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.
Things changed to make the rcu_seq_done() unnecessary, so they could just
as easily change again.
Thanx, Paul
> Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> To: rcu@xxxxxxxxxxxxxxx
> ---
> 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
>
