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Date: Sat, 1 Aug 2015 14:49:21 +0200 From: Ulrich Obergfell <uobergfe@...hat.com> To: linux-kernel@...r.kernel.org Cc: akpm@...ux-foundation.org, dzickus@...hat.com, atomlin@...hat.com, uobergfe@...hat.com, jolsa@...nel.org, mhocko@...e.cz, eranian@...gle.com, cmetcalf@...hip.com, fweisbec@...il.com Subject: [PATCH 0/4] watchdog: avoid races in watchdog_nmi_{en|disable} functions Originally watchdog_nmi_enable(cpu) and watchdog_nmi_disable(cpu) were only called in watchdog thread context. However, the following commits utilize these functions outside of watchdog thread context too. commit 9809b18fcf6b8d8ec4d3643677345907e6b50eca Author: Michal Hocko <mhocko@...e.cz> Date: Tue Sep 24 15:27:30 2013 -0700 watchdog: update watchdog_thresh properly commit b3738d29323344da3017a91010530cf3a58590fc Author: Stephane Eranian <eranian@...gle.com> Date: Mon Nov 17 20:07:03 2014 +0100 watchdog: Add watchdog enable/disable all functions Hence, it is now possible that these functions execute concurrently with the same 'cpu' argument. This concurrency is problematic because per-cpu 'watchdog_ev' can be accessed/modified without adequate synchronization. The patch series aims to address the above problem. However, instead of introducing locks to protect per-cpu 'watchdog_ev' a different approach is taken: Invoke these functions by parking and unparking the watchdog threads (to ensure they are always called in watchdog thread context). static struct smp_hotplug_thread watchdog_threads = { ... .park = watchdog_disable, // calls watchdog_nmi_disable() .unpark = watchdog_enable, // calls watchdog_nmi_enable() }; Both previously mentioned commits call these functions in a similar way and thus in principle contain some duplicate code. The patch series also avoids this duplication by providing a commonly usable mechanism. - Patch 1/4 introduces the watchdog_{park|unpark}_threads functions that park/unpark all watchdog threads specified in 'watchdog_cpumask'. They are intended to be called inside of kernel/watchdog.c only. - Patch 2/4 introduces the watchdog_{suspend|resume} functions which can be utilized by external callers to deactivate the hard and soft lockup detector temporarily. - Patch 3/4 utilizes watchdog_{park|unpark}_threads to replace some code that was introduced by commit 9809b18fcf6b8d8ec4d3643677345907e6b50eca. - Patch 4/4 utilizes watchdog_{suspend|resume} to replace some code that was introduced by commit b3738d29323344da3017a91010530cf3a58590fc. A few corner cases should be mentioned here for completeness. - kthread_park() of watchdog/N could hang if cpu N is already locked up. However, if watchdog is enabled the lockup will be detected anyway. - kthread_unpark() of watchdog/N could hang if cpu N got locked up after kthread_park(). The occurrence of this scenario should be _very_ rare in practice, in particular because it is not expected that temporary deactivation will happen frequently, and if it happens at all it is expected that the duration of deactivation will be short. Ulrich Obergfell (4): watchdog: introduce watchdog_park_threads() and watchdog_unpark_threads() watchdog: introduce watchdog_suspend() and watchdog_resume() watchdog: use park/unpark functions in update_watchdog_all_cpus() watchdog: use suspend/resume interface in fixup_ht_bug() arch/x86/kernel/cpu/perf_event_intel.c | 9 +- include/linux/nmi.h | 2 + include/linux/watchdog.h | 8 -- kernel/watchdog.c | 157 +++++++++++++++++++-------------- 4 files changed, 100 insertions(+), 76 deletions(-) -- 1.7.11.7 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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