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rcu: Make expedited RCU CPU stall warnings detect stall-end races

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If an expedited RCU CPU stall ends just at the stall-warning timeout,
the current code will print an expedited stall-warning message, but one
that doesn't identify any CPUs or tasks causing the stall.  This is most
likely to happen for short-timeout stalls, for example, the 20-millisecond
timeouts that are sometimes used for small embedded devices.  Needless to
say, these semi-empty stall-warning messages can be rather confusing.

One option would be to suppress the stall-warning message entirely in
this case, but the near-miss information can be quite valuable.

Detect this race condition and emits a "INFO: Expedited stall ended
before state dump start" message to clarify matters.

[boqun: Apply feedback from Borislav]

Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
This commit is contained in:
Paul E. McKenney
2025-12-29 11:16:15 -08:00
committed by Boqun Feng
parent 8f0b4cce44
commit 255019537c
1 changed files with 6 additions and 1 deletions
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7
kernel/rcu/tree_exp.h
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@@ -589,7 +589,12 @@ static void synchronize_rcu_expedited_stall(unsigned long jiffies_start, unsigne
pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
j - jiffies_start, rcu_state.expedited_sequence, data_race(rnp_root->expmask),
".T"[!!data_race(rnp_root->exp_tasks)]);
if (ndetected) {
if (!ndetected) {
// This is invoked from the grace-period worker, so
// a new grace period cannot have started. And if this
// worker were stalled, we would not get here. ;-)
pr_err("INFO: Expedited stall ended before state dump start\n");
} else {
pr_err("blocking rcu_node structures (internal RCU debug):");
rcu_for_each_node_breadth_first(rnp) {
if (rnp == rnp_root)