On 2021-06-14 13:33:43 [+0200], Vlastimil Babka wrote: > On 6/14/21 1:16 PM, Sebastian Andrzej Siewior wrote: > > I haven't looked at the series and I have just this tiny question: why > > did migrate_disable() crash for Mel on !RT and why do you expect that it > > does not happen on PREEMPT_RT? > > Right, so it's because __slab_alloc() has this optimization to avoid re-reading > 'c' in case there is no preemption enabled at all (or it's just voluntary). > > #ifdef CONFIG_PREEMPTION > /* > * We may have been preempted and rescheduled on a different > * cpu before disabling preemption. Need to reload cpu area > * pointer. > */ > c = slub_get_cpu_ptr(s->cpu_slab); > #endif > > Mel's config has CONFIG_PREEMPT_VOLUNTARY, which means CONFIG_PREEMPTION is not > enabled. > > But then later in ___slab_alloc() we have > > slub_put_cpu_ptr(s->cpu_slab); > page = new_slab(s, gfpflags, node); > c = slub_get_cpu_ptr(s->cpu_slab); > > And this is not hidden under CONFIG_PREEMPTION, so with the #ifdef bug the > slub_put_cpu_ptr did a migrate_enable() with Mel's config, without prior > migrate_disable(). Ach, right. The update to this field is done with cmpxchg-double (if I remember correctly) but I don't remember if this is also re-entry safe. > If there wasn't the #ifdef PREEMPT_RT bug: > - this slub_put_cpu_ptr() would translate to put_cpu_ptr() thus > preempt_enable(), which on this config is just a barrier(), so it doesn't matter > that there was no matching preempt_disable() before. > - with PREEMPT_RT the CONFIG_PREEMPTION would be enabled, so the > slub_get_cpu_ptr() would do a migrate_disable() and there's no imbalance. > > But now that I dig into this in detail, I can see there might be another > instance of this imbalance bug, if CONFIG_PREEMPTION is disabled, but > CONFIG_PREEMPT_COUNT is enabled, which seems to be possible in some debug > scenarios. Because then preempt_disable()/preempt_enable() still manipulate the > preempt counter and compiling them out in __slab_alloc() will cause imbalance. > > So I think the guards in __slab_alloc() should be using CONFIG_PREEMPT_COUNT > instead of CONFIG_PREEMPT to be correct on all configs. I dare not remove them > completely :) :) Sebastian
