Re: [PATCH][RFC] Signal-per-fd for RT signals; write_lock_bh(file_lock)?

[Dan Kegel <dank@kegel.com>]

Vitaly Luban wrote:
> Could you please try attached one? It's mostly untested, but my home site
> will be down next week.

I just had time to read your new patch; have not yet run it.
I think there may be some problems in send_signal:

1) might still have null pointer dereference.  If files is NULL, following section
+        if( newsignal && q )
+            filep->f_infoptr = q;
+        write_unlock( &files->file_lock);
may crash due to NULL filep and unlocking an already unlocked lock.
BTW, you might be able to use a read lock there.

2) You're using a kind of spinlock that compiles to a no-op on single
processor, but the crash (see the old oops traceback in
http://marc.theaimsgroup.com/?l=linux-kernel&m=100055931808169&w=2 )
happens on a single processor, so your locking shouldn't help.
The conflict is between a bottom half and normal.  Thus write_lock, besides
not helping on uniprocessor, might cause a deadlock on smp.  
Might have to convert the write_lock in get_unused_fd() and friends to be a
write_lock_bh() instead to prevent the bottom half from running until the normal
part is done with the file lock!

This bottom-half-vs-normal issue worries me greatly.  It may take
a lot of thought to fix this.  Then again, maybe I'm just confused;
I've never dealt with kernel spinlocks before, and anyone who reads
my posts regularly knows that I'm wrong most of the time.  Anyone
who actually *understands* this stuff, please speak up with corrections...

3) you still save info in the file even if you don't end up queuing
a signal; your changes in send_signal should be in the default case
of the switch, just to keep things in a good state.

4) collect_signal references the file table, so it needs to lock it;
probably read_lock (but maybe write_lock, I dunno), and probably
_bh, too, to avoid deadlock.

> I'm looking forward to see a test case, all I could come up with happily
> runs on the old version.

I'll try to put it together today.  
- Dan