consumer that now can assume to hold a reference of 
it's own.

If you can access your objects from a external list, you have to hold a global 
lock that protects:
 1) Sanity of the list
 2) Free operations to the member objects

If you access the objects via the list/tree/whatsoever "most of the time" - 
refcounting isn't for you.

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