lkml.org 
[lkml]   [2011]   [Oct]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v5 3.1.0-rc4-tip 3/26] Uprobes: register/unregister probes.
    On 10/05, Srikar Dronamraju wrote:
    >
    > Agree. Infact I encountered this problem last week and had fixed it.
    > In mycase, I had mapped the file read and write while trying to insert
    > probes.
    > The changed code looks like this
    >
    > if (!vma)
    > return NULL;

    This is unneeded, vma_prio_tree_foreach() stops when vma_prio_tree_next()
    returns NULL. IOW, you can never see vma == NULL.

    > if (!valid_vma(vma))
    > continue;

    Yes.

    > > > + mutex_lock(&inode->i_mutex);
    > > > + uprobe = alloc_uprobe(inode, offset);
    > >
    > > Looks like, alloc_uprobe() doesn't need ->i_mutex.
    >
    >
    > Actually this was pointed out by you in the last review.
    > https://lkml.org/lkml/2011/7/24/91

    OOPS ;) may be deserves a comment...

    > > > +void unregister_uprobe(struct inode *inode, loff_t offset,
    > > > + struct uprobe_consumer *consumer)
    > > > +{
    > > > + struct uprobe *uprobe;
    > > > +
    > > > + inode = igrab(inode);
    > > > + if (!inode || !consumer)
    > > > + return;
    > > > +
    > > > + if (offset > inode->i_size)
    > > > + return;
    > > > +
    > > > + uprobe = find_uprobe(inode, offset);
    > > > + if (!uprobe)
    > > > + return;
    > > > +
    > > > + if (!del_consumer(uprobe, consumer)) {
    > > > + put_uprobe(uprobe);
    > > > + return;
    > > > + }
    > > > +
    > > > + mutex_lock(&inode->i_mutex);
    > > > + if (!uprobe->consumers)
    > > > + __unregister_uprobe(inode, offset, uprobe);
    > >
    > > It seemes that del_consumer() should be done under ->i_mutex. If it
    > > removes the last consumer, we can race with register_uprobe() which
    > > takes ->i_mutex before us and does another __register_uprobe(), no?
    >
    > We should still be okay, because we check for the consumers before we
    > do the actual unregister in form of __unregister_uprobe.
    > since the consumer is again added by the time we get the lock, we dont
    > do the actual unregistration and go as if del_consumer deleted one
    > consumer but not the last.

    Yes, but I meant in this case register_uprobe() does the unnecessary
    __register_uprobe() because it sees ->consumers == NULL (add_consumer()
    returns NULL).

    I guess this is probably harmless because of is_bkpt_insn/-EEXIST
    logic, but still.


    Btw. __register_uprobe() does

    ret = install_breakpoint(mm, uprobe, vma, vi->vaddr);
    if (ret && (ret != -ESRCH || ret != -EEXIST)) {
    up_read(&mm->mmap_sem);
    mmput(mm);
    break;
    }
    ret = 0;
    up_read(&mm->mmap_sem);
    mmput(mm);

    Yes, this is cosmetic, but why do we duplicate up_read/mmput ?

    Up to you, but

    ret = install_breakpoint(mm, uprobe, vma, vi->vaddr);
    up_read(&mm->mmap_sem);
    mmput(mm);

    if (ret) {
    if (ret != -ESRCH && ret != -EEXIST)
    break;
    ret = 0;
    }

    Looks a bit simpler.

    Oh, wait. I just noticed that the original code does

    (ret != -ESRCH || ret != -EEXIST)

    this expression is always true ;)

    Oleg.



    \
     
     \ /
      Last update: 2011-10-05 20:57    [W:0.029 / U:0.456 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site