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rcu/tree: Defer setting of jiffies during stall reset 

commit b96e7a5fa0ba9cda32888e04f8f4bac42d49a7f8 upstream.

There are instances where rcu_cpu_stall_reset() is called when jiffies
did not get a chance to update for a long time. Before jiffies is
updated, the CPU stall detector can go off triggering false-positives
where a just-started grace period appears to be ages old. In the past,
we disabled stall detection in rcu_cpu_stall_reset() however this got
changed [1]. This is resulting in false-positives in KGDB usecase [2].

Fix this by deferring the update of jiffies to the third run of the FQS
loop. This is more robust, as, even if rcu_cpu_stall_reset() is called
just before jiffies is read, we would end up pushing out the jiffies
read by 3 more FQS loops. Meanwhile the CPU stall detection will be
delayed and we will not get any false positives.

[1] https://lore.kernel.org/all/20210521155624.174524-2-senozhatsky@chromium.org/
[2] https://lore.kernel.org/all/20230814020045.51950-2-chenhuacai@loongson.cn/

Tested with rcutorture.cpu_stall option as well to verify stall behavior
with/without patch.

Tested-by: Huacai Chen <chenhuacai@loongson.cn>
Reported-by: Binbin Zhou <zhoubinbin@loongson.cn>
Closes: https://lore.kernel.org/all/20230814020045.51950-2-chenhuacai@loongson.cn/
Suggested-by: Paul  McKenney <paulmck@kernel.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Fixes: a80be428fb ("rcu: Do not disable GP stall detection in rcu_cpu_stall_reset()")
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Joel Fernandes (Google) 2023-09-05 00:02:11 +00:00 committed by Greg Kroah-Hartman
parent 6aa3cab6be
commit 8d837850d3
3 changed files with 34 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
kernel/rcu/tree.c
View File

@ -1604,10 +1604,22 @@ static bool rcu_gp_fqs_check_wake(int *gfp)
*/
static void rcu_gp_fqs(bool first_time)
{
int nr_fqs = READ_ONCE(rcu_state.nr_fqs_jiffies_stall);
struct rcu_node *rnp = rcu_get_root();
WRITE_ONCE(rcu_state.gp_activity, jiffies);
WRITE_ONCE(rcu_state.n_force_qs, rcu_state.n_force_qs + 1);
WARN_ON_ONCE(nr_fqs > 3);
/* Only countdown nr_fqs for stall purposes if jiffies moves. */
if (nr_fqs) {
if (nr_fqs == 1) {
WRITE_ONCE(rcu_state.jiffies_stall,
jiffies + rcu_jiffies_till_stall_check());
}
WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, --nr_fqs);
}
if (first_time) {
/* Collect dyntick-idle snapshots. */
force_qs_rnp(dyntick_save_progress_counter);

4
kernel/rcu/tree.h
View File

@ -365,6 +365,10 @@ struct rcu_state {
/* in jiffies. */
unsigned long jiffies_stall; /* Time at which to check */
/* for CPU stalls. */
int nr_fqs_jiffies_stall; /* Number of fqs loops after
* which read jiffies and set
* jiffies_stall. Stall
* warnings disabled if !0. */
unsigned long jiffies_resched; /* Time at which to resched */
/* a reluctant CPU. */
unsigned long n_force_qs_gpstart; /* Snapshot of n_force_qs at */

20
kernel/rcu/tree_stall.h
View File

@ -149,12 +149,17 @@ static void panic_on_rcu_stall(void)
/**
* rcu_cpu_stall_reset - restart stall-warning timeout for current grace period
*
* To perform the reset request from the caller, disable stall detection until
* 3 fqs loops have passed. This is required to ensure a fresh jiffies is
* loaded. It should be safe to do from the fqs loop as enough timer
* interrupts and context switches should have passed.
*
* The caller must disable hard irqs.
*/
void rcu_cpu_stall_reset(void)
{
WRITE_ONCE(rcu_state.jiffies_stall,
jiffies + rcu_jiffies_till_stall_check());
WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 3);
WRITE_ONCE(rcu_state.jiffies_stall, ULONG_MAX);
}
//////////////////////////////////////////////////////////////////////////////
@ -170,6 +175,7 @@ static void record_gp_stall_check_time(void)
WRITE_ONCE(rcu_state.gp_start, j);
j1 = rcu_jiffies_till_stall_check();
smp_mb(); // ->gp_start before ->jiffies_stall and caller's ->gp_seq.
WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 0);
WRITE_ONCE(rcu_state.jiffies_stall, j + j1);
rcu_state.jiffies_resched = j + j1 / 2;
rcu_state.n_force_qs_gpstart = READ_ONCE(rcu_state.n_force_qs);
@ -694,6 +700,16 @@ static void check_cpu_stall(struct rcu_data *rdp)
!rcu_gp_in_progress())
return;
rcu_stall_kick_kthreads();
/*
* Check if it was requested (via rcu_cpu_stall_reset()) that the FQS
* loop has to set jiffies to ensure a non-stale jiffies value. This
* is required to have good jiffies value after coming out of long
* breaks of jiffies updates. Not doing so can cause false positives.
*/
if (READ_ONCE(rcu_state.nr_fqs_jiffies_stall) > 0)
return;
j = jiffies;
/*