Re: [Xen-devel] [PATCH] Fix shared entry status for grant copy operation on paged out gfn

["Jan Beulich" <JBeulich@xxxxxxxx>]

 >>> On 21.08.12 at 22:13, Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx> wrote:
> xen/common/grant_table.c |  33 ++++++++++++++++++++++-----------
>  1 files changed, 22 insertions(+), 11 deletions(-)
> 
> 
> The unwind path was not clearing the shared entry status bits. This was
> BSOD-ing guests on network activity under certain configurations.
> 
> Also:
>  * sed the fixup method name to signal it's related to grant copy.
>  * use atomic clear flag ops during fixup.

Is that last thing really needed? I remember having looked at
these non-atomic operations too a little while back, and came to
the conclusion that probably the authors intentionally coded it
that way.

jan

> Signed-off-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
> 
> diff -r 5267f78c8a1e -r 84a23ae853a5 xen/common/grant_table.c
> --- a/xen/common/grant_table.c
> +++ b/xen/common/grant_table.c
> @@ -1751,14 +1751,14 @@ __release_grant_for_copy(
>     under the domain's grant table lock. */
>  /* Only safe on transitive grants.  Even then, note that we don't
>     attempt to drop any pin on the referent grant. */
> -static void __fixup_status_for_pin(const struct active_grant_entry *act,
> +static void __fixup_status_for_copy_pin(const struct active_grant_entry 
> *act,
>                                     uint16_t *status)
>  {
>      if ( !(act->pin & GNTPIN_hstw_mask) )
> -        *status &= ~GTF_writing;
> +        gnttab_clear_flag(_GTF_writing, status);
>  
>      if ( !(act->pin & GNTPIN_hstr_mask) )
> -        *status &= ~GTF_reading;
> +        gnttab_clear_flag(_GTF_reading, status);
>  }
>  
>  /* Grab a frame number from a grant entry and update the flags and pin
> @@ -1834,7 +1834,7 @@ __acquire_grant_for_copy(
>          if ( sha2 && (shah->flags & GTF_type_mask) == GTF_transitive )
>          {
>              if ( !allow_transitive )
> -                PIN_FAIL(unlock_out, GNTST_general_error,
> +                PIN_FAIL(unlock_out_clear, GNTST_general_error,
>                           "transitive grant when transitivity not 
> allowed\n");
>  
>              trans_domid = sha2->transitive.trans_domid;
> @@ -1842,7 +1842,7 @@ __acquire_grant_for_copy(
>              barrier(); /* Stop the compiler from re-loading
>                            trans_domid from shared memory */
>              if ( trans_domid == rd->domain_id )
> -                PIN_FAIL(unlock_out, GNTST_general_error,
> +                PIN_FAIL(unlock_out_clear, GNTST_general_error,
>                           "transitive grants cannot be self-referential\n");
>  
>              /* We allow the trans_domid == ldom case, which
> @@ -1855,7 +1855,7 @@ __acquire_grant_for_copy(
>              /* We need to leave the rrd locked during the grant copy */
>              td = rcu_lock_domain_by_id(trans_domid);
>              if ( td == NULL )
> -                PIN_FAIL(unlock_out, GNTST_general_error,
> +                PIN_FAIL(unlock_out_clear, GNTST_general_error,
>                           "transitive grant referenced bad domain %d\n",
>                           trans_domid);
>              spin_unlock(&rgt->lock);
> @@ -1866,7 +1866,7 @@ __acquire_grant_for_copy(
>  
>              spin_lock(&rgt->lock);
>              if ( rc != GNTST_okay ) {
> -                __fixup_status_for_pin(act, status);
> +                __fixup_status_for_copy_pin(act, status);
>                  rcu_unlock_domain(td);
>                  spin_unlock(&rgt->lock);
>                  return rc;
> @@ -1878,7 +1878,7 @@ __acquire_grant_for_copy(
>                 and try again. */
>              if ( act->pin != old_pin )
>              {
> -                __fixup_status_for_pin(act, status);
> +                __fixup_status_for_copy_pin(act, status);
>                  rcu_unlock_domain(td);
>                  spin_unlock(&rgt->lock);
>                  put_page(*page);
> @@ -1897,7 +1897,7 @@ __acquire_grant_for_copy(
>          {
>              rc = __get_paged_frame(sha1->frame, &grant_frame, page, 
> readonly, 
> rd);
>              if ( rc != GNTST_okay )
> -                goto unlock_out;
> +                goto unlock_out_clear;
>              act->gfn = sha1->frame;
>              is_sub_page = 0;
>              trans_page_off = 0;
> @@ -1907,7 +1907,7 @@ __acquire_grant_for_copy(
>          {
>              rc = __get_paged_frame(sha2->full_page.frame, &grant_frame, 
> page, 
> readonly, rd);
>              if ( rc != GNTST_okay )
> -                goto unlock_out;
> +                goto unlock_out_clear;
>              act->gfn = sha2->full_page.frame;
>              is_sub_page = 0;
>              trans_page_off = 0;
> @@ -1917,7 +1917,7 @@ __acquire_grant_for_copy(
>          {
>              rc = __get_paged_frame(sha2->sub_page.frame, &grant_frame, page, 
> readonly, rd);
>              if ( rc != GNTST_okay )
> -                goto unlock_out;
> +                goto unlock_out_clear;
>              act->gfn = sha2->sub_page.frame;
>              is_sub_page = 1;
>              trans_page_off = sha2->sub_page.page_off;
> @@ -1948,6 +1948,17 @@ __acquire_grant_for_copy(
>      *length = act->length;
>      *frame = act->frame;
>  
> +    spin_unlock(&rgt->lock);
> +    return rc;
> + 
> + unlock_out_clear:
> +    if ( !(readonly) &&
> +         !(act->pin & GNTPIN_hstw_mask) )
> +        gnttab_clear_flag(_GTF_writing, status);
> +
> +    if ( !act->pin )
> +        gnttab_clear_flag(_GTF_reading, status);
> +
>   unlock_out:
>      spin_unlock(&rgt->lock);
>      return rc;



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