svn commit: r266775 - head/sys/x86/x86

[Attilio Rao]

On Fri, May 30, 2014 at 6:44 PM, John Baldwin <jhb at freebsd.org> wrote:
> On Friday, May 30, 2014 11:51:38 am Attilio Rao wrote:
>> On Fri, May 30, 2014 at 5:47 PM, John Baldwin <jhb at freebsd.org> wrote:
>> > On Friday, May 30, 2014 11:39:24 am Attilio Rao wrote:
>> >> On Fri, May 30, 2014 at 5:03 PM, John Baldwin <jhb at freebsd.org> wrote:
>> >> > On Friday, May 30, 2014 10:54:06 am Attilio Rao wrote:
>> >> >> On Tue, May 27, 2014 at 11:31 PM, Scott Long <scottl at freebsd.org> wrote:
>> >> >> > Author: scottl
>> >> >> > Date: Tue May 27 21:31:11 2014
>> >> >> > New Revision: 266775
>> >> >> > URL: http://svnweb.freebsd.org/changeset/base/266775
>> >> >> >
>> >> >> > Log:
>> >> >> >   Eliminate the fake contig_dmamap and replace it with a new flag,
>> >> >> >   BUS_DMA_KMEM_ALLOC.  They serve the same purpose, but using the flag
>> >> >> >   means that the map can be NULL again, which in turn enables significant
>> >> >> >   optimizations for the common case of no bouncing.
>> >> >>
>> >> >> While I think this is in general a good idea, unfortunately our
>> >> >> drivers do so many dumb things when freeing DMA allocated buffers that
>> >> >> having a NULL map is going to cause some "turbolence" and make such
>> >> >> bugs more visible.
>> >> >> An example is with ATA, where I think this fix is needed:
>> >> >> http://www.freebsd.org/~attilio/dmamem_free-ata.patch
>> >> >>
>> >> >> Otherwise, what can happen with bounce buffers, is that the allocated
>> >> >> memory via contig malloc was not going to be freed anytime.
>> >> >>
>> >> >> I tried to look around and I found questionable (read broken) code in
>> >> >> basically every driver which allocates DMA buffers, so I really don't
>> >> >> feel I want to fix the majority of our drivers. I just think such
>> >> >> paths are not excercised enough to be seen in practice often or the
>> >> >> bugs just get unnoticed.
>> >> >
>> >> > Eh, many maps for static allocations were already NULL and have been for a
>> >> > long time.  This is nothign new.  Plus, the diff you posted has a bug
>> >> > regardless of explicitly destroying a map created by bus_dmamem_alloc().
>> >>
>> >> Did you notice that I *removed* the destroy not *added*?
>> >
>> > Yes, my point was that that bug in the original code you are fixing was there
>> > regardless of Scott's change.
>>
>> And when I did say something different?
>> I don't understand what's the point of your messages, besides showing
>> that you didn't read correctly my patch.
>
> I read yours correctly but worded mine poorly.  My point is that Scott's
> change does not introduce anything new.  We've had NULL maps for static
> allocations for many, many years.  It's only been recently that we've
> had more maps not be NULL for this.  However, even if you discounted
> the whole NULL vs non-NULL maps thing, the driver in question that you
> are fixing was broken regardless.  That is, due to the extra
> bus_dmamap_destroy() the driver was broken regardless of whether the map
> was NULL or non-NULL.

To be honest, pre-266775 the kernel would actually panic for this
specific driver, because we were going to free memory that was never
allocated (by having a valid mapping but an invalid dma memory
pointer).
That was prompted to look at the dma_alloc_*() bits of drivers.
We need to make a real sweep at drivers on these bits.

>
> TL;DR:
>
> - Scott's change did not introduce any new behavior so we don't need to
>   worry about a spate of new bugs uncovered in drivers because of it.

Not entirely true. For ATA it was before a panic and now it is a
silent memory leak. For other drivers it may be the opposite.
I could just find this one becaue I got bitten by it.

Attilio


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