Re: [PATCH] switch rangeset's lock to rwlock

[Keir Fraser <keir.xen@gmail.com>]

On 25/03/2011 17:52, "Dan Magenheimer" <dan.magenheimer@oracle.com> wrote:

> Tmem (in Xen) does use rwlocks.  After hearing from Jeremy that
> Linux maintainers wouldn't approve of new users of rwlocks, I
> redid the locking structure in the in-Linux-kernel version of tmem
> to avoid using them.  I am fairly sure that the same approach used in
> zcache can be used in Xen, but have not tried, and it's likely
> to be a fairly big coding/testing effort that I can't undertake
> right now.
> 
> I am also fairly sure that the current Xen tmem locking structure
> is not suitable for switching to normal spinlocks nor RCU,
> but am far from an expert in this area.

Why would a normal spinlock not work?

 -- Keir

> Dan
> 
>> -----Original Message-----
>> From: Keir Fraser [mailto:keir.xen@gmail.com]
>> Sent: Friday, March 25, 2011 11:09 AM
>> To: Jan Beulich; xen-devel@lists.xensource.com
>> Subject: Re: [Xen-devel] [PATCH] switch rangeset's lock to rwlock
>> 
>> I'd rather get rid of rwlocks altogether and use RCU in any cases where
>> we
>> really have contention. Rwlocks don't help unless the read-side
>> critical
>> sections are large enough to amortise the cache ping-pong cost of the
>> locking/unlocking operations. And in Xen we have very few if any
>> significantly sized critical sections.
>> 
>> I need to double check, but I believe we have only a couple of rwlock
>> users
>> now, and none of the read-side critical sections are large, so in that
>> case
>> I suggest we switch them to use spinlocks and kill our rwlock
>> implementation.
>> 
>>  -- Keir
>> 
>> On 25/03/2011 16:49, "Jan Beulich" <JBeulich@novell.com> wrote:
>> 
>>> As a general library routine, it should behave as efficiently as
>>> possible, even if at present no significant contention is known here.
>>> 
>>> Signed-off-by: Jan Beulich <jbeulich@novell.com>
>>> 
>>> --- a/xen/common/rangeset.c
>>> +++ b/xen/common/rangeset.c
>>> @@ -25,7 +25,7 @@ struct rangeset {
>>> 
>>>      /* Ordered list of ranges contained in this set, and protecting
>> lock. */
>>>      struct list_head range_list;
>>> -    spinlock_t       lock;
>>> +    rwlock_t         lock;
>>> 
>>>      /* Pretty-printing name. */
>>>      char             name[32];
>>> @@ -103,7 +103,7 @@ int rangeset_add_range(
>>> 
>>>      ASSERT(s <= e);
>>> 
>>> -    spin_lock(&r->lock);
>>> +    write_lock(&r->lock);
>>> 
>>>      x = find_range(r, s);
>>>      y = find_range(r, e);
>>> @@ -159,7 +159,7 @@ int rangeset_add_range(
>>>      }
>>> 
>>>   out:
>>> -    spin_unlock(&r->lock);
>>> +    write_unlock(&r->lock);
>>>      return rc;
>>>  }
>>> 
>>> @@ -175,7 +175,7 @@ int rangeset_remove_range(
>>> 
>>>      ASSERT(s <= e);
>>> 
>>> -    spin_lock(&r->lock);
>>> +    write_lock(&r->lock);
>>> 
>>>      x = find_range(r, s);
>>>      y = find_range(r, e);
>>> @@ -231,7 +231,7 @@ int rangeset_remove_range(
>>>      }
>>> 
>>>   out:
>>> -    spin_unlock(&r->lock);
>>> +    write_unlock(&r->lock);
>>>      return rc;
>>>  }
>>> 
>>> @@ -243,10 +243,10 @@ int rangeset_contains_range(
>>> 
>>>      ASSERT(s <= e);
>>> 
>>> -    spin_lock(&r->lock);
>>> +    read_lock(&r->lock);
>>>      x = find_range(r, s);
>>>      contains = (x && (x->e >= e));
>>> -    spin_unlock(&r->lock);
>>> +    read_unlock(&r->lock);
>>> 
>>>      return contains;
>>>  }
>>> @@ -259,10 +259,10 @@ int rangeset_overlaps_range(
>>> 
>>>      ASSERT(s <= e);
>>> 
>>> -    spin_lock(&r->lock);
>>> +    read_lock(&r->lock);
>>>      x = find_range(r, e);
>>>      overlaps = (x && (s <= x->e));
>>> -    spin_unlock(&r->lock);
>>> +    read_unlock(&r->lock);
>>> 
>>>      return overlaps;
>>>  }
>>> @@ -274,13 +274,13 @@ int rangeset_report_ranges(
>>>      struct range *x;
>>>      int rc = 0;
>>> 
>>> -    spin_lock(&r->lock);
>>> +    read_lock(&r->lock);
>>> 
>>>      for ( x = find_range(r, s); x && (x->s <= e) && !rc; x =
>> next_range(r, x)
>>> )
>>>          if ( x->e >= s )
>>>              rc = cb(max(x->s, s), min(x->e, e), ctxt);
>>> 
>>> -    spin_unlock(&r->lock);
>>> +    read_unlock(&r->lock);
>>> 
>>>      return rc;
>>>  }
>>> @@ -318,7 +318,7 @@ struct rangeset *rangeset_new(
>>>      if ( r == NULL )
>>>          return NULL;
>>> 
>>> -    spin_lock_init(&r->lock);
>>> +    rwlock_init(&r->lock);
>>>      INIT_LIST_HEAD(&r->range_list);
>>> 
>>>      BUG_ON(flags & ~RANGESETF_prettyprint_hex);
>>> @@ -403,7 +403,7 @@ void rangeset_printk(
>>>      int nr_printed = 0;
>>>      struct range *x;
>>> 
>>> -    spin_lock(&r->lock);
>>> +    read_lock(&r->lock);
>>> 
>>>      printk("%-10s {", r->name);
>>> 
>>> @@ -422,7 +422,7 @@ void rangeset_printk(
>>> 
>>>      printk(" }");
>>> 
>>> -    spin_unlock(&r->lock);
>>> +    read_unlock(&r->lock);
>>>  }
>>> 
>>>  void rangeset_domain_printk(
>>> 
>>> 
>>> 
>>> _______________________________________________
>>> Xen-devel mailing list
>>> Xen-devel@lists.xensource.com
>>> http://lists.xensource.com/xen-devel
>> 
>> 
>> 
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