Messages in this thread | | | From | Jason Andryuk <> | Date | Wed, 2 Feb 2022 14:48:48 -0500 | Subject | Re: [PATCH] kcmp: Comment get_file_raw_ptr() RCU usage |
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On Wed, Feb 2, 2022 at 12:44 PM Cyrill Gorcunov <gorcunov@gmail.com> wrote: > > On Wed, Feb 02, 2022 at 10:17:34AM -0500, Jason Andryuk wrote: > > This usage of RCU appears wrong since the pointer is passed outside the > > RCU region. However, it is not dereferenced, so it is "okay". Leave a > > comment for the next reader. > > > > Without a reference, these comparisons are racy, but even with their use > > inside an RCU region, the result could go stale. > > > > Signed-off-by: Jason Andryuk <jandryuk@gmail.com> > > --- > > I was looking for examples of task_lookup_fd_rcu()/files_lookup_fd_rcu() > > and found this. It differed from the example given in > > Documentation/filesystems/files.rst, so I was initially confused. A > > comment seemed appropriate to avoid confusion. > > > > kernel/kcmp.c | 3 +++ > > 1 file changed, 3 insertions(+) > > > > diff --git a/kernel/kcmp.c b/kernel/kcmp.c > > index 5353edfad8e1..4fb23f242e0f 100644 > > --- a/kernel/kcmp.c > > +++ b/kernel/kcmp.c > > @@ -63,6 +63,9 @@ get_file_raw_ptr(struct task_struct *task, unsigned int idx) > > { > > struct file *file; > > > > + /* This RCU locking is only present to silence warnings. The pointer > > + * value is only used for comparison and not dereferenced, so it is > > + * acceptable. */ > > rcu_read_lock(); > > file = task_lookup_fd_rcu(task, idx); > > rcu_read_unlock(); > > They are not wrong, this is just such a bit weird semantics where > we fetch the pointers and strictly speaking map them into numbers > set to compare. But I agree that such tricks might confuse. How about > > /* > * Fetching file pointers inside RCU read-lock section > * and reuse them as plain numbers is done in a sake > * of speed. But make sure never dereference them after. > */
I would tweak it a little to "Fetch file pointers inside RCU read-lock section, but skip additional locking for speed. The pointer values will be used as integers, and must not be dereferenced."
One other idea I had was to switch the return value to "void *". That way it isn't a struct file, and it isn't readily dereference-able. But I wasn't sure if that would be overkill. What do you think?
Thanks, Jason
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