[PATCH 2/2 v3] lib: debugobjects: touch watchdog to avoid softlockup when !CONFIG_PREEMPT

["Yang Shi" <yang.s@alibaba-inc.com>]

There are nested loops on debug objects free path, sometimes it may take
over hundred thousands of loops, then cause soft lockup with !CONFIG_PREEMPT
occasionally, like below:

 NMI watchdog: BUG: soft lockup - CPU#15 stuck for 22s! [stress-ng-getde:110342]
 Modules linked in: binfmt_misc(E) tcp_diag(E)
inet_diag(E) bonding(E) intel_rapl(E) iosf_mbi(E)
x86_pkg_temp_thermal(E) coretemp(E) iTCO_wdt(E) iTCO_vendor_support(E)
kvm_intel(E) kvm(E) irqbypass(E) crct10dif_pclmul(E) crc32_pclmul(E)
dcdbas(E) ghash_clmulni_intel(E) aesni_intel(E) lrw(E) gf128mul(E)
glue_helper(E) ablk_helper(E) ipmi_devintf(E) sg(E) cryptd(E) pcspkr(E)
mei_me(E) lpc_ich(E) ipmi_si(E) mfd_core(E) mei(E) shpchp(E) wmi(E)
ipmi_msghandler(E) acpi_power_meter(E) nfsd(E) auth_rpcgss(E) nfs_acl(E)
lockd(E) grace(E) sunrpc(E) ip_tables(E) ext4(E) jbd2(E) mbcache(E)
sd_mod(E) mgag200(E) igb(E) drm_kms_helper(E) ixgbe(E) syscopyarea(E)
mdio(E) sysfillrect(E) sysimgblt(E) ptp(E) fb_sys_fops(E) pps_core(E)
ttm(E) drm(E) crc32c_intel(E) i2c_algo_bit(E) i2c_core(E)
megaraid_sas(E)
  dca(E)
 irq event stamp: 4340444
 hardirqs last  enabled at (4340443): [<ffffffff817d6476>]
_raw_spin_unlock_irqrestore+0x36/0x60
 hardirqs last disabled at (4340444): [<ffffffff817d8c81>]
apic_timer_interrupt+0x91/0xa0
 softirqs last  enabled at (4340398): [<ffffffff817da179>]
__do_softirq+0x349/0x50e
 softirqs last disabled at (4340391): [<ffffffff810a5255>]
irq_exit+0xf5/0x110
 CPU: 15 PID: 110342 Comm: stress-ng-getde Tainted: G
E   4.9.44-003.ali3000.alios7.x86_64.debug #1
 Hardware name: Dell Inc. PowerEdge R720xd/0X6FFV, BIOS
1.6.0 03/07/2013
 task: ffff884cbb0d0000 task.stack: ffff884cabc70000
 RIP: 0010:[<ffffffff817d647b>]  [<ffffffff817d647b>]
_raw_spin_unlock_irqrestore+0x3b/0x60
 RSP: 0018:ffff884cabc77b78  EFLAGS: 00000292
 RAX: ffff884cbb0d0000 RBX: 0000000000000292 RCX: 0000000000000000
 RDX: ffff884cbb0d0000 RSI: 0000000000000001 RDI: 0000000000000292
 RBP: ffff884cabc77b88 R08: 0000000000000000 R09: 0000000000000000
 R10: 0000000000000001 R11: 0000000000000001 R12: ffffffff8357a0d8
 R13: ffff884cabc77bc8 R14: ffffffff8357a0d0 R15: 00000000000000fc
 FS:  00002aee845fd2c0(0000) GS:ffff8852bd400000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000002991808 CR3: 0000005123abf000 CR4: 00000000000406e0
 Stack:
  ffff884ff4fe0000 ffff884ff4fd8000 ffff884cabc77c00 ffffffff8141177e
  0000000000000202 ffff884cbb0d0000 ffff884cabc77bc8 0000000000000006
  ffff884ff4fda000 ffffffff8357a0d8 0000000000000000 91f5d976f6020b6c
 Call Trace:
  [<ffffffff8141177e>] debug_check_no_obj_freed+0x13e/0x220
  [<ffffffff811f8751>] __free_pages_ok+0x1f1/0x5c0
  [<ffffffff811fa785>] __free_pages+0x25/0x40
  [<ffffffff812638db>] __free_slab+0x19b/0x270
  [<ffffffff812639e9>] discard_slab+0x39/0x50
  [<ffffffff812679f7>] __slab_free+0x207/0x270
  [<ffffffff81269966>] ___cache_free+0xa6/0xb0
  [<ffffffff8126c267>] qlist_free_all+0x47/0x80
  [<ffffffff8126c5a9>] quarantine_reduce+0x159/0x190
  [<ffffffff8126b3bf>] kasan_kmalloc+0xaf/0xc0
  [<ffffffff8126b8a2>] kasan_slab_alloc+0x12/0x20
  [<ffffffff81265e8a>] kmem_cache_alloc+0xfa/0x360
  [<ffffffff812abc8f>] ? getname_flags+0x4f/0x1f0
  [<ffffffff812abc8f>] getname_flags+0x4f/0x1f0
  [<ffffffff812abe42>] getname+0x12/0x20
  [<ffffffff81298da9>] do_sys_open+0xf9/0x210
  [<ffffffff81298ede>] SyS_open+0x1e/0x20
  [<ffffffff817d6e01>] entry_SYSCALL_64_fastpath+0x1f/0xc2
 Code: 7f 18 53 48 8b 55 08 48 89 f3 be 01 00 00 00 e8 3c
cd 92 ff 4c 89 e7 e8 f4 0e 93 ff f6 c7 02 74 1b e8 3a ac 92 ff 48 89 df
57 9d <66> 66 90 66 90 65 ff 0d d1 ff 83 7e 5b 41 5c 5d c3 48 89 df 57

