> Il giorno 12 nov 2018, alle ore 13:28, Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> ha scritto: > > On Mon, Nov 12, 2018 at 10:56:21AM +0100, Paolo Valente wrote: >> From: Angelo Ruocco <angeloruocco90@xxxxxxxxx> >> >> 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
