On Wed, Feb 2, 2022 at 12:44 PM Cyrill Gorcunov <[email protected]> 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 <[email protected]>
> > ---
> > 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
On Wed, Feb 02, 2022 at 02:48:48PM -0500, Jason Andryuk wrote:
> On Wed, Feb 2, 2022 at 12:44 PM Cyrill Gorcunov <[email protected]> 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 <[email protected]>
> > > ---
> > > 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."
Up to you. Initially I use _raw_ptr suffix in function name trying to
point out that the pointer obtained should not be considered in any
way except a natural number. So I'm fine with any comment which helps
readability.
>
> 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?
Lets better stick with a comment I think. Thanks!