Peter Zijlstra wrote: > On Sat, 2008-08-30 at 08:32 -0400, Gregory Haskins wrote: > >> Peter Zijlstra wrote: >> >>> On Fri, 2008-08-29 at 14:03 -0400, Gregory Haskins wrote: >>> >>> >>>> *Patch submitted for inclusion in PREEMPT_RT 26-rt4. Applies to 2.6.26.3-rt3* >>>> >>>> Hi Ingo, Steven, Thomas, >>>> Please consider for -rt4. This fixes a nasty deadlock on my systems under >>>> heavy load. >>>> >>>> [ >>>> Changelog: >>>> v2: only touch seqlock_t because raw_seqlock_t doesn't require >>>> serialization and userspace cannot modify data during a read >>>> >>>> v1: initial release >>>> ] >>>> >>>> -Greg >>>> >>>> ---- >>>> seqlock: serialize against writers >>>> >>>> Seqlocks have always advertised that readers do not "block", but this was >>>> never really true. Readers have always logically blocked at the head of >>>> the critical section under contention with writers, regardless of whether >>>> they were allowed to run code or not. >>>> >>>> Recent changes in this space (88a411c07b6fedcfc97b8dc51ae18540bd2beda0) >>>> have turned this into a more explicit blocking operation in mainline. >>>> However, this change highlights a short-coming in -rt because the >>>> normal seqlock_ts are preemptible. This means that we can potentially >>>> deadlock should a reader spin waiting for a write critical-section to end >>>> while the writer is preempted. >>>> >>>> >>> Ah, the point I was missing is higher-priority realtime task, in which >>> case the write side will never run because it wont preempt. >>> >>> >> Yep >> >>> >>> >>>> This patch changes the internal implementation to use a rwlock and forces >>>> the readers to serialize with the writers under contention. This will >>>> have the advantage that -rt seqlocks_t will sleep the reader if deadlock >>>> were imminent, and it will pi-boost the writer to prevent inversion. >>>> >>>> This fixes a deadlock discovered under testing where all high prioritiy >>>> readers were hogging the cpus and preventing a writer from releasing the >>>> lock. >>>> >>>> Since seqlocks are designed to be used as rarely-write locks, this should >>>> not affect the performance in the fast-path >>>> >>>> >>> Still dont like this patch, once you have a rwlock you might as well go >>> all the way. >>> >> Why? >> > > Because the second point. > > >> A full rwlock will still be much slower since the readers will >> always need an atomic op. This construct only uses atomic ops in the >> slow path under contention, which should be rare, and is thus still >> superior when retries are permissible to the design. >> >> >>> Esp since this half-arsed construct defeats PI in certain >>> cases. >>> >>> >> Ouch. While I admit that you can still get into inversion scenarios >> once the reader leaves the seqbegin, this is the nature of seqlocks. >> The only ways I can think of to get around this involve atomic ops in >> the fast path, which I think should be avoided. What would you suggest >> otherwise? >> > > Since we're talking -rt here, determinism rules, so bite the bullet and > do full PI. > > The only reason we made all that stuff preemptable is to gain > determinism, that also means we have to do the PI thing. > Yeah, you have a point. I still think this patch will solve the deadlock thing, so please consider it in the interim. I will whip up a full PI solution next week. -Greg
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