Messages in this thread | | | Subject | Re: [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race | From | Manfred Spraul <> | Date | Thu, 23 Jun 2016 20:55:06 +0200 |
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On 06/21/2016 01:04 AM, Andrew Morton wrote: > On Sat, 18 Jun 2016 22:02:21 +0200 Manfred Spraul <manfred@colorfullife.com> 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?) > I've had this in -mm (and -next) since January 4, without issues. I > put it on hold because Davidlohr expressed concern about performance > regressions. I had several ideas how to fix it. The initial ideas probably had performance issue.
The current one doesn't have any issues. It just took longer than expected to test it. > Your [2/2] should prevent those regressions (yes?) so I assume that any > kernel which has [1/2] really should have [2/2] as well. But without > any quantitative information, this is all mad guesswork. > > What to do? [2/2] is an improvement, it handles one case better than the current code. If you want: 3.10 improved scalability, but it introduced a performance regression for one use case. [with 3.10, simple ops got parallel, but complex ops had to perform a "for_each_sem() {spin_unlock_wait()}"] The patch fixes that.
-- Manfred
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