Messages in this thread | | | From | Mark Gross <> | Subject | Re: PATCH Multithreaded core dump support for the 2.5.14 (and 15) kernel. | Date | Thu, 16 May 2002 14:08:10 -0400 |
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On Thursday 16 May 2002 01:36 pm, Daniel Jacobowitz wrote: > On Thu, May 16, 2002 at 07:27:59PM +0200, Andi Kleen wrote: > > On Thu, May 16, 2002 at 10:13:40AM -0400, Mark Gross wrote: > > > Also, does anyone know WHY the mmap_sem is needed in the elf_core_dump > > > code, and is this need still valid if I've suspended all the other > > > processes that could even touch that mm? I.e. can I fix this by > > > removing the down_write / up_write in elf_core_dump? > > > > The mmap_sem is needed to access current->mm (especially the vma list) > > safely. Otherwise someone else sharing the mm_struct could modify it. > > If you make sure all others sharing the mm_struct are killed first > > (including now way for them to start new clones inbetween) then > > the only loophole left would be remote access using /proc/pid/mem or > > ptrace. If you handle that too then it is probably safe to drop it. > > Unfortunately I don't see a way to handle these remote users without at > > least > > taking it temporarily. > > > > Of course there are other semaphores in involved in dumping too (e.g. the > > VFS ->write code may take the i_sem or other private ones). I guess they > > won't be a big problem if you first kill and then dump later. > > Except unfortunately we don't kill; the other threads are resumed > afterwards for cleanup. They're just suspended.
Yes, they start back up after the dump.
It certainly seems that with the processes paused that the use of the current->mm->mm_sem could be obsolete for core dumps. I'm not so sure protecting the core file data from ptrace or /proc/pid/mem is important in the case of core dumping.
I just don't want the kernel to lock up dumping the multithreaded core file.
I'm still not sure we have a problem yet. (wishful thinking I suppose). Also I've seen zero lock ups from semaphore being held by one of the processes getting pauses temporarily in my testing on the patch I posted.
To restate: the only way I see that my design gets into trouble is when a semaphore is HELD, not getting waited on, by one of the processes that gets put onto the phantom runqueue, AND that semaphore is needed in the processing of elf_core_dump(...).
For this to happen that semaphore would have to held across schedule()'s. The ONLY place I've seen that in the kernel is set_CPUs_allowed + migration_thread.
Can someone point me at other critical sections that have non-deterministic life times as a function of when the process holding the semaphore gets scheduled onto a CPU? That type of code seems very risky to me. This is the only type of code that could get my design into trouble.
--mgross
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