Messages in this thread | | | Date | Sun, 23 Nov 2008 12:07:14 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 2/3] tracing/stack-tracer: fix locking |
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* Török Edwin <edwintorok@gmail.com> wrote:
> On 2008-11-23 13:01, Ingo Molnar wrote: > > * 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). > > > > It calls d_path, that access only kernel structures AFAICT, so it > can't provoke a *user* pagefault. Not sure about kernel pagefaults, > but they shouldn't matter.
yeah - kernel pagefaults can happen in rare circumstances here but they are all atomic and transparent to most other parts of the kernel so they indeed do not matter.
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