* Peter Zijlstra <peterz@xxxxxxxxxxxxx> [2011-06-16 14:00:26]: > On Thu, 2011-06-16 at 08:56 +0530, Srikar Dronamraju wrote: > > * Peter Zijlstra <peterz@xxxxxxxxxxxxx> [2011-06-15 20:11:26]: > > > > > On Tue, 2011-06-07 at 18:29 +0530, Srikar Dronamraju wrote: > > > > + up_write(&mm->mmap_sem); > > > > + mutex_lock(&uprobes_mutex); > > > > + down_read(&mm->mmap_sem); > > > > > > egads, and all that without a comment explaining why you think that is > > > even remotely sane. > > > > > > I'm not at all convinced, it would expose the mmap() even though you > > > could still decide to tear it down if this function were to fail, I bet > > > there's some funnies there. > > > > The problem is with lock ordering. register/unregister operations > > acquire uprobes_mutex (which serializes register unregister and the > > mmap_hook) and then holds mmap_sem for read before they insert a > > breakpoint. > > > > But the mmap hook would be called with mmap_sem held for write. So > > acquiring uprobes_mutex can result in deadlock. Hence we release the > > mmap_sem, take the uprobes_mutex and then again hold the mmap_sem. > > Sure, I saw why you wanted to do it, I'm just not quite convinced its > safe to do and something like this definitely wants a comment explaining > why its safe to drop mmap_sem. > > > After we re-acquire the mmap_sem, we do check if the vma is valid. > > But you don't on the return path, and if !ret > mmap_region():unmap_and_free_vma will be touching vma again to remove > it. > Agree. > > Do we have better solutions? > > /me kicks the brain into gear and walks off to get a fresh cup of tea. > > So the reason we take uprobes_mutex there is to avoid probes from going > away while you're installing them, right? It serializes register/unregister/mmap operations. > > So we start by doing this add_to_temp_list() thing (horrid name), which > iterates the probes on this inode under uprobes_treelock and adds them > to a list. > > Then we iterate the list, installing the probles. > > How about we make the initial pass under uprobes_treelock take a > references on the probe, and then after install_breakpoint() succeeds we > again take uprobes_treelock and validate the uprobe still exists in the > tree and drop the extra reference, if not we simply remove the > breakpoint again and continue like it never existed. > > That should avoid the need to take uprobes_mutex and not require > dropping mmap_sem, right? Now since a register and mmap operations can run in parallel, we could have subtle race conditions like this: 1. register_uprobe inserts the uprobe in RB tree. 2. register_uprobe loops thro vmas and inserts breakpoints. 3. mmap is called for same inode, mmap_uprobe() takes reference; 4. mmap completes insertion and releases reference. 5. register uprobe tries to install breakpoint on one vma fails and not due to -ESRCH or -EEXIST. 6. register_uprobe rolls back all install breakpoints except the one inserted by mmap. We end up with breakpoints that we have inserted by havent cleared. Similarly unregister_uprobe might be looping to remove the breakpoints when mmap comes in installs the breakpoint and returns. unregister_uprobe might erase the uprobe from rbtree after mmap is done. -- Thanks and Regards Srikar -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>