On Mon, May 05, 2008 at 07:14:34PM +0200, Andrea Arcangeli wrote: > On Mon, May 05, 2008 at 11:21:13AM -0500, Jack Steiner wrote: > > The GRU does the registration/deregistration of mmu notifiers from > > mmap/munmap. > > At this point, the mmap_sem is already held writeable. I hit a deadlock > > in mm_lock. > > It'd been better to know about this detail earlier,
Agree. My apologies... I should have caught it. > but frankly this > is a minor problem, the important thing is we all agree together on > the more difficult parts ;). > > > A quick fix would be to do one of the following: > > > > - move the mmap_sem locking to the caller of the [de]registration > > routines. > > Since the first/last thing done in mm_lock/mm_unlock is to > > acquire/release mmap_sem, this change does not cause major changes. > > I don't like this solution very much. Nor GRU nor KVM will call > mmu_notifier_register inside the mmap_sem protected sections, so I > think the default mmu_notifier_register should be smp safe by itself > without requiring additional locks to be artificially taken externally > (especially because the need for mmap_sem in write mode is a very > mmu_notifier internal detail). > > > - add a flag to mmu_notifier_[un]register routines to indicate > > if mmap_sem is already locked. > > The interface would change like this: > > #define MMU_NOTIFIER_REGISTER_MMAP_SEM (1<<0) > void mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm, > unsigned long mmu_notifier_flags); That works... > > A third solution is to add: > > /* > * This must can be called instead of mmu_notifier_register after > * taking the mmap_sem in write mode (read mode isn't enough). > */ > void __mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm); > > Do you still prefer the bitflag or you prefer > __mmu_notifier_register. It's ok either ways, except > __mmu_notifier_reigster could be removed in a backwards compatible > way, the bitflag can't. > > > I've temporarily deleted the mm_lock locking of mmap_sem and am continuing > > to > > test. More later.... __mmu_notifier_register/__mmu_notifier_unregister seems like a better way to go, although either is ok. > > Sure! In the meantime go ahead this way. > > Another very minor change I've been thinking about is to make > ->release not mandatory. It happens that with KVM ->release isn't > strictly required because after mm_users reaches 0, no guest could > possibly run anymore. So I'm using ->release only for debugging by > placing -1UL in the root shadow pagetable, to be sure ;). So because > at least one user won't strictly require ->release being consistent in > having all method optional may be nicer. Alternatively we could make > them all mandatory and if somebody doesn't need one of the methods it > should implement it as a dummy function. Both ways have pros and cons, > but they don't make any difference to us in practice. If I've to > change the patch for the mmap_sem taken during registration I may as > well cleanup this minor bit. Let me finish my testing. At one time, I did not use ->release but with all the locking & teardown changes, I need to do some reverification. --- jack ------------------------------------------------------------------------- This SF.net email is sponsored by the 2008 JavaOne(SM) Conference Don't miss this year's exciting event. There's still time to save $100. Use priority code J8TL2D2. http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
