Re: [PATCH] bcachefs: grab s_umount only if snapshotting

[Brian Foster <bfoster@redhat.com>]

On Tue, Jan 16, 2024 at 10:10:53PM +0800, Su Yue wrote:
> 
> On Tue 16 Jan 2024 at 08:47, Brian Foster <bfoster@redhat.com> wrote:
> 
> > On Mon, Jan 15, 2024 at 10:21:25AM +0800, Su Yue wrote:
> > > When I was testing mongodb over bcachefs with compression,
> > > there is a lockdep warning when snapshotting mongodb data volume.
> > > 
> > > $ cat test.sh
> > > prog=bcachefs
> > > 
> > > $prog subvolume create /mnt/data
> > > $prog subvolume create /mnt/data/snapshots
> > > 
> > > while true;do
> > >     $prog subvolume snapshot /mnt/data
> > > /mnt/data/snapshots/$(date +%s)
> > >     sleep 1s
> > > done
> > > 
> > > $ cat /etc/mongodb.conf
> > > systemLog:
> > >   destination: file
> > >   logAppend: true
> > >   path: /mnt/data/mongod.log
> > > 
> > > storage:
> > >   dbPath: /mnt/data/
> > > 
> > > lockdep reports:
> > > [ 3437.452330]
> > > ======================================================
> > > [ 3437.452750] WARNING: possible circular locking dependency
> > > detected
> > > [ 3437.453168] 6.7.0-rc7-custom+ #85 Tainted: G            E
> > > [ 3437.453562]
> > > ------------------------------------------------------
> > > [ 3437.453981] bcachefs/35533 is trying to acquire lock:
> > > [ 3437.454325] ffffa0a02b2b1418 (sb_writers#10){.+.+}-{0:0}, at:
> > > filename_create+0x62/0x190
> > > [ 3437.454875]
> > >                but task is already holding lock:
> > > [ 3437.455268] ffffa0a02b2b10e0
> > > (&type->s_umount_key#48){.+.+}-{3:3}, at:
> > > bch2_fs_file_ioctl+0x232/0xc90 [bcachefs]
> > > [ 3437.456009]
> > >                which lock already depends on the new lock.
> > > 
> > > [ 3437.456553]
> > >                the existing dependency chain (in reverse order)
> > > is:
> > > [ 3437.457054]
> > >                -> #3 (&type->s_umount_key#48){.+.+}-{3:3}:
> > > [ 3437.457507]        down_read+0x3e/0x170
> > > [ 3437.457772]        bch2_fs_file_ioctl+0x232/0xc90 [bcachefs]
> > > [ 3437.458206]        __x64_sys_ioctl+0x93/0xd0
> > > [ 3437.458498]        do_syscall_64+0x42/0xf0
> > > [ 3437.458779]        entry_SYSCALL_64_after_hwframe+0x6e/0x76
> > > [ 3437.459155]
> > >                -> #2 (&c->snapshot_create_lock){++++}-{3:3}:
> > > [ 3437.459615]        down_read+0x3e/0x170
> > > [ 3437.459878]        bch2_truncate+0x82/0x110 [bcachefs]
> > > [ 3437.460276]        bchfs_truncate+0x254/0x3c0 [bcachefs]
> > > [ 3437.460686]        notify_change+0x1f1/0x4a0
> > > [ 3437.461283]        do_truncate+0x7f/0xd0
> > > [ 3437.461555]        path_openat+0xa57/0xce0
> > > [ 3437.461836]        do_filp_open+0xb4/0x160
> > > [ 3437.462116]        do_sys_openat2+0x91/0xc0
> > > [ 3437.462402]        __x64_sys_openat+0x53/0xa0
> > > [ 3437.462701]        do_syscall_64+0x42/0xf0
> > > [ 3437.462982]        entry_SYSCALL_64_after_hwframe+0x6e/0x76
> > > [ 3437.463359]
> > >                -> #1 (&sb->s_type->i_mutex_key#15){+.+.}-{3:3}:
