Re: [PATCH] ipc/sem.c: Fix complex_count vs. simple op race

[Manfred Spraul]

This is a multi-part message in MIME format.
On 01/04/2016 02:02 PM, Davidlohr Bueso wrote:
> On Sat, 02 Jan 2016, Manfred Spraul wrote:
>
> > Commit 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()") introduced a 
> > race:
> >
> > sem_lock has a fast path that allows parallel simple operations.
> > There are two reasons why a simple operation cannot run in parallel:
> > - a non-simple operations is ongoing (sma->sem_perm.lock held)
> > - a complex operation is sleeping (sma->complex_count != 0)
> >
> > As both facts are stored independently, a thread can bypass the current
> > checks by sleeping in the right positions. See below for more details
> > (or kernel bugzilla 105651).
> >
> > The patch fixes that by creating one variable (complex_mode)
> > that tracks both reasons why parallel operations are not possible.
> >
> > The patch also updates stale documentation regarding the locking.
> >
> > With regards to stable kernels:
> > The patch is required for all kernels that include the commit 
> > 6d07b68ce16a
> > ("ipc/sem.c: optimize sem_lock()") (3.10?)
> >
> > The alternative is to revert the patch that introduced the race.
>
> I am just now catching up with this, but a quick thought is that we 
> probably
> want to keep 6d07b68ce16a as waiting on unlocking all sem->lock should be
> much worse for performance than keeping track of the complex 'mode'.
Keeping track is simple - and the fast path gets simpler compared to 
ncurrent code.
> Specially
> if we have a large array.
Yes, I would prefer my patch as a backport.
But if someone has objections, then a revert would be an alternative.

> Also, any idea what workload exposed this race? Anyway, will take a 
> closer look
> at the patch/issue.
It was found by a theoretical review:
The sem_lock code was used as an exercise at University Bremen.

--
    Manfred
P.S.: If we replace the "bool" with an "int", we could even introduce a 
hysteresis, to further reduce the amount of array scans.