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Date: Mon, 13 Apr 2015 13:39:04 +0200 From: Patrick Marlier <patrick.marlier@...il.com> To: Paul McKenney <paulmck@...ux.vnet.ibm.com> Cc: linux-kernel@...r.kernel.org, Josh Triplett <josh@...htriplett.org> Subject: Re: [PATCH 1/3] rculist: Fix list_entry_rcu to read ptr with rcu_dereference_raw Strange I don't get any conflict. Maybe due to my email client so I attached the patches to this email. Thanks, -- Patrick On Sat, Mar 28, 2015 at 11:47 AM, Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote: > On Sat, Mar 28, 2015 at 03:42:10AM -0700, Paul E. McKenney wrote: >> On Wed, Mar 25, 2015 at 04:01:24PM +0100, Patrick Marlier wrote: >> > On 03/25/2015 03:30 PM, Paul E. McKenney wrote: >> > >On Tue, Mar 24, 2015 at 11:31:38AM +0100, Patrick Marlier wrote: >> > >>Change to read effectively ptr with rcu_dereference_raw and not the >> > >>__ptr variable on the stack. >> > >> >> > >>Signed-off-by: Patrick Marlier <patrick.marlier@...il.com> >> > >Avoiding an extra load could be worthwhile in a number of situations, >> > >agreed. >> > Not only a load. It adds a store and a load on the stack and I think >> > this creates a dependency in the processor pipeline. >> > >> > >However, won't this change cause sparse to complain if invoked on a >> > >non-RCU-protected pointer? The ability to use list-RCU API >> > >members on both RCU and non-RCU pointers was one of the points >> > >of the previous commit, right? >> > Probably we can put back the cast but I am not familiar enough with >> > the RCU API. >> > >> > Also, the problem here is that you probably want ACCESS_ONCE to >> > happen on the content of 'ptr' and not on the stack variable >> > '__ptr'. >> > >> > (you have to follow this chain: rcu_dereference_raw -> >> > rcu_dereference_check -> __rcu_dereference_check -> >> > lockless_dereference -> ACCESS_ONCE) >> > >> > #define lockless_dereference(p) \ >> > ({ \ >> > typeof(p) _________p1 = ACCESS_ONCE(p); \ >> > smp_read_barrier_depends(); /* Dependency order vs. p above. */ \ >> > (_________p1); \ >> > }) >> > >> > #define __ACCESS_ONCE(x) ({ \ >> > __maybe_unused typeof(x) __var = (__force typeof(x)) 0; \ >> > (volatile typeof(x) *)&(x); }) >> > #define ACCESS_ONCE(x) (*__ACCESS_ONCE(x)) >> > >> > Note that ACCESS_ONCE is doing "&" on x. >> > >> > IMHO, I would prefer saving some useless instructions for better >> > performance rather than giving too much flexibility on the API (also >> > pretty sure the cast can be still done). >> >> OK, what I am going to do is to apply your patches for testing purposes. >> If there are no complaints, they will likely go into v4.3 or thereabouts. > > Except that I hit conflicts. Could you please rebase to rcu/dev at > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git? > > Thanx, Paul > View attachment "0001-rculist-Fix-list_entry_rcu-to-read-ptr-with-rcu_dere.patch" of type "text/x-patch" (1157 bytes) View attachment "0002-netfilter-fix-list_entry_rcu-usage.patch" of type "text/x-patch" (948 bytes) View attachment "0003-md-bitmap-fix-list_entry_rcu-usage.patch" of type "text/x-patch" (915 bytes)
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