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. > 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. Esp since this half-arsed construct defeats PI in certain cases. -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
