Re: [PATCH 1/1] percpu_rw_semaphore: reimplement to not block thereaders unnecessarily

[Oleg Nesterov]

Thanks!

I'll send v2 tomorrow.

On 11/01, Linus Torvalds wrote:
> On Wed, Oct 31, 2012 at 12:41 PM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> > Currently the writer does msleep() plus synchronize_sched() 3 times
> > to acquire/release the semaphore, and during this time the readers
> > are blocked completely. Even if the "write" section was not actually
> > started or if it was already finished.
> >
> > With this patch down_read/up_read does synchronize_sched() twice and
> > down_read/up_read are still possible during this time, just they use
> > the slow path.
> 
> The changelog is wrong (it's the write path, not read path, that does
> the synchronize_sched).
> 
> >  struct percpu_rw_semaphore {
> > -       unsigned __percpu *counters;
> > -       bool locked;
> > -       struct mutex mtx;
> > +       int __percpu            *fast_read_ctr;
> 
> This change is wrong.
> 
> You must not make the 'fast_read_ctr' thing be an int. Or at least you
> need to be a hell of a lot more careful about it.
> 
> Why?
> 
> Because the readers update the counters while possibly moving around
> cpu's, the increment and decrement of the counters may be on different
> CPU's. But that means that when you add all the counters together,
> things can overflow (only the final sum is meaningful). And THAT in
> turn means that you should not use a signed count, for the simple
> reason that signed integers don't have well-behaved overflow behavior
> in C.
> 
> Now, I doubt you'll find an architecture or C compiler where this will
> actually ever make a difference, but the fact remains that you
> shouldn't use signed integers for counters like this. You should use
> unsigned, and you should rely on the well-defined modulo-2**n
> semantics.
> 
> I'd also like to see a comment somewhere in the source code about the
> whole algorithm and the rules.
> 
> Other than that, I guess it looks ok.
> 
>             Linus

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