From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Subject: Re: [rfc] SLOB memory ordering issue Date: Thu, 16 Oct 2008 05:12:28 +1100 References: <200810160334.13082.nickpiggin@yahoo.com.au> <200810160445.28781.nickpiggin@yahoo.com.au> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200810160512.28443.nickpiggin@yahoo.com.au> Sender: owner-linux-mm@kvack.org Return-Path: To: Linus Torvalds Cc: Matt Mackall , Pekka Enberg , linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: On Thursday 16 October 2008 05:03, Linus Torvalds wrote: > 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. Yes, as I said, I do not want to add a branch and/or barrier to the allocator for this. I just want to flag the issue and discuss whether there is anything that can be done about it. > 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. They will be in the minority or non-existant, but obviously there only need be one "counterexample" bug to disprove a claim that it never matters. > 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. When I said "I'd really hate to add a branch to the slab fastpath", it wasn't a tacit acknowlegement that the barrier is the only way to go, if it sounded that way. I meant: I'd *really* hate to add a branch to the slab fastpath :) -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org