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McKenney" Cc: rcu@vger.kernel.org, Lai Jiangshan , Frederic Weisbecker , Josh Triplett , Steven Rostedt , Mathieu Desnoyers Subject: Re: [PATCH 1/2] srcu: Remove needless rcu_seq_done() check while holding read lock Message-ID: References: <20221120034014.7390-1-kernelfans@gmail.com> <20221120034014.7390-2-kernelfans@gmail.com> <20221122011345.GS4001@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221122011345.GS4001@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Mon, Nov 21, 2022 at 05:13:45PM -0800, Paul E. McKenney wrote: > On Sun, Nov 20, 2022 at 11:40:13AM +0800, Pingfan Liu wrote: > > As the code changes, now, srcu_gp_start_if_needed() holds the srcu read > > lock, gets the gp_seq snap and call srcu_funnel_gp_start() passing that > > snap value. > > > > As the rcu_seq_snap() promises "a full grace period has elapsed since > > the current time". In srcu_funnel_gp_start(), the statement > > rcu_seq_done(&ssp->srcu_gp_seq, s) > > always return false. > > > > The same condition stands for srcu_funnel_exp_start(). Hence removing > > all the checks of rcu_seq_done(). > > > > Test info: > > Running "tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 10h --configs 18*SRCU-P" > > without any failure. > > Nice catch!!! And I do appreciate the testing. > > But if we are going to do this, let's please also place lockdep assertions > in srcu_funnel_exp_start() and srcu_funnel_gp_start() to verify that these > functions are in fact invoked within an SRCU read-side critical section. > Also a WARN_ON_ONCE(rcu_seq_done(&ssp->srcu_gp_seq, s)), please. > OK, I will do both. > Things changed to make the rcu_seq_done() unnecessary, so they could just > as easily change again. > Yes. It is wise to prevent from the future variation. Thanks, Pingfan > Thanx, Paul > > > Signed-off-by: Pingfan Liu > > Cc: Lai Jiangshan > > Cc: "Paul E. McKenney" > > Cc: Frederic Weisbecker > > Cc: Josh Triplett > > Cc: Steven Rostedt > > Cc: Mathieu Desnoyers > > To: rcu@vger.kernel.org > > --- > > kernel/rcu/srcutree.c | 13 ++++++------- > > 1 file changed, 6 insertions(+), 7 deletions(-) > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > > index 1c304fec89c0..7df59fc8073e 100644 > > --- a/kernel/rcu/srcutree.c > > +++ b/kernel/rcu/srcutree.c > > @@ -851,8 +851,7 @@ static void srcu_funnel_exp_start(struct srcu_struct *ssp, struct srcu_node *snp > > if (snp) > > for (; snp != NULL; snp = snp->srcu_parent) { > > sgsne = READ_ONCE(snp->srcu_gp_seq_needed_exp); > > - if (rcu_seq_done(&ssp->srcu_gp_seq, s) || > > - (!srcu_invl_snp_seq(sgsne) && ULONG_CMP_GE(sgsne, s))) > > + if (!srcu_invl_snp_seq(sgsne) && ULONG_CMP_GE(sgsne, s)) > > return; > > spin_lock_irqsave_rcu_node(snp, flags); > > sgsne = snp->srcu_gp_seq_needed_exp; > > @@ -878,6 +877,9 @@ static void srcu_funnel_exp_start(struct srcu_struct *ssp, struct srcu_node *snp > > * > > * Note that this function also does the work of srcu_funnel_exp_start(), > > * in some cases by directly invoking it. > > + * > > + * The srcu read lock should be hold around this function. And s is a seq snap > > + * after holding that lock. > > */ > > static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp, > > unsigned long s, bool do_norm) > > @@ -898,8 +900,6 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp, > > if (snp_leaf) > > /* Each pass through the loop does one level of the srcu_node tree. */ > > for (snp = snp_leaf; snp != NULL; snp = snp->srcu_parent) { > > - if (rcu_seq_done(&ssp->srcu_gp_seq, s) && snp != snp_leaf) > > - return; /* GP already done and CBs recorded. */ > > spin_lock_irqsave_rcu_node(snp, flags); > > snp_seq = snp->srcu_have_cbs[idx]; > > if (!srcu_invl_snp_seq(snp_seq) && ULONG_CMP_GE(snp_seq, s)) { > > @@ -935,9 +935,8 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp, > > if (!do_norm && ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, s)) > > WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, s); > > > > - /* If grace period not already done and none in progress, start it. */ > > - if (!rcu_seq_done(&ssp->srcu_gp_seq, s) && > > - rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) { > > + /* If grace period not already in progress, start it. */ > > + if (rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) { > > WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed)); > > srcu_gp_start(ssp); > > > > -- > > 2.31.1 > >