On 10/31/24 15:02, Jens Axboe wrote:
On 10/31/24 7:54 AM, Jens Axboe wrote:
On 10/31/24 5:20 AM, Peter Mann wrote:
Hello,
it appears that there is a high probability of a deadlock occuring when performing fsfreeze on a filesystem which is currently performing multiple io_uring O_DIRECT writes.
Steps to reproduce:
1. Mount xfs or ext4 filesystem on /mnt
2. Start writing to the filesystem. Must use io_uring, direct io and iodepth>1 to reproduce:
fio --ioengine=io_uring --direct=1 --bs=4k --size=100M --rw=randwrite --loops=100000 --iodepth=32 --name=test --filename=/mnt/fio_test
3. Run this in another shell. For me it deadlocks almost immediately:
while true; do fsfreeze -f /mnt/; echo froze; fsfreeze -u /mnt/; echo unfroze; done
4. Fsfreeze and all tasks attempting to write /mnt get stuck:
At this point all stuck processes cannot be killed by SIGKILL and they are stuck in uninterruptible sleep.
If you try 'touch /mnt/a' for example, the new process gets stuck in the exact same way as well.
This gets printed when running 6.11.4 with some debug options enabled:
[ 539.586122] Showing all locks held in the system:
[ 539.612972] 1 lock held by khungtaskd/35:
[ 539.626204] #0: ffffffffb3b1c100 (rcu_read_lock){....}-{1:2}, at: debug_show_all_locks+0x32/0x1e0
[ 539.640561] 1 lock held by dmesg/640:
[ 539.654282] #0: ffff9fd541a8e0e0 (&user->lock){+.+.}-{3:3}, at: devkmsg_read+0x74/0x2d0
[ 539.669220] 2 locks held by fio/647:
[ 539.684253] #0: ffff9fd54fe720b0 (&ctx->uring_lock){+.+.}-{3:3}, at: __do_sys_io_uring_enter+0x5c2/0x820
[ 539.699565] #1: ffff9fd541a8d450 (sb_writers#15){++++}-{0:0}, at: io_issue_sqe+0x9c/0x780
[ 539.715587] 2 locks held by fio/648:
[ 539.732293] #0: ffff9fd54fe710b0 (&ctx->uring_lock){+.+.}-{3:3}, at: __do_sys_io_uring_enter+0x5c2/0x820
[ 539.749121] #1: ffff9fd541a8d450 (sb_writers#15){++++}-{0:0}, at: io_issue_sqe+0x9c/0x780
[ 539.765484] 2 locks held by fio/649:
[ 539.781483] #0: ffff9fd541a8f0b0 (&ctx->uring_lock){+.+.}-{3:3}, at: __do_sys_io_uring_enter+0x5c2/0x820
[ 539.798785] #1: ffff9fd541a8d450 (sb_writers#15){++++}-{0:0}, at: io_issue_sqe+0x9c/0x780
[ 539.815466] 2 locks held by fio/650:
[ 539.831966] #0: ffff9fd54fe740b0 (&ctx->uring_lock){+.+.}-{3:3}, at: __do_sys_io_uring_enter+0x5c2/0x820
[ 539.849527] #1: ffff9fd541a8d450 (sb_writers#15){++++}-{0:0}, at: io_issue_sqe+0x9c/0x780
[ 539.867469] 1 lock held by fsfreeze/696:
[ 539.884565] #0: ffff9fd541a8d450 (sb_writers#15){++++}-{0:0}, at: freeze_super+0x20a/0x600
I reproduced this bug on nvme, sata ssd, virtio disks and lvm logical volumes.
It deadlocks on all kernels that I tried (all on amd64):
6.12-rc5 (compiled from kernel.org)
6.11.4 (compiled from kernel.org)
6.10.11-1~bpo12+1 (debian)
6.1.0-23 (debian)
5.14.0-427.40.1.el9_4.x86_64 (rocky linux)
5.10.0-33-amd64 (debian)
I tried to compile some older ones to check if it's a regression, but
those either didn't compile or didn't boot in my VM, sorry about that.
If you have anything specific for me to try, I'm happy to help.
Found this issue as well, so it seems like it's not just me:
https://gitlab.com/qemu-project/qemu/-/issues/881
Note that mariadb 10.6 adds support for io_uring, and that proxmox backups perform fsfreeze in the guest VM.
Originally I discovered this after a scheduled lvm snapshot of mariadb
got stuck. It appears that lvm calls dm_suspend, which then calls
freeze_super, so it looks like the same bug to me. I discovered the
simpler fsfreeze/fio reproduction method when I tried to find a
workaround.
Thanks for the report! I'm pretty sure this is due to the freezing not
allowing task_work to run, which prevents completions from being run.
Hence you run into a situation where freezing isn't running the very IO
completions that will free up the rwsem, with IO issue being stuck on
the freeze having started.
I'll take a look...
Can you try the below? Probably easiest on 6.12-rc5 as you already
tested that and should apply directly.
diff --git a/io_uring/rw.c b/io_uring/rw.c
index 30448f343c7f..ea057ec4365f 100644
--- a/io_uring/rw.c
+++ b/io_uring/rw.c
@@ -1013,6 +1013,18 @@ int io_read_mshot(struct io_kiocb *req, unsigned int issue_flags)
return IOU_OK;
}
+static bool io_kiocb_start_write(struct io_kiocb *req, struct kiocb *kiocb)
+{
+ if (!(req->flags & REQ_F_ISREG))
+ return true;
+ if (!(kiocb->ki_flags & IOCB_NOWAIT)) {
+ kiocb_start_write(kiocb);
+ return true;
+ }
+
+ return sb_start_write_trylock(file_inode(kiocb->ki_filp)->i_sb);
+}
+
int io_write(struct io_kiocb *req, unsigned int issue_flags)
{
bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;
@@ -1050,8 +1062,8 @@ int io_write(struct io_kiocb *req, unsigned int issue_flags)
if (unlikely(ret))
return ret;
- if (req->flags & REQ_F_ISREG)
- kiocb_start_write(kiocb);
+ if (unlikely(!io_kiocb_start_write(req, kiocb)))
+ return -EAGAIN;
kiocb->ki_flags |= IOCB_WRITE;
if (likely(req->file->f_op->write_iter))
I can confirm this fixes both the fsfreeze and lvm snapshot issues.
Thank you very much!
--
Peter Mann
