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Message-ID: <20211022152104.215612498@infradead.org>
Date: Fri, 22 Oct 2021 17:09:35 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: keescook@...omium.org, x86@...nel.org
Cc: linux-kernel@...r.kernel.org, peterz@...radead.org,
juri.lelli@...hat.com, vincent.guittot@...aro.org,
dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
mgorman@...e.de, bristot@...hat.com, akpm@...ux-foundation.org,
mark.rutland@....com, zhengqi.arch@...edance.com,
linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org,
mpe@...erman.id.au, paul.walmsley@...ive.com, palmer@...belt.com,
hca@...ux.ibm.com, gor@...ux.ibm.com, borntraeger@...ibm.com,
linux-arch@...r.kernel.org, ardb@...nel.org
Subject: [PATCH 2/7] stacktrace,sched: Make stack_trace_save_tsk() more robust
Recent patches to get_wchan() made it more robust by only doing the
unwind when the task was blocked and serialized against wakeups.
Extract this functionality as a simpler companion to task_call_func()
named task_try_func() that really only cares about blocked tasks. Then
employ this new function to implement the same robustness for
ARCH_STACKWALK based stack_trace_save_tsk().
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
include/linux/wait.h | 1
kernel/sched/core.c | 62 ++++++++++++++++++++++++++++++++++++++++++++-------
kernel/stacktrace.c | 13 ++++++----
3 files changed, 63 insertions(+), 13 deletions(-)
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -1162,5 +1162,6 @@ int autoremove_wake_function(struct wait
typedef int (*task_call_f)(struct task_struct *p, void *arg);
extern int task_call_func(struct task_struct *p, task_call_f func, void *arg);
+extern int task_try_func(struct task_struct *p, task_call_f func, void *arg);
#endif /* _LINUX_WAIT_H */
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1966,21 +1966,21 @@ bool sched_task_on_rq(struct task_struct
return task_on_rq_queued(p);
}
+static int try_get_wchan(struct task_struct *p, void *arg)
+{
+ unsigned long *wchan = arg;
+ *wchan = __get_wchan(p);
+ return 0;
+}
+
unsigned long get_wchan(struct task_struct *p)
{
unsigned long ip = 0;
- unsigned int state;
if (!p || p == current)
return 0;
- /* Only get wchan if task is blocked and we can keep it that way. */
- raw_spin_lock_irq(&p->pi_lock);
- state = READ_ONCE(p->__state);
- smp_rmb(); /* see try_to_wake_up() */
- if (state != TASK_RUNNING && state != TASK_WAKING && !p->on_rq)
- ip = __get_wchan(p);
- raw_spin_unlock_irq(&p->pi_lock);
+ task_try_func(p, try_get_wchan, &ip);
return ip;
}
@@ -4184,6 +4184,52 @@ int task_call_func(struct task_struct *p
return ret;
}
+/*
+ * task_try_func - Invoke a function on task in blocked state
+ * @p: Process for which the function is to be invoked
+ * @func: Function to invoke
+ * @arg: Argument to function
+ *
+ * Fix the task in a blocked state, when possible. And if so, invoke @func on it.
+ *
+ * Returns:
+ * -EBUSY or whatever @func returns
+ */
+int task_try_func(struct task_struct *p, task_call_f func, void *arg)
+{
+ unsigned long flags;
+ unsigned int state;
+ int ret = -EBUSY;
+
+ raw_spin_lock_irqsave(&p->pi_lock, flags);
+
+ state = READ_ONCE(p->__state);
+
+ /*
+ * Ensure we load p->on_rq after p->__state, otherwise it would be
+ * possible to, falsely, observe p->on_rq == 0.
+ *
+ * See try_to_wake_up() for a longer comment.
+ */
+ smp_rmb();
+
+ /*
+ * Since pi->lock blocks try_to_wake_up(), we don't need rq->lock when
+ * the task is blocked. Make sure to check @state since ttwu() can drop
+ * locks at the end, see ttwu_queue_wakelist().
+ */
+ if (state != TASK_RUNNING && state != TASK_WAKING && !p->on_rq) {
+ /*
+ * The task is blocked and we're holding off wakeupsr. For any
+ * of the other task states, see task_call_func().
+ */
+ ret = func(p, arg);
+ }
+
+ raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+ return ret;
+}
+
/**
* wake_up_process - Wake up a specific process
* @p: The process to be woken up.
--- a/kernel/stacktrace.c
+++ b/kernel/stacktrace.c
@@ -123,6 +123,13 @@ unsigned int stack_trace_save(unsigned l
}
EXPORT_SYMBOL_GPL(stack_trace_save);
+static int try_arch_stack_walk_tsk(struct task_struct *tsk, void *arg)
+{
+ stack_trace_consume_fn consume_entry = stack_trace_consume_entry_nosched;
+ arch_stack_walk(consume_entry, arg, tsk, NULL);
+ return 0;
+}
+
/**
* stack_trace_save_tsk - Save a task stack trace into a storage array
* @task: The task to examine
@@ -135,7 +142,6 @@ EXPORT_SYMBOL_GPL(stack_trace_save);
unsigned int stack_trace_save_tsk(struct task_struct *tsk, unsigned long *store,
unsigned int size, unsigned int skipnr)
{
- stack_trace_consume_fn consume_entry = stack_trace_consume_entry_nosched;
struct stacktrace_cookie c = {
.store = store,
.size = size,
@@ -143,11 +149,8 @@ unsigned int stack_trace_save_tsk(struct
.skip = skipnr + (current == tsk),
};
- if (!try_get_task_stack(tsk))
- return 0;
+ task_try_func(tsk, try_arch_stack_walk_tsk, &c);
- arch_stack_walk(consume_entry, &c, tsk, NULL);
- put_task_stack(tsk);
return c.len;
}
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