From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 15 Oct 2008 11:03:08 -0700 (PDT) From: Linus Torvalds Subject: Re: [rfc] SLOB memory ordering issue In-Reply-To: <200810160445.28781.nickpiggin@yahoo.com.au> Message-ID: References: <200810160334.13082.nickpiggin@yahoo.com.au> <200810160410.49894.nickpiggin@yahoo.com.au> <200810160445.28781.nickpiggin@yahoo.com.au> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: Nick Piggin Cc: Matt Mackall , Pekka Enberg , linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: 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 -- 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