Re: [Xen-devel] [PATCH 2 of 3] Make p2m lookups fully synchronized wrt modifications

[Tim Deegan <tim@xxxxxxx>]

At 16:42 -0500 on 08 Nov (1320770545), Andres Lagar-Cavilla wrote:
>  xen/arch/x86/mm/mm-locks.h |  13 +++++++------
>  xen/arch/x86/mm/p2m.c      |  18 +++++++++++++++++-
>  xen/include/asm-x86/p2m.h  |  39 ++++++++++++++++++++++++---------------
>  3 files changed, 48 insertions(+), 22 deletions(-)
> 
> 
> We achieve this by locking/unlocking the global p2m_lock in get/put_gfn.
> This brings about a few consequences for the p2m_lock:
> 
>  - not ordered anymore in mm-locks.h: unshare does get_gfn -> shr_lock,
>    there are code paths that do paging_lock -> get_gfn. All of these
>    would cause mm-locks.h to panic.

In that case there's a potential deadlock in the sharing code, and
turning off the safety catches is not an acceptable response to that. :)

ISTR you had a plan to get rid of the shr_lock entirely, or am
I misremembering?

Tim.

>  - the lock is always taken recursively, as there are many paths that
>    do get_gfn -> p2m_lock
> 
> Signed-off-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
> 
> diff -r a0c55cc5d696 -r 6203a0549d8a xen/arch/x86/mm/mm-locks.h
> --- a/xen/arch/x86/mm/mm-locks.h
> +++ b/xen/arch/x86/mm/mm-locks.h
> @@ -165,14 +165,15 @@ declare_mm_lock(nestedp2m)
>  
>  /* P2M lock (per-p2m-table)
>   * 
> - * This protects all updates to the p2m table.  Updates are expected to
> - * be safe against concurrent reads, which do *not* require the lock. */
> + * This protects all queries and updates to the p2m table. Because queries
> + * can happen interleaved with other locks in here, we do not enforce
> + * ordering, and make all locking recursive. */
>  
> -declare_mm_lock(p2m)
> -#define p2m_lock(p)           mm_lock(p2m, &(p)->lock)
> -#define p2m_lock_recursive(p) mm_lock_recursive(p2m, &(p)->lock)
> -#define p2m_unlock(p)         mm_unlock(&(p)->lock)
> +#define p2m_lock(p)           spin_lock_recursive(&(p)->lock.lock)
> +#define p2m_lock_recursive(p) spin_lock_recursive(&(p)->lock.lock)
> +#define p2m_unlock(p)         spin_unlock_recursive(&(p)->lock.lock)
>  #define p2m_locked_by_me(p)   mm_locked_by_me(&(p)->lock)
> +#define gfn_locked_by_me(p,g) mm_locked_by_me(&(p)->lock)
>  
>  /* Page alloc lock (per-domain)
>   *
> diff -r a0c55cc5d696 -r 6203a0549d8a xen/arch/x86/mm/p2m.c
> --- a/xen/arch/x86/mm/p2m.c
> +++ b/xen/arch/x86/mm/p2m.c
> @@ -158,6 +158,9 @@ mfn_t gfn_to_mfn_type_p2m(struct p2m_dom
>          return _mfn(gfn);
>      }
>  
> +    /* Grab the lock here, don't release until put_gfn */
> +    p2m_lock(p2m);
> +
>      mfn = p2m->get_entry(p2m, gfn, t, a, q, page_order);
>  
>  #ifdef __x86_64__
> @@ -182,6 +185,19 @@ mfn_t gfn_to_mfn_type_p2m(struct p2m_dom
>      return mfn;
>  }
>  
> +void put_gfn(struct domain *d, unsigned long gfn)
> +{
> +    struct p2m_domain *p2m = p2m_get_hostp2m(d);
> +
> +    if ( !p2m || !paging_mode_translate(d) )
> +        /* Nothing to do in this case */
> +        return;
> +
> +    ASSERT(p2m_locked_by_me(p2m));
> +
> +    p2m_unlock(p2m);
> +}
> +
>  int set_p2m_entry(struct p2m_domain *p2m, unsigned long gfn, mfn_t mfn, 
>                    unsigned int page_order, p2m_type_t p2mt, p2m_access_t 
