On Fri, Feb 26, 2016 at 01:47:39PM -0800, Yang Shi wrote:
> commit 5634cc2aa9aebc77bc862992e7805469dcf83dac ("writeback: update writeback
> tracepoints to report cgroup") made writeback tracepoints report cgroup
> writeback, but it may trigger the below bug on -rt kernel since kernfs_path
> and kernfs_path_len are called by tracepoints, which acquire sleeping lock.
>
> BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:930
> in_atomic(): 1, irqs_disabled(): 0, pid: 625, name: kworker/u16:3
> INFO: lockdep is turned off.
> Preemption disabled at:[<ffffffc000374a5c>] wb_writeback+0xec/0x830
>
> CPU: 7 PID: 625 Comm: kworker/u16:3 Not tainted 4.4.1-rt5 #20
> Hardware name: Freescale Layerscape 2085a RDB Board (DT)
> Workqueue: writeback wb_workfn (flush-7:0)
> Call trace:
> [<ffffffc00008d708>] dump_backtrace+0x0/0x200
> [<ffffffc00008d92c>] show_stack+0x24/0x30
> [<ffffffc0007b0f40>] dump_stack+0x88/0xa8
> [<ffffffc000127d74>] ___might_sleep+0x2ec/0x300
> [<ffffffc000d5d550>] rt_spin_lock+0x38/0xb8
> [<ffffffc0003e0548>] kernfs_path_len+0x30/0x90
> [<ffffffc00036b360>] trace_event_raw_event_writeback_work_class+0xe8/0x2e8
> [<ffffffc000374f90>] wb_writeback+0x620/0x830
> [<ffffffc000376224>] wb_workfn+0x61c/0x950
> [<ffffffc000110adc>] process_one_work+0x3ac/0xb30
> [<ffffffc0001112fc>] worker_thread+0x9c/0x7a8
> [<ffffffc00011a9e8>] kthread+0x190/0x1b0
> [<ffffffc000086ca0>] ret_from_fork+0x10/0x30
>
> Since kernfs_* functions are heavily used by cgroup, so it sounds not
> reasonable to convert kernfs_rename_lock to raw lock.
>
> Call synchronize_sched() when kernfs_node is updated since tracepoints are
> protected by rcu_read_lock_sched.
>
> Create raw version kernfs_path, kernfs_path_len and cgroup_path, which don't
> acquire lock and are used by tracepoints only.
>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxx>
> ---
> It should be applicable to both mainline and -rt kernel.
> The change survives ftrace stress test in ltp.
>
> v2:
> * Adopted Steven's suggestion to call synchronize_sched in kernfs_rename_ns.
>
> fs/kernfs/dir.c | 30 +++++++++++++++++++++++++++---
> include/linux/cgroup.h | 10 ++++++++++
> include/linux/kernfs.h | 10 ++++++++++
> include/trace/events/writeback.h | 4 ++--
> 4 files changed, 49 insertions(+), 5 deletions(-)
>
> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> index 996b774..70a9687 100644
> --- a/fs/kernfs/dir.c
> +++ b/fs/kernfs/dir.c
> @@ -44,7 +44,7 @@ static int kernfs_name_locked(struct kernfs_node *kn, char *buf, size_t buflen)
> return strlcpy(buf, kn->parent ? kn->name : "/", buflen);
> }
>
> -static char * __must_check kernfs_path_locked(struct kernfs_node *kn, char *buf,
> +static char * __must_check __kernfs_path(struct kernfs_node *kn, char *buf,
> size_t buflen)
> {
> char *p = buf + buflen;
> @@ -131,12 +131,33 @@ char *kernfs_path(struct kernfs_node *kn, char *buf, size_t buflen)
> char *p;
>
> spin_lock_irqsave(&kernfs_rename_lock, flags);
> - p = kernfs_path_locked(kn, buf, buflen);
> + p = __kernfs_path(kn, buf, buflen);
> spin_unlock_irqrestore(&kernfs_rename_lock, flags);
> return p;
> }
> EXPORT_SYMBOL_GPL(kernfs_path);
>
> +/* Raw version. Used by tracepoints only without acquiring lock */
> +size_t _kernfs_path_len(struct kernfs_node *kn)
> +{
> + size_t len = 0;
> +
> + do {
> + len += strlen(kn->name) + 1;
> + kn = kn->parent;
> + } while (kn && kn->parent);
> +
> + return len;
> +}
> +
> +char *_kernfs_path(struct kernfs_node *kn, char *buf, size_t buflen)
> +{
> + char *p;
> +
> + p = __kernfs_path(kn, buf, buflen);
> + return p;
> +}
> +
> /**
> * pr_cont_kernfs_name - pr_cont name of a kernfs_node
> * @kn: kernfs_node of interest
> @@ -168,7 +189,7 @@ void pr_cont_kernfs_path(struct kernfs_node *kn)
>
> spin_lock_irqsave(&kernfs_rename_lock, flags);
>
> - p = kernfs_path_locked(kn, kernfs_pr_cont_buf,
> + p = __kernfs_path(kn, kernfs_pr_cont_buf,
> sizeof(kernfs_pr_cont_buf));
> if (p)
> pr_cont("%s", p);
> @@ -1397,6 +1418,9 @@ int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,
> kn->hash = kernfs_name_hash(kn->name, kn->ns);
> kernfs_link_sibling(kn);
>
> + /* Tracepoints are protected by rcu_read_lock_sched */
> + synchronize_sched();
> +
> kernfs_put(old_parent);
> kfree_const(old_name);
>
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 2162dca..bbd88d3 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -538,6 +538,16 @@ static inline char * __must_check cgroup_path(struct cgroup *cgrp, char *buf,
> return kernfs_path(cgrp->kn, buf, buflen);
> }
>
> +/*
> + * Without acquiring kernfs_rename_lock in _kernfs_path.
> + * Used by tracepoints only.
> + */
> +static inline char * __must_check _cgroup_path(struct cgroup *cgrp, char *buf,
> + size_t buflen)
> +{
> + return _kernfs_path(cgrp->kn, buf, buflen);
> +}
> +
> static inline void pr_cont_cgroup_name(struct cgroup *cgrp)
> {
> pr_cont_kernfs_name(cgrp->kn);
> diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
> index af51df3..14c01cdc 100644
> --- a/include/linux/kernfs.h
> +++ b/include/linux/kernfs.h
> @@ -267,8 +267,11 @@ static inline bool kernfs_ns_enabled(struct kernfs_node *kn)
>
> int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen);
> size_t kernfs_path_len(struct kernfs_node *kn);
> +size_t _kernfs_path_len(struct kernfs_node *kn);
> char * __must_check kernfs_path(struct kernfs_node *kn, char *buf,
> size_t buflen);
> +char * __must_check _kernfs_path(struct kernfs_node *kn, char *buf,
> + size_t buflen);
I despise functions with just a _ in the front of them, you don't give
anyone a clue as to what they are for, or why to use one vs. the other.
Trust me, you will not remember it either in 1 month, let alone 5 years.
Please change the whole function name to be obvious as to what to use,
or not use.
thanks,
greg k-h
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