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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.



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