On Fri, Jan 10, 2014 at 04:34:59PM +0100, Oleg Nesterov wrote:
> On 01/08, Paul E. McKenney wrote:
> >
> > On Wed, Jan 08, 2014 at 04:19:18PM +0100, Oleg Nesterov wrote:
> > > On 01/08, Paul E. McKenney wrote:
> > > >
> > > > Another approach would be to add an argument to files_fdtable()
> > > > that is zero normally and one for "we know we don't need RCU
> > > > protection." Then rcu_dereference_check() could be something
> > > > like the following:
> > > >
> > > > #define files_fdtable(files, c) \
> > > > (rcu_dereference_check_fdtable((files), (files)->fdt)
> > > > || c)
> > > >
> > > > Would that work?
> > >
> > > Yes, I considered this optiion, but this needs much more uglifications^W
> > > changes.
> > >
> > > Either we need to change all users of files_fdtable(), or we need
> > > something
> > > like
> >
> > There are only about 20 uses of files_fdtable() in 3.12, with almost all
> > of them in fs/file.c. So is changing all the users really all that
> > problematic?
>
> But only one user, close_files(), needs files_fdtable(files, true). Why
> complicate the patch and the code? I think it would be better to simply
> change close_files() to use rcu_dereference_raw().
>
> And note that rcu_dereference_check_fdtable() needs the new argument too.
>
> And we should also take care of fcheck_files(),
>
> > > static inline struct file *__fcheck_files(struct files_struct *files,
> > > unsigned int fd)
> > > {
> > > struct fdtable *fdt = rcu_dereference_raw(files->fdt);
> > > struct file *file = NULL;
> > >
> > > if (fd < fdt->max_fds)
> > > file = rcu_dereference_raw(fdt->fd[fd]);
> > >
> > > return file;
> > > }
> > >
> > > static inline struct file *fcheck_files(struct files_struct *files,
> > > unsigned int fd)
> > > {
> > > rcu_lockdep_assert(rcu_read_lock_held() ||
> > > lockdep_is_held(files->file_lock),
> > > "message");
> > > return __fcheck_files(files, fd);
> > > }
>
> doesn't this look much simpler than adding the "bool unshared" argument
> and changing the callers?
I might be being too paranoid, but my concern with using rcu_lock_acquire()
and rcu_lock_release() is the possibility of code needing rcu_read_lock()
appearing somewhere in the function-call graph between rcu_lock_acquire()
and rcu_lock_release(). In that case, lockdep would be happy, but the
required RCU protection would not be present.
Sort of like my experience with people using RCU from idle.
Thanx, Paul
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