> > > [ 3437.463843]        down_write+0x3b/0xc0
> > > [ 3437.464223]        bch2_write_iter+0x5b/0xcc0 [bcachefs]
> > > [ 3437.464493]        vfs_write+0x21b/0x4c0
> > > [ 3437.464653]        ksys_write+0x69/0xf0
> > > [ 3437.464839]        do_syscall_64+0x42/0xf0
> > > [ 3437.465009]        entry_SYSCALL_64_after_hwframe+0x6e/0x76
> > > [ 3437.465231]
> > >                -> #0 (sb_writers#10){.+.+}-{0:0}:
> > > [ 3437.465471]        __lock_acquire+0x1455/0x21b0
> > > [ 3437.465656]        lock_acquire+0xc6/0x2b0
> > > [ 3437.465822]        mnt_want_write+0x46/0x1a0
> > > [ 3437.465996]        filename_create+0x62/0x190
> > > [ 3437.466175]        user_path_create+0x2d/0x50
> > > [ 3437.466352]        bch2_fs_file_ioctl+0x2ec/0xc90 [bcachefs]
> > > [ 3437.466617]        __x64_sys_ioctl+0x93/0xd0
> > > [ 3437.466791]        do_syscall_64+0x42/0xf0
> > > [ 3437.466957]        entry_SYSCALL_64_after_hwframe+0x6e/0x76
> > > [ 3437.467180]
> > >                other info that might help us debug this:
> > > 
> > > [ 3437.469670] 2 locks held by bcachefs/35533:
> > >                other info that might help us debug this:
> > > 
> > > [ 3437.467507] Chain exists of:
> > >                  sb_writers#10 --> &c->snapshot_create_lock -->
> > > &type->s_umount_key#48
> > > 
> > > [ 3437.467979]  Possible unsafe locking scenario:
> > > 
> > > [ 3437.468223]        CPU0                    CPU1
> > > [ 3437.468405]        ----                    ----
> > > [ 3437.468585]   rlock(&type->s_umount_key#48);
> > > [ 3437.468758] lock(&c->snapshot_create_lock);
> > > [ 3437.469030] lock(&type->s_umount_key#48);
> > > [ 3437.469291]   rlock(sb_writers#10);
> > > [ 3437.469434]
> > >                 *** DEADLOCK ***
> > > 
> > > [ 3437.469670] 2 locks held by bcachefs/35533:
> > > [ 3437.469838]  #0: ffffa0a02ce00a88
> > > (&c->snapshot_create_lock){++++}-{3:3}, at:
> > > bch2_fs_file_ioctl+0x1e3/0xc90 [bcachefs]
> > > [ 3437.470294]  #1: ffffa0a02b2b10e0
> > > (&type->s_umount_key#48){.+.+}-{3:3}, at:
> > > bch2_fs_file_ioctl+0x232/0xc90 [bcachefs]
> > > [ 3437.470744]
> > >                stack backtrace:
> > > [ 3437.470922] CPU: 7 PID: 35533 Comm: bcachefs Kdump: loaded
> > > Tainted: G            E      6.7.0-rc7-custom+ #85
> > > [ 3437.471313] Hardware name: QEMU Standard PC (i440FX + PIIX,
> > > 1996), BIOS Arch Linux 1.16.3-1-1 04/01/2014
> > > [ 3437.471694] Call Trace:
> > > [ 3437.471795]  <TASK>
> > > [ 3437.471884]  dump_stack_lvl+0x57/0x90
> > > [ 3437.472035]  check_noncircular+0x132/0x150
> > > [ 3437.472202]  __lock_acquire+0x1455/0x21b0
> > > [ 3437.472369]  lock_acquire+0xc6/0x2b0
> > > [ 3437.472518]  ? filename_create+0x62/0x190
> > > [ 3437.472683]  ? lock_is_held_type+0x97/0x110
> > > [ 3437.472856]  mnt_want_write+0x46/0x1a0
> > > [ 3437.473025]  ? filename_create+0x62/0x190
> > > [ 3437.473204]  filename_create+0x62/0x190
> > > [ 3437.473380]  user_path_create+0x2d/0x50
