[struct mount] Unprotected access to mnt_secondary_writes

[Alexander Lochmann]

Hi folks!

According to the documentation [1], and to our findings, mnt_mtx is used 
to protect mnt_secondary_writes in struct mount.
However, our data shows that it is accessed in vn_fsync_buf() [2] 
without the mnt_mtx lock. I attached the stack trace at the end of this 
mail.
Our data shows as well that the vnode.v_lock and the 
vnode.v_bufobj.bo_lock are being held.
Is this a valid access that ignores the locking on purpose?
Or is it a bug?

Regards,
Alex

[1] https://github.com/freebsd/freebsd-src/blob/main/sys/sys/mount.h#L236
[2] 
https://github.com/freebsd/freebsd-src/blob/main/sys/kern/vfs_vnops.c#L2805

Stacktrace:
syscall
syscallenter
sys_nmount
vfs_donmount
vfs_domount
vfs_domount_update
ffs_mount
vfs_write_suspend_umnt
vfs_write_suspend
ffs_sync
softdep_flushworklist
VOP_FSYNC
vop_stdfsync
vn_fsync_buf

-- 
Technische Universität Dortmund
Alexander Lochmann                PGP key: 0xBC3EF6FD
Otto-Hahn-Str. 16                 phone:  +49.231.7556141
D-44227 Dortmund                  fax:    +49.231.7556116
http://ess.cs.tu-dortmund.de/Staff/al

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