On Wed, Nov 23, 2022 at 09:56:37PM +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().
>
> 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
Thank you! I have queued this with some wordsmithing and modifications,
as shown below. Please check these.
Also, please test future patches with the debug options shown in
Documentation/process/submit-checklist.rst. In this case, testing with
CONFIG_PROVE_RCU=y would have located a bug with this repeat-by:
tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 2m --configs "SRCU-P SRCU-N" --trust-make
Thanx, Paul
------------------------------------------------------------------------
commit 17a874a71faed9e38b69c6ec7bae6aa309a2530e
Author: Pingfan Liu <kernelfans@xxxxxxxxx>
Date: Wed Nov 23 21:56:37 2022 +0800
srcu: Remove needless rcu_seq_done() check while holding read lock
The srcu_gp_start_if_needed() function now read-holds the srcu_struct
whose grace period is being started, which means that the corresponding
SRCU grace period cannot end. This in turn means that the SRCU
grace-period sequence number returned by rcu_seq_snap() cannot expire
during this time. And that means that the calls to rcu_seq_done() in
srcu_funnel_exp_start() and srcu_funnel_gp_start() can never return true.
This commit therefore removes these rcu_seq_done() checks, but adds checks
in kernels built with CONFIG_PROVE_RCU=y that splats if rcu_seq_done()
does somehow return true.
[ paulmck: Rearrange checks to handle kernels built with lockdep. ]
Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
To: rcu@xxxxxxxxxxxxxxx
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 6af0312005801..b25f77df9e5e7 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -915,8 +915,9 @@ 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)))
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) &&
+ rcu_seq_done(&ssp->srcu_gp_seq, 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;
@@ -942,6 +943,9 @@ 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)
@@ -962,8 +966,8 @@ 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. */
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) &&
+ rcu_seq_done(&ssp->srcu_gp_seq, s));
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)) {
@@ -999,9 +1003,9 @@ 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. */
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && rcu_seq_done(&ssp->srcu_gp_seq, s));
+ 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);
