On Fri, Nov 23, 2018 at 12:06:11PM +0100, Sebastian Andrzej Siewior wrote: > On 2018-11-23 12:02:55 [+0100], Andrea Parri wrote: > > > is this an RT-only problem? Because mainline should not allow read->read > > > locking or read->write locking for reader-writer locks. If this only > > > happens on v4.18 and not on v4.19 then something must have fixed it. > > > > Probably misunderstanding, but I'd say that read->read locking is "the > > norm"...? > > > > If you don't use qrwlock, readers are also "recursive", in part., > > > > P0 P1 > > read_lock(l) > > write_lock(l) > > read_lock(l) > > > > won't block P0 on the second read_lock(). (qrwlock somehow complicate > > the analysis; IIUC, they are recursive if and only if in_interrupt().). > > ehm, peterz, is that true? My memory on that is that all readers will > block if there is a writer pending. With qwrlocks, the readers will normally block if there is a pending writer (to avoid starving the writer), unless in_interrupt() when the readers are allowed to starve a pending writer. TLA+/PlusCal model here: ;) https://git.kernel.org/pub/scm/linux/kernel/git/cmarinas/kernel-tla.git/tree/qrwlock.tla -- Catalin