> p2ma)
>  {
> @@ -190,7 +206,7 @@ int set_p2m_entry(struct p2m_domain *p2m
>      unsigned int order;
>      int rc = 1;
>  
> -    ASSERT(p2m_locked_by_me(p2m));
> +    ASSERT(gfn_locked_by_me(p2m, gfn));
>  
>      while ( todo )
>      {
> diff -r a0c55cc5d696 -r 6203a0549d8a xen/include/asm-x86/p2m.h
> --- a/xen/include/asm-x86/p2m.h
> +++ b/xen/include/asm-x86/p2m.h
> @@ -305,9 +305,10 @@ struct p2m_domain *p2m_get_p2m(struct vc
>  
>  #define p2m_get_pagetable(p2m)  ((p2m)->phys_table)
>  
> -/**** p2m query accessors. After calling any of the variants below, you
> - * need to call put_gfn(domain, gfn). If you don't, you'll lock the
> - * hypervisor. ****/
> +/**** p2m query accessors. They lock p2m_lock, and thus serialize
> + * lookups wrt modifications. They _do not_ release the lock on exit.
> + * After calling any of the variants below, caller needs to use
> + * put_gfn. ****/
>  
>  /* Read a particular P2M table, mapping pages as we go.  Most callers
>   * should _not_ call this directly; use the other get_gfn* functions
> @@ -349,19 +350,27 @@ static inline unsigned long get_gfn_unty
>      return INVALID_MFN;
>  }
>  
> -/* This is a noop for now. */
> -static inline void put_gfn(struct domain *d, unsigned long gfn)
> -{
> -}
> +/* Will release the p2m_lock and put the page behind this mapping. */
> +void put_gfn(struct domain *d, unsigned long gfn);
>  
> -/* These are identical for now. The intent is to have the caller not worry 
> - * about put_gfn. To only be used in printk's, crash situations, or to 
> - * peek at a type. You're not holding the p2m entry exclsively after calling
> - * this. */
> -#define get_gfn_unlocked(d, g, t)         get_gfn_type((d), (g), (t), 
> p2m_alloc)
> -#define get_gfn_query_unlocked(d, g, t)   get_gfn_type((d), (g), (t), 
> p2m_query)
> -#define get_gfn_guest_unlocked(d, g, t)   get_gfn_type((d), (g), (t), 
> p2m_guest)
> -#define get_gfn_unshare_unlocked(d, g, t) get_gfn_type((d), (g), (t), 
> p2m_unshare)
> +/* The intent is to have the caller not worry about put_gfn. They apply to 
> + * very specific situations: debug printk's, dumps during a domain crash,
> + * or to peek at a p2m entry/type. Caller is not holding the p2m entry 
> + * exclusively after calling this. */
> +#define build_unlocked_accessor(name)                                   \
> +    static inline mfn_t get_gfn ## name ## _unlocked(struct domain *d,  \
> +                                                unsigned long gfn,      \
> +                                                p2m_type_t *t)          \
> +    {                                                                   \
> +        mfn_t mfn = get_gfn ##name (d, gfn, t);                         \
> +        put_gfn(d, gfn);                                                \
> +        return mfn;                                                     \
> +    }
> +
> +build_unlocked_accessor()
> +build_unlocked_accessor(_query)
> +build_unlocked_accessor(_guest)
> +build_unlocked_accessor(_unshare)
>  
>  /* General conversion function from mfn to gfn */
>  static inline unsigned long mfn_to_gfn(struct domain *d, mfn_t mfn)
> 
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