Re: [RFC, PATCH] avoid some atomics in open()/close() for monothreaded processes

[Andrew Morton <akpm@osdl.org>]

Eric Dumazet <dada1@cosmosbay.com> wrote:
>
> Goal : Avoid some locking/unlocking 'struct files_struct'->file_lock for mono 
> threaded processes.
> 
> We define files_multithreaded() function .
> 
> static inline int files_multithreaded(const struct files_struct *files)
> {
>         return sizeof(files->file_lock) > 0 && atomic_read(&files->count) > 1;
> }

That's bascially sizeof(spinlock_t).  That's architecture dependent and
varies wildly according to the day of week.

It _might_ work in all situations - probably you checked that.  But I still
wouldn't do it because it might break in the future.  Let's be explicit and
stick the appropriate ifdefs in there.

I'd also consider renaming it to files_shared() - processes are
multithreaded, not data structures.

Once you're done with that we should change fget_light() and fput_light() to
use this helper.  Separate patch.