[{"id":3674858,"web_url":"http://patchwork.ozlabs.org/comment/3674858/","msgid":"","list_archive_url":null,"date":"2026-04-08T16:53:16","subject":"Re: [patch V2 01/11] hrtimer: Provide hrtimer_start_range_ns_user()","submitter":{"id":79411,"url":"http://patchwork.ozlabs.org/api/people/79411/","name":"Frederic Weisbecker","email":"frederic@kernel.org"},"content":"Le Wed, Apr 08, 2026 at 01:53:46PM +0200, Thomas Gleixner a écrit :\n> Calvin reported an odd NMI watchdog lockup which claims that the CPU locked\n> up in user space. He provided a reproducer, which set's up a timerfd based\n> timer and then rearms it in a loop with an absolute expiry time of 1ns.\n> \n> As the expiry time is in the past, the timer ends up as the first expiring\n> timer in the per CPU hrtimer base and the clockevent device is programmed\n> with the minimum delta value. If the machine is fast enough, this ends up\n> in a endless loop of programming the delta value to the minimum value\n> defined by the clock event device, before the timer interrupt can fire,\n> which starves the interrupt and consequently triggers the lockup detector\n> because the hrtimer callback of the lockup mechanism is never invoked.\n> \n> The clockevents code already has a last resort mechanism to prevent that,\n> but it's sensible to catch such issues before trying to reprogram the clock\n> event device.\n> \n> Provide a variant of hrtimer_start_range_ns(), which sanity checks the\n> timer after queueing it. It does not so before because the timer might be\n> armed and therefore needs to be dequeued. also we optimize for the latest\n> possible point to check, so that the clock event prevention is avoided as\n> much as possible.\n> \n> If the timer is already expired _before_ the clock event is reprogrammed,\n> remove the timer from the queue and signal to the caller that the operation\n> failed by returning false.\n> \n> That allows the caller to take immediate action without going through the\n> loops and hoops of the hrtimer interrupt.\n> \n> The queueing code can't invoke the timer callback as the caller might hold\n> a lock which is taken in the callback.\n> \n> Add a tracepoint which allows to analyze the expired at start situation.\n> \n> Reported-by: Calvin Owens \n> Signed-off-by: Thomas Gleixner \n> Cc: Anna-Maria Behnsen \n> Cc: Frederic Weisbecker \n\nReviewed-by: Frederic Weisbecker ","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","netfilter-devel@vger.kernel.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256\n header.s=k20201202 header.b=eKj5Xyn4;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=vger.kernel.org\n (client-ip=2600:3c04:e001:36c::12fc:5321; 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