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: a80be428fbc1 ("rcu: Do not disable GP stall detection in rcu_cpu_stall_reset()") Signed-off-by: Joel Fernandes (Google) joel@joelfernandes.org --- v1->v2: Just a resend with reported-by fixed.

kernel/rcu/tree.c | 12 ++++++++++++ kernel/rcu/tree.h | 4 ++++ kernel/rcu/tree_stall.h | 20 ++++++++++++++++++-- 3 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 1449cb69a0e0..b695c0eb515a 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1552,10 +1552,22 @@ 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); diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index 192536916f9a..e9821a8422db 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -386,6 +386,10 @@ 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 */ diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h index b10b8349bb2a..a2fa6b22e248 100644 --- a/kernel/rcu/tree_stall.h +++ b/kernel/rcu/tree_stall.h @@ -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); @@ -725,6 +731,16 @@ 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;

/*