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McKenney" Subject: Re: [PATCH] events: Annotate parent_ctx with __rcu Message-ID: <20200210125948.GA16485@workstation-portable> References: <20200208144648.18833-1-frextrite@gmail.com> <20200210093624.GB14879@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200210093624.GB14879@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 10, 2020 at 10:36:24AM +0100, Peter Zijlstra wrote: > On Sat, Feb 08, 2020 at 08:16:49PM +0530, Amol Grover wrote: > > parent_ctx is used under RCU context in kernel/events/core.c, > > tell sparse about it aswell. > > > > Fixes the following instances of sparse error: > > kernel/events/core.c:3221:18: error: incompatible types in comparison > > kernel/events/core.c:3222:23: error: incompatible types in comparison > > > > This introduces the following two sparse errors: > > kernel/events/core.c:3117:18: error: incompatible types in comparison > > kernel/events/core.c:3121:30: error: incompatible types in comparison > > > > Hence, use rcu_dereference() to access parent_ctx and silence > > the newly introduced errors. > > > > Signed-off-by: Amol Grover > > --- > > include/linux/perf_event.h | 2 +- > > kernel/events/core.c | 11 ++++++++--- > > 2 files changed, 9 insertions(+), 4 deletions(-) > > > > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h > > index 6d4c22aee384..e18a7bdc6f98 100644 > > --- a/include/linux/perf_event.h > > +++ b/include/linux/perf_event.h > > @@ -810,7 +810,7 @@ struct perf_event_context { > > * These fields let us detect when two contexts have both > > * been cloned (inherited) from a common ancestor. > > */ > > - struct perf_event_context *parent_ctx; > > + struct perf_event_context __rcu *parent_ctx; > > u64 parent_gen; > > u64 generation; > > int pin_count; > > diff --git a/kernel/events/core.c b/kernel/events/core.c > > index 2173c23c25b4..2a8c5670b254 100644 > > --- a/kernel/events/core.c > > +++ b/kernel/events/core.c > > @@ -3106,26 +3106,31 @@ static void ctx_sched_out(struct perf_event_context *ctx, > > static int context_equiv(struct perf_event_context *ctx1, > > struct perf_event_context *ctx2) > > { > > + struct perf_event_context *parent_ctx1, *parent_ctx2; > > + > > lockdep_assert_held(&ctx1->lock); > > lockdep_assert_held(&ctx2->lock); > > > > + parent_ctx1 = rcu_dereference(ctx1->parent_ctx); > > + parent_ctx2 = rcu_dereference(ctx2->parent_ctx); > > Bah. > > Why are you fixing all this sparse crap and making the code worse? Hi Peter, Sparse is quite noisy and we need to eliminate false-positives, right? __rcu will tell the developer, this pointer could change and he needs to take the required steps to make sure the code doesn't break. Thanks Amol