Messages in this thread | | | Date | Sun, 10 Oct 1999 18:45:51 +0200 | From | Manfred Spraul <> | Subject | Re: locking question: do_mmap(), do_munmap() |
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Alexander Viro wrote: > > On Sun, 10 Oct 1999, Alexander Viro wrote: > > > > > [Cc'd to mingo] > > > > On Sun, 10 Oct 1999, Manfred Spraul wrote: > > > > > I've started adding "assert_down()" and "assert_kernellocked()" macros, > > > and now I don't see the login prompt any more... > > > > > > eg. sys_mprotect calls merge_segments without lock_kernel(). > > > > Manfred, Andrea - please stop it. Yes, it does and yes, it should.
Yes, it should cause oops?
> > Plonking the big lock around every access to VM is _not_ a solution
I never did that, I'll never do that, I only notice that the current code is filled with races.
> >. If > > swapper doesn't use mmap_sem - _swapper_ should be fixed. How the hell > > does lock_kernel() have smaller deadlock potential than > > down(&mm->mmap_sem)?
lock_kernel() is dropped on thread switch, the semaphore is not dropped.
> > OK, folks. Code in swapper (unuse_process(), right?) is called only from > sys_swapoff(). It's a syscall. Andrea, could you show a scenario for > deadlock here? OK, some process (but not the process doing swapoff()) may > have the map locked So? it is not going to release the thing - we are > seriously screwed anyway (read: we already are in deadlock). We don't hold > the semaphore ourselves.
AFAIK the problem is OOM: * a process accesses a not-present, ie page fault: ... handle_mm_fault(): this process own mm->mmap_sem. ->handle_pte_fault(). -> (eg.) do_wp_page(). -> get_free_page(). now get_free_page() notices that there is no free memory. --> wakeup kswapd.
* the swapper runs, and it tries to swap out data from that process. mm->mmap_sem is already acquired --> lock-up.
-- Manfred
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