Re: [v3 04/11] ASoC: Intel: sst: Add IPC handling

From: Vinod Koul <vinod.koul@intel.com>
To: Mark Brown <broonie@kernel.org>
Cc: alsa-devel@alsa-project.org, Lars-Peter Clausen <lars@metafoo.de>,
	"Subhransu S. Prusty" <subhransu.s.prusty@intel.com>,
	lgirdwood@gmail.com
Subject: Re: [v3 04/11] ASoC: Intel: sst: Add IPC handling
Date: Tue, 2 Sep 2014 10:52:25 +0530	[thread overview]
Message-ID: <20140902052225.GA1610@intel.com> (raw)
In-Reply-To: <20140901144134.GY29327@sirena.org.uk>

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On Mon, Sep 01, 2014 at 03:41:34PM +0100, Mark Brown wrote:
> On Mon, Sep 01, 2014 at 07:27:07PM +0530, Subhransu S. Prusty wrote:
> > On Mon, Sep 01, 2014 at 01:51:14PM +0100, Mark Brown wrote:
> > > On Mon, Sep 01, 2014 at 05:47:53PM +0530, Subhransu S. Prusty wrote:
> 
> > > > > I'd expect much louder complaints if we try to free something that's not
> > > > > allocated - what happens if we end up reallocating something quickly and
> > > > > then double freeing?  Better to complain if we hit such a code path.
> > > 
> > > > "freed" is a block which is passed by the caller to be freed up. Will add a
> > > > comment.
> 
> > > How would that address the problem?  Obviously the caller is trying to
> > > free what they're passing in.
> 
> > sst_create_block() which allocates the memory and sst_free_block() which
> > frees the memory are called in a synchronous way. A single thread who is
> > allocating waits till a response arrives, if that response is valid then
> > after processing the response the sst_free_block() is called to free up the
> > memory. So the double freeing will not happen. Does this address your concern?
> 
> No.  You've described what happens when things are working and
> everything is operating correctly and there are no bugs in the kernel,
> the goal with error checking is to provide robustness against the
> possibility that one of those things isn't true so we can tell what went
> wrong more easily than if we get memory corruption.
Lets assume a wrong case here is triggered due to some other issue. So we
get invoked twice for the same pointer.
Since the function holds the lock and searches the object in the list, only
first access will find the object and start to free it and relinquish the
lock.

Now, the second access will not find this and return, so no harm done.

I agree that we need to at least put a log indicating such a scenario
did occur and we failed to find the object. So we can return immediately
after freeing up and then if we hit end of function implying we haven't found
the object we should complain.

Would that help?

-- 
~Vinod

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