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DateSun, 23 Nov 2008 12:01:14 +0100
FromIngo Molnar <>
SubjectRe: [PATCH 2/3] tracing/stack-tracer: fix locking

* Török Edwin <edwintorok@gmail.com> wrote:

> On 2008-11-23 12:52, Ingo Molnar wrote:
> > * Török Edwin <edwintorok@gmail.com> wrote:
> >
> >
> >> + down_read(&mm->mmap_sem);
> >> + vma = find_vma(mm, ip);
> >> if (vma) {
> >> file = vma->vm_file;
> >> vmstart = vma->vm_start;
> >> }
> >> + up_read(&mm->mmap_sem);
> >> }
> >> if (file) {
> >> ret = trace_seq_path(s, &file->f_path);
> >>
> >
> > and now it's "file" that is held without a reference and possibly
> > racy.
> >
>
> I'll move the (file) inside the mmap_sem too, since file is part of
> vma, and you need mmap_sem to access the vma nobody can modify
> (file) if they don't hold mmap_sem. Am I right?

yeah - you can use the mm lock for that as the vma holds a reference
to the file. This works as long as none of the seq functions called
with this lock held can provoke a user pagefault (they shouldnt, but i
havent checked in detail).

Ingo
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