diff options
author | Paul E. McKenney <paulmck@kernel.org> | 2023-11-02 04:28:38 +0300 |
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committer | Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com> | 2023-12-12 00:01:22 +0300 |
commit | 4e58aaeebb3c27993c734c99eae6881b196b1ddb (patch) | |
tree | a3d94fb2b8dd013fa8fc3415026c59511874a12d /kernel/rcu/rcu.h | |
parent | 98b1cc82c4affc16f5598d4fa14b1858671b2263 (diff) | |
download | linux-4e58aaeebb3c27993c734c99eae6881b196b1ddb.tar.xz |
rcu: Restrict access to RCU CPU stall notifiers
Although the RCU CPU stall notifiers can be useful for dumping state when
tracking down delicate forward-progress bugs where NUMA effects cause
cache lines to be delivered to a given CPU regularly, but always in a
state that prevents that CPU from making forward progress. These bugs can
be detected by the RCU CPU stall-warning mechanism, but in some cases,
the stall-warnings printk()s disrupt the forward-progress bug before
any useful state can be obtained.
Unfortunately, the notifier mechanism added by commit 5b404fdabacf ("rcu:
Add RCU CPU stall notifier") can make matters worse if used at all
carelessly. For example, if the stall warning was caused by a lock not
being released, then any attempt to acquire that lock in the notifier
will hang. This will prevent not only the notifier from producing any
useful output, but it will also prevent the stall-warning message from
ever appearing.
This commit therefore hides this new RCU CPU stall notifier
mechanism under a new RCU_CPU_STALL_NOTIFIER Kconfig option that
depends on both DEBUG_KERNEL and RCU_EXPERT. In addition, the
rcupdate.rcu_cpu_stall_notifiers=1 kernel boot parameter must also
be specified. The RCU_CPU_STALL_NOTIFIER Kconfig option's help text
contains a warning and explains the dangers of careless use, recommending
lockless notifier code. In addition, a WARN() is triggered each time
that an attempt is made to register a stall-warning notifier in kernels
built with CONFIG_RCU_CPU_STALL_NOTIFIER=y.
This combination of measures will keep use of this mechanism confined to
debug kernels and away from routine deployments.
[ paulmck: Apply Dan Carpenter feedback. ]
Fixes: 5b404fdabacf ("rcu: Add RCU CPU stall notifier")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com>
Diffstat (limited to 'kernel/rcu/rcu.h')
-rw-r--r-- | kernel/rcu/rcu.h | 8 |
1 files changed, 5 insertions, 3 deletions
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index b531c33e9545..f94f65877f2b 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -262,6 +262,8 @@ static inline bool rcu_stall_is_suppressed_at_boot(void) return rcu_cpu_stall_suppress_at_boot && !rcu_inkernel_boot_has_ended(); } +extern int rcu_cpu_stall_notifiers; + #ifdef CONFIG_RCU_STALL_COMMON extern int rcu_cpu_stall_ftrace_dump; @@ -659,10 +661,10 @@ static inline bool rcu_cpu_beenfullyonline(int cpu) { return true; } bool rcu_cpu_beenfullyonline(int cpu); #endif -#ifdef CONFIG_RCU_STALL_COMMON +#if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER) int rcu_stall_notifier_call_chain(unsigned long val, void *v); -#else // #ifdef CONFIG_RCU_STALL_COMMON +#else // #if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER) static inline int rcu_stall_notifier_call_chain(unsigned long val, void *v) { return NOTIFY_DONE; } -#endif // #else // #ifdef CONFIG_RCU_STALL_COMMON +#endif // #else // #if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER) #endif /* __LINUX_RCU_H */ |