> On Feb 21, 2019, at 16:30, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > > On Thu, Feb 21, 2019 at 03:45:06PM +0800, Fam Zheng wrote: >> We’ve noticed an issue that frequent open,close,open,close... of /dev/ptmx eventually causes soft lockup. >> >> Here is a summary of what happened. >> >> Upon user opening /dev/ptmx, devpts_pty_new calls d_alloc_name and d_add, the new dentry is inserted into dcache_hashtable. >> >> Later when closing, devpts_pty_kill calls d_delete and dput, > > ... since it looks like a normal negative dentry, to be eventually > evicted upon memory pressure. That's not the worst problem in > there, actually - consider > touch /tmp/a > mount --bind /tmp/a /dev/pts/<number> > followed by devpts_pty_kill() taking that sucker out... I think then the pty dentry stops us from operating the real mount point, is that what you mean? Once devpts_pty_kill() returns, I’ll have no way to unmount the bind mount... Fam
