Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

[Boqun Feng <boqun.feng@gmail.com>]

On Wed, Sep 25, 2024 at 12:11:52PM +0200, Jonas Oberhauser wrote:
> 
> 
> Am 9/25/2024 um 12:02 PM schrieb Boqun Feng:
> > Hi Jonas,
> > 
> > Of
> > course, if we are really worried about compilers being too "smart"
> 
> Ah, I see you know me better and better...
> 
> > we can always do the comparison in asm code, then compilers don't know
> > anything of the equality between 'ptr' and 'head - head_offset'.
> Yes, but then a simple compiler barrier between the comparison and returning
> ptr would also do the trick, right? And maybe easier on the eyes.
> 

The thing about putting a compiler barrier is that it will prevent all
compiler reorderings, and some of the reordering may contribute to
better codegen. (I know in this case, we have a smp_mb(), but still
compilers can move unrelated code upto the second load for optimization
purpose). Asm comparison is cheaper in this way. But TBH, compilers
should provide a way to compare pointer values without using the result
for pointer equality proof, if "convert to unsigned long" doesn't work,
some other ways should work.

Regards,
Boqun

> 
> Have fun,
>    jonas
>