On Tue, Nov 13, 2018 at 06:53:59PM +0100, Paolo Valente wrote: > > > > 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. I can guarantee once you define that function, it will be used in an unsafe way :( So just don't create it, use the put calls, it's fast and should never be a performance issue. > 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). Nope, do not create it. > 2) Do not define such a new function, and, in the other patches, use > the already-available find_and_get. Yes, please do that. thanks, greg k-h
