sched/debug: Fix cgroup_path[] serialization

[ Upstream commit ad789f84c9a145f8a18744c0387cec22ec51651e ]

The handling of sysrq key can be activated by echoing the key to
/proc/sysrq-trigger or via the magic key sequence typed into a terminal
that is connected to the system in some way (serial, USB or other mean).
In the former case, the handling is done in a user context. In the
latter case, it is likely to be in an interrupt context.

Currently in print_cpu() of kernel/sched/debug.c, sched_debug_lock is
taken with interrupt disabled for the whole duration of the calls to
print_*_stats() and print_rq() which could last for the quite some time
if the information dump happens on the serial console.

If the system has many cpus and the sched_debug_lock is somehow busy
(e.g. parallel sysrq-t), the system may hit a hard lockup panic
depending on the actually serial console implementation of the
system.

The purpose of sched_debug_lock is to serialize the use of the global
cgroup_path[] buffer in print_cpu(). The rests of the printk calls don't
need serialization from sched_debug_lock.

Calling printk() with interrupt disabled can still be problematic if
multiple instances are running. Allocating a stack buffer of PATH_MAX
bytes is not feasible because of the limited size of the kernel stack.

The solution implemented in this patch is to allow only one caller at a
time to use the full size group_path[], while other simultaneous callers
will have to use shorter stack buffers with the possibility of path
name truncation. A "..." suffix will be printed if truncation may have
happened.  The cgroup path name is provided for informational purpose
only, so occasional path name truncation should not be a big problem.

Fixes: efe25c2c7b3a ("sched: Reinstate group names in /proc/sched_debug")
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210415195426.6677-1-longman@redhat.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 2357921..6264584 100644 (file)
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -8,8 +8,6 @@
  */
 #include "sched.h"
 
-static DEFINE_SPINLOCK(sched_debug_lock);
-
 /*
  * This allows printing both to /proc/sched_debug and
  * to the console
@@ -470,16 +468,37 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group
 #endif
 
 #ifdef CONFIG_CGROUP_SCHED
+static DEFINE_SPINLOCK(sched_debug_lock);
 static char group_path[PATH_MAX];
 
-static char *task_group_path(struct task_group *tg)
+static void task_group_path(struct task_group *tg, char *path, int plen)
 {
-       if (autogroup_path(tg, group_path, PATH_MAX))
-               return group_path;
+       if (autogroup_path(tg, path, plen))
+               return;
 
-       cgroup_path(tg->css.cgroup, group_path, PATH_MAX);
+       cgroup_path(tg->css.cgroup, path, plen);
+}
 
-       return group_path;
+/*
+ * Only 1 SEQ_printf_task_group_path() caller can use the full length
+ * group_path[] for cgroup path. Other simultaneous callers will have
+ * to use a shorter stack buffer. A "..." suffix is appended at the end
+ * of the stack buffer so that it will show up in case the output length
+ * matches the given buffer size to indicate possible path name truncation.
+ */
+#define SEQ_printf_task_group_path(m, tg, fmt...)                      \
+{                                                                      \
+       if (spin_trylock(&sched_debug_lock)) {                          \
+               task_group_path(tg, group_path, sizeof(group_path));    \
+               SEQ_printf(m, fmt, group_path);                         \
+               spin_unlock(&sched_debug_lock);                         \
+       } else {                                                        \
+               char buf[128];                                          \
+               char *bufend = buf + sizeof(buf) - 3;                   \
+               task_group_path(tg, buf, bufend - buf);                 \
+               strcpy(bufend - 1, "...");                              \
+               SEQ_printf(m, fmt, buf);                                \
+       }                                                               \
 }
 #endif
 
@@ -506,7 +525,7 @@ print_task(struct seq_file *m, struct rq *rq, struct task_struct *p)
        SEQ_printf(m, " %d %d", task_node(p), task_numa_group_id(p));
 #endif
 #ifdef CONFIG_CGROUP_SCHED
-       SEQ_printf(m, " %s", task_group_path(task_group(p)));
+       SEQ_printf_task_group_path(m, task_group(p), " %s")
 #endif
 
        SEQ_printf(m, "\n");
@@ -543,7 +562,7 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
        SEQ_printf(m, "\n");
-       SEQ_printf(m, "cfs_rq[%d]:%s\n", cpu, task_group_path(cfs_rq->tg));
+       SEQ_printf_task_group_path(m, cfs_rq->tg, "cfs_rq[%d]:%s\n", cpu);
 #else
        SEQ_printf(m, "\n");
        SEQ_printf(m, "cfs_rq[%d]:\n", cpu);
@@ -614,7 +633,7 @@ void print_rt_rq(struct seq_file *m, int cpu, struct rt_rq *rt_rq)
 {
 #ifdef CONFIG_RT_GROUP_SCHED
        SEQ_printf(m, "\n");
-       SEQ_printf(m, "rt_rq[%d]:%s\n", cpu, task_group_path(rt_rq->tg));
+       SEQ_printf_task_group_path(m, rt_rq->tg, "rt_rq[%d]:%s\n", cpu);
 #else
        SEQ_printf(m, "\n");
        SEQ_printf(m, "rt_rq[%d]:\n", cpu);
@@ -666,7 +685,6 @@ void print_dl_rq(struct seq_file *m, int cpu, struct dl_rq *dl_rq)
 static void print_cpu(struct seq_file *m, int cpu)
 {
        struct rq *rq = cpu_rq(cpu);
-       unsigned long flags;
 
 #ifdef CONFIG_X86
        {
@@ -717,13 +735,11 @@ do {                                                                      \
        }
 #undef P
 
-       spin_lock_irqsave(&sched_debug_lock, flags);
        print_cfs_stats(m, cpu);
        print_rt_stats(m, cpu);
        print_dl_stats(m, cpu);
 
        print_rq(m, rq, cpu);
-       spin_unlock_irqrestore(&sched_debug_lock, flags);
        SEQ_printf(m, "\n");
 }