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rcu: Provide optional RCU-reader exit delay for strict GPs

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The goal of this series is to increase the probability of tools like
KASAN detecting that an RCU-protected pointer was used outside of its
RCU read-side critical section.  Thus far, the approach has been to make
grace periods and callback processing happen faster.  Another approach
is to delay the pointer leaker.  This commit therefore allows a delay
to be applied to exit from RCU read-side critical sections.

This slowdown is specified by a new rcutree.rcu_unlock_delay kernel boot
parameter that specifies this delay in microseconds, defaulting to zero.

Reported-by Jann Horn <jannh@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2020-08-07 13:44:10 -07:00
parent 4e025f52a1
commit 3d29aaf1ef
2 changed files with 19 additions and 2 deletions
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9
Documentation/admin-guide/kernel-parameters.txt
View File

@ -4152,6 +4152,15 @@
This wake_up() will be accompanied by a
WARN_ONCE() splat and an ftrace_dump().
rcutree.rcu_unlock_delay= [KNL]
In CONFIG_RCU_STRICT_GRACE_PERIOD=y kernels,
this specifies an rcu_read_unlock()-time delay
in microseconds. This defaults to zero.
Larger delays increase the probability of
catching RCU pointer leaks, that is, buggy use
of RCU-protected pointers after the relevant
rcu_read_unlock() has completed.
rcutree.sysrq_rcu= [KNL]
Commandeer a sysrq key to dump out Tree RCU's
rcu_node tree with an eye towards determining

12
kernel/rcu/tree_plugin.h
View File

@ -430,6 +430,12 @@ static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
return !list_empty(&rnp->blkd_tasks);
}
// Add delay to rcu_read_unlock() for strict grace periods.
static int rcu_unlock_delay;
#ifdef CONFIG_RCU_STRICT_GRACE_PERIOD
module_param(rcu_unlock_delay, int, 0444);
#endif
/*
* Report deferred quiescent states. The deferral time can
* be quite short, for example, in the case of the call from
@ -460,11 +466,13 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
}
t->rcu_read_unlock_special.s = 0;
if (special.b.need_qs) {
if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) {
rcu_report_qs_rdp(rdp->cpu, rdp);
else
udelay(rcu_unlock_delay);
} else {
rcu_qs();
}
}
/*
* Respond to a request by an expedited grace period for a