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Date: Thu, 14 Jul 2022 08:01:15 -0400
From: Brian Foster <bfoster@...hat.com>
To: Zhihao Cheng <chengzhihao1@...wei.com>
Cc: ebiederm@...ssion.com, willy@...radead.org, bhe@...hat.com,
akpm@...ux-foundation.org, kaleshsingh@...gle.com,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
yukuai3@...wei.com
Subject: Re: [PATCH v5] proc: Fix a dentry lock race between release_task and
lookup
On Wed, Jul 13, 2022 at 09:00:29PM +0800, Zhihao Cheng wrote:
> Commit 7bc3e6e55acf06 ("proc: Use a list of inodes to flush from proc")
> moved proc_flush_task() behind __exit_signal(). Then, process systemd
> can take long period high cpu usage during releasing task in following
> concurrent processes:
>
> systemd ps
> kernel_waitid stat(/proc/tgid)
> do_wait filename_lookup
> wait_consider_task lookup_fast
> release_task
> __exit_signal
> __unhash_process
> detach_pid
> __change_pid // remove task->pid_links
> d_revalidate -> pid_revalidate // 0
> d_invalidate(/proc/tgid)
> shrink_dcache_parent(/proc/tgid)
> d_walk(/proc/tgid)
> spin_lock_nested(/proc/tgid/fd)
> // iterating opened fd
> proc_flush_pid |
> d_invalidate (/proc/tgid/fd) |
> shrink_dcache_parent(/proc/tgid/fd) |
> shrink_dentry_list(subdirs) ↓
> shrink_lock_dentry(/proc/tgid/fd) --> race on dentry lock
>
> Function d_invalidate() will remove dentry from hash firstly, but why does
> proc_flush_pid() process dentry '/proc/tgid/fd' before dentry '/proc/tgid'?
> That's because proc_pid_make_inode() adds proc inode in reverse order by
> invoking hlist_add_head_rcu(). But proc should not add any inodes under
> '/proc/tgid' except '/proc/tgid/task/pid', fix it by adding inode into
> 'pid->inodes' only if the inode is /proc/tgid or /proc/tgid/task/pid.
>
> Performance regression:
> Create 200 tasks, each task open one file for 50,000 times. Kill all
> tasks when opened files exceed 10,000,000 (cat /proc/sys/fs/file-nr).
>
> Before fix:
> $ time killall -wq aa
> real 4m40.946s # During this period, we can see 'ps' and 'systemd'
> taking high cpu usage.
>
> After fix:
> $ time killall -wq aa
> real 1m20.732s # During this period, we can see 'systemd' taking
> high cpu usage.
>
> Fixes: 7bc3e6e55acf06 ("proc: Use a list of inodes to flush from proc")
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=216054
> Signed-off-by: Zhihao Cheng <chengzhihao1@...wei.com>
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
> Suggested-by: Brian Foster <bfoster@...hat.com>
> ---
LGTM, and thanks for the tweaks:
Reviewed-by: Brian Foster <bfoster@...hat.com>
> v1->v2: Add new helper proc_pid_make_base_inode that performs the extra
> work of adding to the pid->list.
> v2->v3: Add performance regression in commit message.
> v3->v4: Make proc_pid_make_base_inode() static
> v4->v5: Add notes to explain what proc_pid_make_base_inode() does
> fs/proc/base.c | 46 ++++++++++++++++++++++++++++++++++++++--------
> 1 file changed, 38 insertions(+), 8 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 8dfa36a99c74..93f7e3d971e4 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1885,7 +1885,7 @@ void proc_pid_evict_inode(struct proc_inode *ei)
> put_pid(pid);
> }
>
> -struct inode *proc_pid_make_inode(struct super_block * sb,
> +struct inode *proc_pid_make_inode(struct super_block *sb,
> struct task_struct *task, umode_t mode)
> {
> struct inode * inode;
> @@ -1914,11 +1914,6 @@ struct inode *proc_pid_make_inode(struct super_block * sb,
>
> /* Let the pid remember us for quick removal */
> ei->pid = pid;
> - if (S_ISDIR(mode)) {
> - spin_lock(&pid->lock);
> - hlist_add_head_rcu(&ei->sibling_inodes, &pid->inodes);
> - spin_unlock(&pid->lock);
> - }
>
> task_dump_owner(task, 0, &inode->i_uid, &inode->i_gid);
> security_task_to_inode(task, inode);
> @@ -1931,6 +1926,39 @@ struct inode *proc_pid_make_inode(struct super_block * sb,
> return NULL;
> }
>
> +/*
> + * Generating an inode and adding it into @pid->inodes, so that task will
> + * invalidate inode's dentry before being released.
> + *
> + * This helper is used for creating dir-type entries under '/proc' and
> + * '/proc/<tgid>/task'. Other entries(eg. fd, stat) under '/proc/<tgid>'
> + * can be released by invalidating '/proc/<tgid>' dentry.
> + * In theory, dentries under '/proc/<tgid>/task' can also be released by
> + * invalidating '/proc/<tgid>' dentry, we reserve it to handle single
> + * thread exiting situation: Any one of threads should invalidate its
> + * '/proc/<tgid>/task/<pid>' dentry before released.
> + */
> +static struct inode *proc_pid_make_base_inode(struct super_block *sb,
> + struct task_struct *task, umode_t mode)
> +{
> + struct inode *inode;
> + struct proc_inode *ei;
> + struct pid *pid;
> +
> + inode = proc_pid_make_inode(sb, task, mode);
> + if (!inode)
> + return NULL;
> +
> + /* Let proc_flush_pid find this directory inode */
> + ei = PROC_I(inode);
> + pid = ei->pid;
> + spin_lock(&pid->lock);
> + hlist_add_head_rcu(&ei->sibling_inodes, &pid->inodes);
> + spin_unlock(&pid->lock);
> +
> + return inode;
> +}
> +
> int pid_getattr(struct user_namespace *mnt_userns, const struct path *path,
> struct kstat *stat, u32 request_mask, unsigned int query_flags)
> {
> @@ -3369,7 +3397,8 @@ static struct dentry *proc_pid_instantiate(struct dentry * dentry,
> {
> struct inode *inode;
>
> - inode = proc_pid_make_inode(dentry->d_sb, task, S_IFDIR | S_IRUGO | S_IXUGO);
> + inode = proc_pid_make_base_inode(dentry->d_sb, task,
> + S_IFDIR | S_IRUGO | S_IXUGO);
> if (!inode)
> return ERR_PTR(-ENOENT);
>
> @@ -3671,7 +3700,8 @@ static struct dentry *proc_task_instantiate(struct dentry *dentry,
> struct task_struct *task, const void *ptr)
> {
> struct inode *inode;
> - inode = proc_pid_make_inode(dentry->d_sb, task, S_IFDIR | S_IRUGO | S_IXUGO);
> + inode = proc_pid_make_base_inode(dentry->d_sb, task,
> + S_IFDIR | S_IRUGO | S_IXUGO);
> if (!inode)
> return ERR_PTR(-ENOENT);
>
> --
> 2.31.1
>
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