The code path might be called in either atomic or non-atomic context,
so touching softlockup watchdog instead of calling cond_resched() which
might fall asleep. However, it is unnecessary to touch the watchdog
every loop, so just touch the watchdog at every 10000 (best estimate) loops.

And, introduce a new knob: /sys/kernel/debug/debug_objects/suppress_lockup.
0 value means not suppress the softlockup message by touching the
watchdog, non-zero value means suppress the softlockup message.
The default value is zero.

Signed-off-by: Yang Shi <yang.s@alibaba-inc.com>
CC: Waiman Long <longman@redhat.com>
CC: Thomas Gleixner <tglx@linutronix.de>
---
v2 --> v3:
 * Use debugfs_create_u32() helper API per Waiman's suggestion
v1 --> v2:
 * Added suppress_lockup knob in debugfs per Waiman's suggestion

 lib/debugobjects.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/lib/debugobjects.c b/lib/debugobjects.c
index 166488d..c960221 100644
--- a/lib/debugobjects.c
+++ b/lib/debugobjects.c
@@ -19,6 +19,7 @@
 #include <linux/slab.h>
 #include <linux/hash.h>
 #include <linux/kmemleak.h>
+#include <linux/nmi.h>
 
 #define ODEBUG_HASH_BITS	14
 #define ODEBUG_HASH_SIZE	(1 << ODEBUG_HASH_BITS)
@@ -67,6 +68,8 @@ struct debug_bucket {
 static int			debug_objects_allocated;
 static int			debug_objects_freed;
 
+static int			suppress_lockup;
+
 static void free_obj_work(struct work_struct *work);
 static DECLARE_WORK(debug_obj_work, free_obj_work);
 
@@ -768,6 +771,10 @@ static void __debug_check_no_obj_freed(const void *address, unsigned long size)
 			debug_objects_maxchain = cnt;
 
 		max_loops += cnt;
+
+		if (max_loops > 10000 && ((max_loops % 10000) == 0)
+		    && suppress_lockup != 0)
+			touch_softlockup_watchdog();
 	}
 
 	if (max_loops > debug_objects_maxloops)
@@ -812,7 +819,7 @@ static int debug_stats_open(struct inode *inode, struct file *filp)
 
 static int __init debug_objects_init_debugfs(void)
 {
-	struct dentry *dbgdir, *dbgstats;
+	struct dentry *dbgdir, *dbgstats, *dbglockup;
 
 	if (!debug_objects_enabled)
 		return 0;
@@ -826,10 +833,15 @@ static int __init debug_objects_init_debugfs(void)
 	if (!dbgstats)
 		goto err;
 
+	dbglockup = debugfs_create_u32("suppress_lockup", 0644, dbgdir,
+					&suppress_lockup);
+	if (!dbglockup)
+		goto err;
+
 	return 0;
 
 err:
-	debugfs_remove(dbgdir);
+	debugfs_remove_recursive(dbgdir);
 
 	return -ENOMEM;
 }
-- 
1.8.3.1