crash> bt
PID: 20412    TASK: ffff9b55dc432800  CPU: 0    COMMAND: "mount"
 #0 [ffffc0d4e40238c8] __schedule at ffffffff91beafe6
 #1 [ffffc0d4e4023920] schedule at ffffffff91beb735
 #2 [ffffc0d4e4023930] xfs_extent_busy_flush at ffffffffc03b49aa [xfs]
 #3 [ffffc0d4e4023980] xfs_alloc_ag_vextent_size at ffffffffc03622c5 [xfs]
 #4 [ffffc0d4e40239f0] xfs_alloc_ag_vextent at ffffffffc0362f16 [xfs]
 #5 [ffffc0d4e4023a00] xfs_alloc_fix_freelist at ffffffffc036369b [xfs]
 #6 [ffffc0d4e4023b08] xfs_free_extent_fix_freelist at ffffffffc0363e54 [xfs]
 #7 [ffffc0d4e4023bb8] __xfs_free_extent at ffffffffc0363ef8 [xfs]
 #8 [ffffc0d4e4023c18] xfs_trans_free_extent at ffffffffc03df456 [xfs]
 #9 [ffffc0d4e4023c70] xfs_efi_item_recover at ffffffffc03df6c1 [xfs]
#10 [ffffc0d4e4023cb8] xlog_recover_process_intents at ffffffffc03e7433 [xfs]
#11 [ffffc0d4e4023d60] xlog_recover_finish at ffffffffc03e794c [xfs]
#12 [ffffc0d4e4023d78] xfs_log_mount_finish at ffffffffc03d93d2 [xfs]
#13 [ffffc0d4e4023da0] xfs_mountfs at ffffffffc03cbf92 [xfs]
#14 [ffffc0d4e4023df8] xfs_fs_fill_super at ffffffffc03d08ac [xfs]
#15 [ffffc0d4e4023e30] get_tree_bdev at ffffffff915603ff
#16 [ffffc0d4e4023e70] vfs_get_tree at ffffffff9155ead5
#17 [ffffc0d4e4023e90] do_mount at ffffffff91585209
#18 [ffffc0d4e4023ef8] ksys_mount at ffffffff91585c1e
#19 [ffffc0d4e4023f30] __x64_sys_mount at ffffffff91585c61
#20 [ffffc0d4e4023f38] do_syscall_64 at ffffffff912043ab
#21 [ffffc0d4e4023f50] entry_SYSCALL_64_after_hwframe at ffffffff91c000a9
    RIP: 00007f1878cb135e  RSP: 00007fff7025cd78  RFLAGS: 00000246
    RAX: ffffffffffffffda  RBX: 00005633b04ada40  RCX: 00007f1878cb135e
    RDX: 00005633b04b5250  RSI: 00005633b04adc40  RDI: 00005633b04adc20
    RBP: 00007f1879b23184   R8: 0000000000000000   R9: 0000000000000002
    R10: 00000000c0ed0000  R11: 0000000000000246  R12: 0000000000000000
    R13: 00000000c0ed0000  R14: 00005633b04adc20  R15: 00005633b04b5250
    ORIG_RAX: 00000000000000a5  CS: 0033  SS: 002b

// here :

581 void
582 xfs_extent_busy_flush(
583         struct xfs_mount        *mp,
584         struct xfs_perag        *pag,
585         unsigned                busy_gen)
586 {
587         DEFINE_WAIT             (wait);
588         int                     error;
589 
590         error = xfs_log_force(mp, XFS_LOG_SYNC);
591         if (error)
592                 return;
593 
594         do {
595                 prepare_to_wait(&pag->pagb_wait, &wait, TASK_KILLABLE);
596                 if  (busy_gen != READ_ONCE(pag->pagb_gen))
597                         break;
598                 schedule(); <<<<---
599         } while (1);

// we've been running for a long time :

crash> ps -t 20412
PID: 20412    TASK: ffff9b55dc432800  CPU: 0    COMMAND: "mount"
    RUN TIME: 02:49:31
  START TIME: 304267682759
       UTIME: 5832582
       STIME: 9204315314778


// for some reason on vmcore process is on RUN state, but i suspect is not the case
// if i look for busy extents i don't find any but on current transaction :

crash> xfs_cil_ctx.busy_extents 0xffff9b5528f60300
  busy_extents = {
    next = 0xffff9b5528f60330,
    prev = 0xffff9b5528f60330
  },
crash> xfs_cil_ctx.busy_extents 0xffff9b5528f60300 -ox
struct xfs_cil_ctx {
  [ffff9b5528f60330] struct list_head busy_extents;
}


crash> xfs_trans.t_busy ffff9b54c9bfbed0 -ox
struct xfs_trans {
  [ffff9b54c9bfbf90] struct list_head t_busy;
}

crash> list -H ffff9b54c9bfbf90 -o xfs_extent_busy.list
ffff9b5598c14980
crash> xfs_extent_busy ffff9b5598c14980
struct xfs_extent_busy {
  rb_node = {
    __rb_parent_color = 1,
    rb_right = 0x0,
    rb_left = 0x0
  },
  list = {
    next = 0xffff9b54c9bfbf90,
    prev = 0xffff9b54c9bfbf90
  },
  agno = 13,
  bno = 14770,
  length = 19,
  flags = 0
}