Il giorno 12 nov 2018, alle ore 13:28, Greg Kroah-Hartman [off-list ref] ha scritto:

On Mon, Nov 12, 2018 at 10:56:21AM +0100, Paolo Valente wrote:

quoted

From: Angelo Ruocco <redacted>

The kernfs pseudo file system doesn't export any function to only find
a node by name, without also getting a reference on it.
But in some cases it is useful to just locate a kernfs node, while
using it or not depends on some other condition.

This commit adds a function to just look for a node, without getting
a reference on it.

Eeek, that sounds really bad.  So you save off a pointer to something,
and have no idea if that pointer now really is valid or not?  It can
instantly disappear right afterwards.

Hi Greg,
that function is invoked only in functions executed with cgroup_mutex
held.  This guarantees that nothing disappears or becomes
inconsistent.  That's why we decided to go for this optimization,
instead of doing useless gets&puts pairs.  Still, I'm not expert
enough to state whether it is impossible that, once we have defined
that function, it may then get used in some unsafe way.

So, I seem to see two options:
1) Add a comment on the function, saying that cgroup_mutex must be
   held while invoking it (I guess you won't like this one).
2) Do not define such a new function, and, in the other patches, use
   the already-available find_and_get.

Looking forward to your feedback (or of other knowledgeable people on
this issue) before proceeding to a rebased V2,
Paolo

This feels wrong, what is the problem of having a properly reference
counted object passed back to you that you have to create a dangerous
function like this?

greg k-h