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

[Davidlohr Bueso]

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'. Specially
if we have a large array.

Also, any idea what workload exposed this race? Anyway, will take a closer look
at the patch/issue.

Thanks,
Davidlohr
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