I tested this series using next-20220315.
I can't test locking dependency because lockdep is turned off upon boot.
[ 30.251160] ======================================================
[ 30.251162] WARNING: possible circular locking dependency detected
[ 30.251164] 5.17.0-rc8-next-20220315+ #22 Not tainted
[ 30.251166] ------------------------------------------------------
[ 30.251168] mount/407 is trying to acquire lock:
[ 30.251170] ffff8881003c8898 (&p->pi_lock){-.-.}-{2:2}, at: try_to_wake_up+0x4f/0x4d0
[ 30.251185]
[ 30.251185] but task is already holding lock:
[ 30.251186] ffffffff830f7798 ((console_sem).lock){-.-.}-{2:2}, at: up+0xd/0x50
[ 30.251195]
[ 30.251195] which lock already depends on the new lock.
(...snipped...)
[ 30.251433] Chain exists of:
[ 30.251433] &p->pi_lock --> &rq->__lock --> (console_sem).lock
[ 30.251433]
[ 30.251440] Possible unsafe locking scenario:
[ 30.251440]
[ 30.251441] CPU0 CPU1
[ 30.251443] ---- ----
[ 30.251444] lock((console_sem).lock);
[ 30.251446] lock(&rq->__lock);
[ 30.251458] lock((console_sem).lock);
[ 30.251461] lock(&p->pi_lock);
[ 30.251464]
[ 30.251464] *** DEADLOCK ***
[ 30.251464]
[ 30.251465] 2 locks held by mount/407:
[ 30.251467] #0: ffffffff830c7878 (low_water_lock){+.+.}-{2:2}, at: do_exit+0x7ff/0xeb0
[ 30.251486] #1: ffffffff830f7798 ((console_sem).lock){-.-.}-{2:2}, at: up+0xd/0x50
But I can see that due to no longer waiting for lo->lo_mutex from lo_open(),
there are occasional I/O errors. What is your plan to avoid this?
----------------------------------------
[ 148.444639] loop56: detected capacity change from 0 to 2048
[ 148.448298] I/O error, dev loop56, sector 0 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
[ 148.456250] loop57: detected capacity change from 0 to 2048
--
[ 149.264210] loop70: detected capacity change from 0 to 2048
[ 149.267449] I/O error, dev loop70, sector 0 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
[ 149.314558] loop71: detected capacity change from 0 to 2048
--
[ 154.708948] loop172: detected capacity change from 0 to 2048
[ 154.712403] I/O error, dev loop172, sector 0 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
[ 154.760841] loop173: detected capacity change from 0 to 2048
[ 154.763728] I/O error, dev loop173, sector 0 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
[ 154.789921] loop174: detected capacity change from 0 to 2048
--
[ 155.469135] loop185: detected capacity change from 0 to 2048
[ 155.470800] I/O error, dev loop185, sector 0 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
[ 155.483457] loop186: detected capacity change from 0 to 2048
--
[ 155.568911] loop190: detected capacity change from 0 to 2048
[ 155.570783] I/O error, dev loop190, sector 0 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
[ 155.576789] loop191: detected capacity change from 0 to 2048
--
[ 159.671039] loop259: detected capacity change from 0 to 2048
[ 159.674879] I/O error, dev loop259, sector 0 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
[ 159.706059] loop260: detected capacity change from 0 to 2048
--
[ 162.845545] loop309: detected capacity change from 0 to 2048
[ 162.848151] I/O error, dev loop309, sector 0 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
[ 162.873731] loop310: detected capacity change from 0 to 2048
--
[ 162.940326] loop313: detected capacity change from 0 to 2048
[ 162.943770] I/O error, dev loop313, sector 0 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
[ 163.012664] loop314: detected capacity change from 0 to 2048
--
[ 164.725370] loop338: detected capacity change from 0 to 2048
[ 164.728747] I/O error, dev loop338, sector 0 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
[ 164.734669] loop339: detected capacity change from 0 to 2048
--
[ 166.463447] loop370: detected capacity change from 0 to 2048
[ 166.468262] I/O error, dev loop370, sector 0 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
[ 166.476621] loop371: detected capacity change from 0 to 2048
--
[ 169.133011] loop417: detected capacity change from 0 to 2048
[ 169.136747] I/O error, dev loop417, sector 0 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
[ 169.140203] I/O error, dev loop417, sector 0 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
[ 169.152730] Buffer I/O error on dev loop417, logical block 0, async page read
[ 169.299206] loop418: detected capacity change from 0 to 2048
----------------------------------------
By the way, if CONFIG_BLOCK_LEGACY_AUTOLOAD=n,
# mount -o loop,ro isofs.iso isofs/
unconditionally fails with
mount: isofs/: failed to setup loop device for isofs.iso.
message. Commit 451f0b6f4c44d7b6 ("block: default BLOCK_LEGACY_AUTOLOAD to y")
says "if the device node already exists because old scripts created it manually".
But it is not always manual creation of loop devices; I think it is
ioctl(LOOP_CTL_GET_FREE) in my case.
