lkml.org 
[lkml]   [2008]   [Oct]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [rfc] SLOB memory ordering issue


On Thu, 16 Oct 2008, Nick Piggin wrote:
>
> What do you mean by the allocation is stable?

"all writes done to it before it's exposed".

> 2. I think it could be easy to assume that the allocated object that was
> initialised with a ctor for us already will have its initializing stores
> ordered when we get it from slab.

You make tons of assumptions.

You assume that
(a) unlocked accesses are the normal case and should be something the
allocator should prioritize/care about.
(b) that if you have a ctor, it's the only thing the allocator will do.

I don't think either of those assumptions are at all relevant or
interesting. Quite the reverse - I'd expect them to be in a very small
minority.

Now, obviously, on pretty much all machines out there (ie x86[-64] and UP
ARM), smp_wmb() is a no-op, so in that sense we could certainly say that
"sure, this is a total special case, but we can add a smp_wmb() anyway
since it won't cost us anything".

On the other hand, on the machines where it doesn't cost us anything, it
obviously doesn't _do_ anything either, so that argument is pretty
dubious.

And on machines where the memory ordering _can_ matter, it's going to add
cost to the wrong point.

Linus


\
 
 \ /
  Last update: 2008-10-15 20:05    [W:0.065 / U:2.776 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site