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Tue, 10 Apr 2018 13:50:19 -0400 Date: Tue, 10 Apr 2018 10:50:12 -0700 From: Matthew Wilcox To: Christopher Lameter Cc: Eric Dumazet , linux-mm@kvack.org, Matthew Wilcox , Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , linux-kernel@vger.kernel.org, Jan Kara , Jeff Layton , Mel Gorman , stable@vger.kernel.org Subject: Re: [PATCH 1/2] slab: __GFP_ZERO is incompatible with a constructor Message-ID: <20180410175011.GE3614@bombadil.infradead.org> References: <20180410125351.15837-1-willy@infradead.org> <20180410165054.GC3614@bombadil.infradead.org> <20180410173841.GD3614@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.2 (2017-12-15) Sender: stable-owner@vger.kernel.org X-Mailing-List: stable@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On Tue, Apr 10, 2018 at 12:45:56PM -0500, Christopher Lameter wrote: > On Tue, 10 Apr 2018, Matthew Wilcox wrote: > > > > How do you envision dealing with the SLAB_TYPESAFE_BY_RCU slab caches? > > > Those must have a defined state of the objects at all times and a constructor is > > > required for that. And their use of RCU is required for numerous lockless > > > lookup algorithms in the kernhel. > > > > Not at all times. Only once they've been used. Re-constructing them > > once they've been used might break the rcu typesafety, I suppose ... > > would need to examine the callers. > > Objects can be freed and reused and still be accessed from code that > thinks the object is the old and not the new object.... Yes, I know, that's the point of RCU typesafety. My point is that an object *which has never been used* can't be accessed. So you don't *need* a constructor.