342898 - Need an atomic pointer swap

Status: NEW

(NSPR :: NSPR, enhancement)

Description

[Ben Turner (not reading bugmail, use the needinfo flag!)]

From bug 204114 it appears that NSPR doesn't have any ability to atomically swap a pointer value. This would be great for implementing RCU-type synchronization (see the URL).

Comment 1

[Benjamin Smedberg]

wtc, assuming that you're ok with this change, what's the process for getting the appropriate platform-specific code (assembly)?

Comment 2

[Wan-Teh Chang]

I suspect the proposed function is the pointer
analog of PR_AtomicSet, but I'm not sure.
Please write down the proposed function prototype
so I can comment.

Comment 3

[Ben Turner (not reading bugmail, use the needinfo flag!)]

(In reply to comment #2)
> I suspect the proposed function is the pointer
> analog of PR_AtomicSet, but I'm not sure.
> Please write down the proposed function prototype
> so I can comment.
> 

Yes, you're absolutely correct. The function would take as arguments the address of the pointer to set and the value for that pointer. It would then return the previous value of the pointer. It's syntax would be just like PR_AtomicSet:

  void* oldValue = PR_AtomicSetPtr(&pointerToSet, newValue);

Comment 4

[Benjamin Smedberg]

NSPR_API(void*) PR_AtomicSetPointer(void **val, void *newval);

And, if reading a pointer-sized value is not an atomic operation on all systems:

NSPR_API(void*) PR_AtomicReadPointer(void** location);

Comment 5

[Wan-Teh Chang]

On 32-bit platforms PR_AtomicSetPointer can be implemented
using PR_AtomicSet with typecasts.  On 64-bit platforms
you should be able to adapt the assembly code of PR_AtomicSet
for 64-bit.

Comment 6

[BugBot [:suhaib / :marco/ :calixte]]

The bug assignee is inactive on Bugzilla, so the assignee is being reset.