Messages in this thread | | | Date | Mon, 4 Jan 2016 05:02:31 -0800 | From | Davidlohr Bueso <> | Subject | Re: [PATCH] ipc/sem.c: Fix complex_count vs. simple op race |
| |
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
| |