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 bOjQuJZn 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Peter Zijlstra writes: > On Wed, Aug 30, 2023 at 11:49:56AM -0700, Ankur Arora wrote: > >> +#ifdef TIF_RESCHED_ALLOW >> +/* >> + * allow_resched() .. disallow_resched() demarcate a preemptible section. >> + * >> + * Used around primitives where it might not be convenient to periodically >> + * call cond_resched(). >> + */ >> +static inline void allow_resched(void) >> +{ >> + might_sleep(); >> + set_tsk_thread_flag(current, TIF_RESCHED_ALLOW); > > So the might_sleep() ensures we're not currently having preemption > disabled; but there's nothing that ensures we don't do stupid things > like: > > allow_resched(); > spin_lock(); > ... > spin_unlock(); > disallow_resched(); > > Which on a PREEMPT_COUNT=n build will cause preemption while holding the > spinlock. I think something like the below will cause sufficient > warnings to avoid growing patterns like that. Yeah, I agree this is a problem. I'll expand on the comment above allow_resched() detailing this scenario. > Index: linux-2.6/kernel/sched/core.c > =================================================================== > --- linux-2.6.orig/kernel/sched/core.c > +++ linux-2.6/kernel/sched/core.c > @@ -5834,6 +5834,13 @@ void preempt_count_add(int val) > { > #ifdef CONFIG_DEBUG_PREEMPT > /* > + * Disabling preemption under TIF_RESCHED_ALLOW doesn't > + * work for PREEMPT_COUNT=n builds. > + */ > + if (WARN_ON(resched_allowed())) > + return; > + > + /* > * Underflow? > */ > if (DEBUG_LOCKS_WARN_ON((preempt_count() < 0))) And, maybe something like this to guard against __this_cpu_read() etc: diff --git a/lib/smp_processor_id.c b/lib/smp_processor_id.c index a2bb7738c373..634788f16e9e 100644 --- a/lib/smp_processor_id.c +++ b/lib/smp_processor_id.c @@ -13,6 +13,9 @@ unsigned int check_preemption_disabled(const char *what1, const char *what2) { int this_cpu = raw_smp_processor_id(); + if (unlikely(resched_allowed())) + goto out_error; + if (likely(preempt_count())) goto out; @@ -33,6 +36,7 @@ unsigned int check_preemption_disabled(const char *what1, const char *what2) if (system_state < SYSTEM_SCHEDULING) goto out; +out_error: /* * Avoid recursion: */ -- ankur