Re: [PATCH] Fix potential OOPS in generic_setlease()

[Pavel Emelyanov <xemul@openvz.org>]

J. Bruce Fields wrote:
> On Wed, Sep 19, 2007 at 06:26:05PM +0400, Pavel Emelyanov wrote:
>> This code is run under lock_kernel(), which is dropped during
>> sleeping operations, so the following race is possible:
>>
>> CPU1:                                CPU2:
>>   vfs_setlease();                    vfs_setlease();
>>   lock_kernel();
>>                                      lock_kernel(); /* spin */
>>   generic_setlease():
>>     ...
>>     for (before = ...)
>>     /* here we found some lease after
>>      * which we will insert the new one
>>      */
>>     fl = locks_alloc_lock();
>>     /* go to sleep in this allocation and
>>      * drop the BKL
>>      */
>>                                      generic_setlease():
>>                                        ...
>>                                        for (before = ...)
>>                                        /* here we find the "before" pointing
>>                                         * at the one we found on CPU1
>>                                         */
>>                                       ->fl_change(my_before, arg);
>>                                               lease_modify();
>>                                                      locks_free_lock();
>>                                                      /* and we freed it */
>>                                      ...
>>                                      unlock_kernel();
>>    locks_insert_lock(before, fl);
>>    /* OOPS! We have just tried to add the lease
>>     * at the tail of already removed one
>>     */
> 
> Thanks for spotting this!
> 
> But--careful-- it looks like "fl" is also used as a temporary variable
> in a loop between the new and old location of that allocation.  Isn't
> that a bug?

OOPS! Good catch, thanks. I will resend the patch shortly.