Re: [PATCH v4 3.0-rc2-tip 4/22] 4: Uprobes: register/unregister probes.

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Oleg Nesterov <oleg@redhat.com> [2011-07-24 20:07:13]:

> Hi Srikar,
> 
> I still hope some day I'll find the time to read the whole series ;)
> Trying to continue from where I have stopped, and it seems that this
> patch has a couple more problems.


Thanks for the review and I sincerely hope you find time and that too at
the earliest. 

> 
> On 06/07, Srikar Dronamraju wrote:
> >
> > A probe is specified by a file:offset.  While registering, a breakpoint
> > is inserted for the first consumer, On subsequent probes, the consumer
> > gets appended to the existing consumers. While unregistering a
> > breakpoint is removed if the consumer happens to be the last consumer.
> > All other unregisterations, the consumer is deleted from the list of
> > consumers.
> >
> > Probe specifications are maintained in a rb tree. A probe specification
> > is converted into a uprobe before store in a rb tree.  A uprobe can be
> > shared by many consumers.
> 
> register/unregister logic looks racy...
> 
> Supose that uprobe U has a single consumer C and register_uprobe()
> is called with the same inode/offset, while another thread does
> unregister(U,C).
> 
> 	- register() calls alloc_uprobe(), finds the entry in rb tree,
> 	  and increments U->ref. But this doesn't add the new consumer.
> 
> 	- uregister() does del_consumer(), and removes the single
> 	  consumer C.
> 
> 	  then it takes uprobes_mutex, sees uprobe->consumers == NULL
> 	  and calls delete_uprobe()->rb_erase()
> 
> 	- register() continues, takes uprobes_mutex, re-inserts the
> 	  breakpoints, finds the new consumer and succeeds.
> 
> 	  However, this uprobe is not in rb-tree, it was deleted
> 	  by unregister.
> 

Agree, 
I will move the alloc_uprobe under the mutex_lock.

On a side_note: As per the current discussions in this thread, I plan to
use inode->i_mutex so that we could serialize register/unregister if
they are for two different files.

> 
> 
> OTOH. Suppose we add the new uprobe. register()->alloc_uprobe() sets
> new_uprobe->ref == 2. If something goes wrong after that, register()
> does delete_uprobe() + put_uprobe(), new_uprobe->ref becomes 1 and
> we leak this uprobe.
> 

Agree:
yes I will add a put_uprobe() just after delete_uprobe() but just before
the goto.
sidenote: Even this code will change based on the discussions we had on
this topic. But I will ensure to make the appropriate changes are taken
care of.

-- 
Thanks and Regards
Srikar

> Oleg.

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