Re: Re: [PATCH v3] Fix one error in mthca_alloc

["kangning18z@ict.ac.cn" <kangning18z@ict.ac.cn>]

> On Tue, Aug 31, 2021 at 06:40:38PM +0800, 康宁 wrote:
> > >
> > > On Fri, Aug 27, 2021 at 08:52:28AM +0800, kangning wrote:
> > > > drivers/infiniband/hw/mthca/mthca_allocator.c: alloc->last left unchanged in mthca_alloc, which
> > > > has impact on performance of function find_next_zero_bit in mthca_alloc.
> > >
> > > I don't know what the sentence above means, but the change is unlikely
> > > to be correct.
> > >
> > > When alloc->last starts to be equal to alloc->max, the
> > > find_next_zero_bit() will always return alloc->max. which will ensure
> > > that the following code is executed.
> > >
> > >    48         if (obj >= alloc->max) {
> > >    49                 alloc->top = (alloc->top + alloc->max) & alloc->mask;
> > >    50                 obj = find_first_zero_bit(alloc->table, alloc->max);
> > >    51         }
> > >
> >
> > Thanks for your review.
> > Yes, your analysis is right. However, this is a bitmap allocator for resource id allocation (like QP, and CQ).
> > We first look at the situation where no resource id is released.
> > When the value of alloc->last starts to reaches alloc->max, the bitmap will
> > be full. In this case, it's normal to return an invalid value.
> >
> > Now, let's add resource releasing into consideration.
> > The following code is part of mthca_free：
> >
> > 70	spin_lock_irqsave(&alloc->lock, flags);
> > 71
> > 72	clear_bit(obj, alloc->table);
> > 73	alloc->last = min(alloc->last, obj);
> > 74	alloc->top = (alloc->top + alloc->max) & alloc->mask;
> >
> > mthca_free() is used to release the allocated resource id. It will update alloc->last when obj is freed.
> > So, if the bitmap has space for allocation, my modification can continuously work.
> 
> After alloc->last starts to be equal to alloc->max, the bitmap is
> searches with find_first_zero_bit() call. That will ensure that we look
> for any bit between 0 and alloc->max.
> 
> So no, your change can't be right.

If you never call function mthca_free(), you will never find a valid bit  in [0, alloc->max]
with find_first_zero_bit() call. Cause every bits between 0 and alloc->max are alloacted.

But if you called mthca_free() before, the freed obj is assigned to alloc->last, and we 
still can allocate the bitmap.

My modification is correct. 

> 
> Thanks
> 
> >
> > >
> > > However the mthca_alloc() function has other error, it returns -1 while
> > > based on its declaration it needs to be unsigned,
> >
> > I think you are right about this. But obj is the return value of u32 type, which is the requirement of resource id
> > (though it may not fully use 32 bits). I have no idea of how to fix it.
> >
> > Thanks.
> >
> > >
> > > Thanks
> > >
> > > >
> > &gt; &gt; Signed-off-by: kangning <kangning18z@ict.ac.cn>
> > &gt; &gt; ---
> > &gt; &gt; 
> > &gt; &gt;  I squashed two commits into one in this version.
> > &gt; &gt; 
> > &gt; &gt;  drivers/infiniband/hw/mthca/mthca_allocator.c | 3 +++
> > &gt; &gt;  1 file changed, 3 insertions(+)
> > &gt; &gt;
> > &gt; &gt; diff --git a/drivers/infiniband/hw/mthca/mthca_allocator.c b/drivers/infiniband/hw/mthca/mthca_allocator.c
> > &gt; &gt; index aef1d274a14e..1141695093e7 100644
> > &gt; &gt; --- a/drivers/infiniband/hw/mthca/mthca_allocator.c
> > &gt; &gt; +++ b/drivers/infiniband/hw/mthca/mthca_allocator.c
> > &gt; &gt; @@ -51,6 +51,9 @@ u32 mthca_alloc(struct mthca_alloc *alloc)
> > &gt; &gt;  }
> > &gt; &gt; 
> > &gt; &gt;  if (obj &lt; alloc-&gt;max) {
> > &gt; &gt; +	alloc-&gt;last = obj + 1;
> > &gt; &gt; +	if (alloc-&gt;last == alloc-&gt;max)
> > &gt; &gt; +	alloc-&gt;last = 0;
> > &gt; &gt;  set_bit(obj, alloc-&gt;table);
> > &gt; &gt;  obj |= alloc-&gt;top;
> > &gt; &gt;  } else
> > &gt; &gt; --
> > &gt; &gt; 2.17.1
> > &gt; &gt;
> > </kangning18z@ict.ac.cn>