Re: PR/38141 CVS commit: src/sys

[yamt%mwd.biglobe.ne.jp@localhost (YAMAMOTO Takashi)]

>  Log Message:
>  PR kern/38141 lookup/vfs_busy acquire rwlock recursively
>  
>  Until the code paths are fixed properly, put in place an ugly workaround
>  to make it safe to recursively acquire a read lock on a mount.

unfortunately, recursive lock was not necessary to triger
a deadlock.  i saw the following while running "build.sh -j128".
(well, the actual deadlock i saw had 10 or more vnodes involved.
the following is a simplified version for explanation.)

LWP1 in vn_open()
        1. lock a directory vnode which is VV_ROOT.
LWP2 in lookup()
        1. vfs_busy(RW_READER)
        2. call VFS_ROOT, which tries to lock the VV_ROOT vnode which
           is already locked by LWP1. => block
LWP3 (syncer) in sync_fsync()
        1. vfs_trybusy(RW_WRITER) => block
now, LWP1 again:
        2. call VOP_CREATE -> getnewvnode -> vfs_busy(RW_READER) => block

YAMAMOTO Takashi