[PATCH] ocfs2: try a blocking lock before return AOP_TRUNCATED_PAGE

[Gang He]

If we can't get inode lock immediately in the function
ocfs2_inode_lock_with_page() when reading a page, we should not
return directly here, since this will lead to a softlockup problem.
The method is to get a blocking lock and immediately unlock before
returning, this can avoid CPU resource waste due to lots of retries,
and benefits fairness in getting lock among multiple nodes, increase
efficiency in case modifying the same file frequently from multiple
nodes.
The softlockup problem looks like,
Kernel panic - not syncing: softlockup: hung tasks
CPU: 0 PID: 885 Comm: multi_mmap Tainted: G L 4.12.14-6.1-default #1
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
Call Trace:
  <IRQ>
  dump_stack+0x5c/0x82
  panic+0xd5/0x21e
  watchdog_timer_fn+0x208/0x210
  ? watchdog_park_threads+0x70/0x70
  __hrtimer_run_queues+0xcc/0x200
  hrtimer_interrupt+0xa6/0x1f0
  smp_apic_timer_interrupt+0x34/0x50
  apic_timer_interrupt+0x96/0xa0
  </IRQ>
 RIP: 0010:unlock_page+0x17/0x30
 RSP: 0000:ffffaf154080bc88 EFLAGS: 00000246 ORIG_RAX: ffffffffffffff10
 RAX: dead000000000100 RBX: fffff21e009f5300 RCX: 0000000000000004
 RDX: dead0000000000ff RSI: 0000000000000202 RDI: fffff21e009f5300
 RBP: 0000000000000000 R08: 0000000000000000 R09: ffffaf154080bb00
 R10: ffffaf154080bc30 R11: 0000000000000040 R12: ffff993749a39518
 R13: 0000000000000000 R14: fffff21e009f5300 R15: fffff21e009f5300
  ocfs2_inode_lock_with_page+0x25/0x30 [ocfs2]
  ocfs2_readpage+0x41/0x2d0 [ocfs2]
  ? pagecache_get_page+0x30/0x200
  filemap_fault+0x12b/0x5c0
  ? recalc_sigpending+0x17/0x50
  ? __set_task_blocked+0x28/0x70
  ? __set_current_blocked+0x3d/0x60
  ocfs2_fault+0x29/0xb0 [ocfs2]
  __do_fault+0x1a/0xa0
  __handle_mm_fault+0xbe8/0x1090
  handle_mm_fault+0xaa/0x1f0
  __do_page_fault+0x235/0x4b0
  trace_do_page_fault+0x3c/0x110
  async_page_fault+0x28/0x30
 RIP: 0033:0x7fa75ded638e
 RSP: 002b:00007ffd6657db18 EFLAGS: 00010287
 RAX: 000055c7662fb700 RBX: 0000000000000001 RCX: 000055c7662fb700
 RDX: 0000000000001770 RSI: 00007fa75e909000 RDI: 000055c7662fb700
 RBP: 0000000000000003 R08: 000000000000000e R09: 0000000000000000
 R10: 0000000000000483 R11: 00007fa75ded61b0 R12: 00007fa75e90a770
 R13: 000000000000000e R14: 0000000000001770 R15: 0000000000000000

Fixes: 1cce4df04f37 ("ocfs2: do not lock/unlock() inode DLM lock")
Signed-off-by: Gang He <ghe@xxxxxxxx>
---
 fs/ocfs2/dlmglue.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
index 4689940..5193218 100644
--- a/fs/ocfs2/dlmglue.c
+++ b/fs/ocfs2/dlmglue.c
@@ -2486,6 +2486,15 @@ int ocfs2_inode_lock_with_page(struct inode *inode,
 	ret = ocfs2_inode_lock_full(inode, ret_bh, ex, OCFS2_LOCK_NONBLOCK);
 	if (ret == -EAGAIN) {
 		unlock_page(page);
+		/*
+		 * If we can't get inode lock immediately, we should not return
+		 * directly here, since this will lead to a softlockup problem.
+		 * The method is to get a blocking lock and immediately unlock
+		 * before returning, this can avoid CPU resource waste due to
+		 * lots of retries, and benefits fairness in getting lock.
+		 */
+		if (ocfs2_inode_lock(inode, ret_bh, ex) == 0)
+			ocfs2_inode_unlock(inode, ex);
 		ret = AOP_TRUNCATED_PAGE;
 	}
 
-- 
1.8.5.6