Re: make LWLockCounter a global variable

Hi,

On Thu, Aug 28, 2025 at 05:56:07PM -0400, Tom Lane wrote:
> Nathan Bossart <nathandbossart@gmail.com> writes:
> > In lwlock.c, uses of LWLockCounter must first calculate its address in
> > shared memory with something like this:
> 
> >     LWLockCounter = (int *) ((char *) MainLWLockArray - sizeof(int));
> 
> > This appears to have been started by commit 82e861f in order to fix
> > EXEC_BACKEND builds, but it could also be fixed by adding it to the
> > BackendParameters struct.  I find the current approach somewhat difficult
> > to read and understand, so I'd like to switch to the latter approach.  This
> > is admittedly just nitpicking...
> 
> No objection here.  As a small improvement, perhaps you could swap
> around the code in LWLockShmemSize so that the order in which it
> considers size contributions matches the physical layout, more
> or less like
> 
>     /* Calculate total number of locks needed in the main array. */
>     numLocks += NumLWLocksForNamedTranches();
> 
> +    /* Space for dynamic allocation counter, plus room for alignment. */
> +    size = sizeof(int) + LWLOCK_PADDED_SIZE;
> +
>     /* Space for the LWLock array. */
> -    size = mul_size(numLocks, sizeof(LWLockPadded));
> +    size = add_size(size, mul_size(numLocks, sizeof(LWLockPadded)));
> 
> -    /* Space for dynamic allocation counter, plus room for alignment. */
> -    size = add_size(size, sizeof(int) + LWLOCK_PADDED_SIZE);
> -
>     /* space for named tranches. */
>     size = add_size(size, mul_size(NamedLWLockTrancheRequests, sizeof(NamedLWLockTranche)));
> 
> I find it a little confusing that that code doesn't line up
> exactly with what CreateLWLocks does.

+1.

Another option could be to not change CreateLWLocks() at all, except removing the
local variable:

@@ -423,7 +424,6 @@ CreateLWLocks(void)
        if (!IsUnderPostmaster)
        {
                Size            spaceLocks = LWLockShmemSize();
-               int                *LWLockCounter;

to use the global variable. That way we preserve the current memory layout and
there is no need to change LWLockShmemSize().

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com