On Tue, Jun 27, 2023 at 11:42 AM Tejun Heo <tj@xxxxxxxxxx> wrote: > > Hello, Christian. > > On Tue, Jun 27, 2023 at 07:30:26PM +0200, Christian Brauner wrote: > ... > > ->release() was added in > > > > commit 0e67db2f9fe91937e798e3d7d22c50a8438187e1 > > kernfs: add kernfs_ops->open/release() callbacks > > > > Add ->open/release() methods to kernfs_ops. ->open() is called when > > the file is opened and ->release() when the file is either released or > > severed. These callbacks can be used, for example, to manage > > persistent caching objects over multiple seq_file iterations. > > > > Signed-off-by: Tejun Heo <tj@xxxxxxxxxx> > > Acked-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > > Acked-by: Acked-by: Zefan Li <lizefan@xxxxxxxxxx> > > > > which mentions "either releases or severed" which imho already points to > > separate methods. > > This is because kernfs has revoking operation which doesn't exist for other > filesystems. Other filesystem implemenations can't just say "I'm done. Bye!" > and go away. Even if the underlying filesystem has completely failed, the > code still has to remain attached and keep aborting operations. > > However, kernfs serves as the midlayer to a lot of device drivers and other > internal subsystems and it'd be really inconvenient for each of them to have > to implement "I want to go away but I gotta wait out this user who's holding > onto my tuning knob file". So, kernfs exposes a revoke or severing semantics > something that's exposing interface through kernfs wants to stop doing so. > > If you look at it from file operation implementation POV, this seems exactly > like ->release. All open files are shutdown and there won't be any future > operations. After all, revoke is forced closing of all fd's. So, for most > users, treating severing just like ->release is the right thing to do. > > The PSI file which caused this is a special case because it attaches > something to its kernfs file which outlives the severing operation bypassing > kernfs infra. A more complete way to fix this would be supporting the > required behavior from kernfs side, so that the PSI file operates on kernfs > interface which knows the severing event and detaches properly. That said, > currently, this is very much an one-off. > > Suren, if you're interested, it might make sense to pipe poll through kernfs > properly so that it has its kernfs operation and kernfs can sever it. That > said, as this is a fix for something which is currently causing crashes, > it'd be better to merge this simpler fix first no matter what. I'm happy to implement the right fix if you go into more details. AFAIKT kernfs_ops already has poll() operation, we are hooking cgroup_file_poll() to it and using kernfs_generic_poll(). I thought this is the right way to pipe poll through kernfs but if that's incorrect, please let me know. I'm happy to fix that. Thanks, Suren. > > Thanks. > > -- > tejun