> > > [ 3437.473555]  bch2_fs_file_ioctl+0x2ec/0xc90 [bcachefs]
> > > [ 3437.473819]  ? lock_acquire+0xc6/0x2b0
> > > [ 3437.474002]  ? __fget_files+0x2a/0x190
> > > [ 3437.474195]  ? __fget_files+0xbc/0x190
> > > [ 3437.474380]  ? lock_release+0xc5/0x270
> > > [ 3437.474567]  ? __x64_sys_ioctl+0x93/0xd0
> > > [ 3437.474764]  ? __pfx_bch2_fs_file_ioctl+0x10/0x10 [bcachefs]
> > > [ 3437.475090]  __x64_sys_ioctl+0x93/0xd0
> > > [ 3437.475277]  do_syscall_64+0x42/0xf0
> > > [ 3437.475454]  entry_SYSCALL_64_after_hwframe+0x6e/0x76
> > > [ 3437.475691] RIP: 0033:0x7f2743c313af
> > > ======================================================
> > > 
> > > In __bch2_ioctl_subvolume_create(), we grab s_umount unconditionally
> > > and unlock it at the end of the function. There is a comment
> > > "why do we need this lock?" about the lock coming from
> > > commit 42d237320e98 ("bcachefs: Snapshot creation, deletion")
> > > The reason is that __bch2_ioctl_subvolume_create() calls
> > > sync_inodes_sb() which enforce locked s_umount to writeback all
> > > dirty
> > > nodes before doing snapshot works.
> > > 
> > > Fix it by read locking s_umount for snapshotting only and unlocking
> > > s_umount after sync_inodes_sb().
> > > 
> > 
> > Heh, seems reasonable to me..
> > 
> > > Signed-off-by: Su Yue <glass.su@suse.com>
> > > ---
> > >  fs/bcachefs/fs-ioctl.c | 11 +++++------
> > >  1 file changed, 5 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/fs/bcachefs/fs-ioctl.c b/fs/bcachefs/fs-ioctl.c
> > > index e0a19a73c8e1..1346861ed944 100644
> > > --- a/fs/bcachefs/fs-ioctl.c
> > > +++ b/fs/bcachefs/fs-ioctl.c
> > > @@ -337,11 +337,12 @@ static long
> > > __bch2_ioctl_subvolume_create(struct bch_fs *c, struct file *filp,
> > >  	if (arg.flags & BCH_SUBVOL_SNAPSHOT_RO)
> > >  		create_flags |= BCH_CREATE_SNAPSHOT_RO;
> > > 
> > > -	/* why do we need this lock? */
> > > -	down_read(&c->vfs_sb->s_umount);
> > > -
> > > -	if (arg.flags & BCH_SUBVOL_SNAPSHOT_CREATE)
> > > +	if (arg.flags & BCH_SUBVOL_SNAPSHOT_CREATE) {
> > > +		/* sync_inodes_sb enforce s_umount is locked */
> > > +		down_read(&c->vfs_sb->s_umount);
> > >  		sync_inodes_sb(c->vfs_sb);
> > > +		up_read(&c->vfs_sb->s_umount);
> > > +	}
> > >  retry:
> > >  	if (arg.src_ptr) {
> > >  		error = user_path_at(arg.dirfd,
> > > @@ -425,8 +426,6 @@ static long __bch2_ioctl_subvolume_create(struct
> > > bch_fs *c, struct file *filp,
> > >  		goto retry;
> > >  	}
> > >  err1:
> > > -	up_read(&c->vfs_sb->s_umount);
> > > -
> > 
> > Can we get rid of this label now that it serves no purpose?
> > 
> I think we can't. Two gotos are still there(under the retry label)
> unless change them to return
> 

Sorry.. yes, that's what I mean. Usually we can just return directly
rather than goto for cases where there is no necessary clean up on exit.

Brian

> --
> Su
> 
> > Brian
> > 
> > >  	return error;
> > >  }
> > > 
> > > --
> > > 2.43.0
> > > 
> > > 
>