[RFC PATCH v2 2/5] mm/show_mem: Avoid blocking on zone->lock during Oops or panic

[Aaron Tomlin <atomlin@atomlin.com>]

When show_free_areas() iterates over populated memory zones, it
unconditionally acquires zone->lock via spin_lock_irqsave(). If
show_mem() is invoked from an Oops or panic context while the crashing
CPU or a wedged remote CPU already holds zone->lock, or from an NMI or
hardirq context under CONFIG_PREEMPT_RT where spinlock_t is a sleeping
rt_mutex, acquiring zone->lock can self-deadlock or fault before
post-mortem recovery (such as kdump) can proceed.

Check can_spin_trylock() and use spin_trylock_irqsave() when
!can_spin_trylock(), in_nmi(), oops_in_progress, or panic_in_progress()
is active, and emit the zone header after releasing zone->lock,
logging a contention notice and skipping the locked zone rather than
spinning indefinitely.

Fixes: d999bd9392de ("panic: add options to print system info when panic happens")
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
 mm/show_mem.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/mm/show_mem.c b/mm/show_mem.c
index b938cbcd774a..77098077523a 100644
--- a/mm/show_mem.c
+++ b/mm/show_mem.c
@@ -367,10 +367,17 @@ static void show_free_areas(unsigned int filter, const nodemask_t *nodemask,
 			continue;
 		if (show_mem_node_skip(filter, zone_to_nid(zone), nodemask))
 			continue;
-		show_node(zone);
-		printk(KERN_CONT "%s: ", zone->name);
 
-		spin_lock_irqsave(&zone->lock, flags);
+		if (!can_spin_trylock() || in_nmi() ||
+		    oops_in_progress || panic_in_progress()) {
+			if (!can_spin_trylock() ||
+			    !spin_trylock_irqsave(&zone->lock, flags)) {
+				pr_warn("zone %s lock contended\n", zone->name);
+				continue;
+			}
+		} else {
+			spin_lock_irqsave(&zone->lock, flags);
+		}
 		for (order = 0; order < NR_PAGE_ORDERS; order++) {
 			struct free_area *area = &zone->free_area[order];
 			int type;
@@ -385,6 +392,8 @@ static void show_free_areas(unsigned int filter, const nodemask_t *nodemask,
 			}
 		}
 		spin_unlock_irqrestore(&zone->lock, flags);
+		show_node(zone);
+		printk(KERN_CONT "%s: ", zone->name);
 		for (order = 0; order < NR_PAGE_ORDERS; order++) {
 			printk(KERN_CONT "%lu*%lukB ",
 			       nr[order], K(1UL) << order);
-- 
2.55.0