Re: 2.6.16-rc regression: m68k CONFIG_RMW_INSNS=n compile broken

[Dipankar Sarma <dipankar@in.ibm.com>]

On Sun, Mar 05, 2006 at 08:44:15PM -0800, Suzanne Wood wrote:
> > From: Adrian Bunk Fri Mar 03 2006 - 18:40:57 EST 
> >
> Do kernel coders value the marking of the rcu read-side critical 
> section for consistency?  In fs/file_table.c, fcheck_files() 

Generally speaking, yes.

> is called by fget_light() without rcu_read_lock() in one case, 
> but with the apparently necessary rcu_read_lock() in place 
> otherwise.  The struct file pointer that fcheck_files() returns 
> is rcu_dereference(fdt->fd[fd]) or NULL. Does the _commented_guarantee 
> of the current task holding the refcnt assure there's no need to 
> check for NULL or to mark the rcu readside section around the first
> call to fcheck_files()?
> 
> This is the code sample:
> /*
>  * Lightweight file lookup - no refcnt increment if fd table isn't shared.
>  * You can use this only if it is guranteed that the current task already
>  * holds a refcnt to that file. That check has to be done at fget() only
>  * and a flag is returned to be passed to the corresponding fput_light().
>  * There must not be a cloning between an fget_light/fput_light pair.
>  */
> struct file fastcall *fget_light(unsigned int fd, int *fput_needed)
> {
> 	struct file *file;
> 	struct files_struct *files = current->files;
> 
> 	*fput_needed = 0;
> 	if (likely((atomic_read(&files->count) == 1))) {
> 		file = fcheck_files(files, fd);
> 	} else {

This means that the fd table is not shared between threads. So,
there can't be any race and no need to protect using
rcu_read_lock()/rcu_read_unlock().

> 
> The attached patch would superficially address the rcu 
> discrepancy, but another underlying question is about the 
> desired extent of the rcu read-side critical section in that 
> fget_light() returns the pointer to the file struct that was 
> flagged for rcu protection by rcu_dereference() in 
> fcheck_files().  In this application, does it make sense to 
> push the rcu_read_lock() into fcheck_files() or add it there
> or to extend it to the calling function?

I think a comment there explaining why rcu_read_lock/unlock
pair is not there should be sufficient. While the are NOP
for non-PREEMPT kernels, they do have a cost otherwise.
Avoiding them if we can is a good idea, IMO.

> Up the call tree, we note that fcheck() uses fcheck_files(), 
> but the only call to fcheck() nested in rcu_read_lock() is 
> in the disparaged irixioctl.c.
> 
> Are the other calls to fcheck() under circumstances that give 
> reason for the rcu_read_lock elision, like 
> spin_lock(&files->file_lock) in fs/fcntl.c, or being in the 
> context of applying locks in fs/locks.c, or calls from assembly 
> code in arch/sparc/kernel/sunos_ioctl.c & solaris/socksys.c.  
> If there is reason to pursue the insertion of the 
> rcu_read_lock/unlock() pairs in these circumstances, any 
> suggestions would be appreciated in order to dispel the question
> altogether or to try to submit a more extensive patch.

It depends on whether the fdtable is shared or not and if
shared whether we are already holding the ->files_lock or
not. The key is that if it is lock-free and if the fdtable
is shared, they rcu_read_lock()/unlock() pair must be
there, otherwise it is a bug.

Thanks
Dipankar
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