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Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1qeVW6-00GFvS-UW; Fri, 08 Sep 2023 07:02:59 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 978B9300472; Fri, 8 Sep 2023 09:02:58 +0200 (CEST) Date: Fri, 8 Sep 2023 09:02:58 +0200 From: Peter Zijlstra To: Ankur Arora Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, willy@infradead.org, mgorman@suse.de, rostedt@goodmis.org, tglx@linutronix.de, jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com, Linus Torvalds Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED Message-ID: <20230908070258.GA19320@noisy.programming.kicks-ass.net> References: <20230830184958.2333078-1-ankur.a.arora@oracle.com> <20230830184958.2333078-8-ankur.a.arora@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230830184958.2333078-8-ankur.a.arora@oracle.com> X-Rspamd-Queue-Id: 02B7D40002 X-Rspam-User: X-Stat-Signature: uke4dthobmeh4iyjm9oap8h5qhb7p5n4 X-Rspamd-Server: rspam01 X-HE-Tag: 1694156598-223253 X-HE-Meta: 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 l1KiaBiD CfewES2BtBPj+2HiBqzEryot9VxQ+bcchRQZSTYG5o/BIpA2ZIJe1YQZ1E51yot0NR4IWvzPVLEIoiiWFN/mWPrgMgRRk/IDcym7/8IaunX+VcOeqAiXSQt6sd8tbpI1zrR1zNcUjo2ShbpmbDS3ZPeJItSVwvo9UTSGSqOawtwlObac9n/LWurZY/w== 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: 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. 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)))