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From: Stephen Brennan <stephen.s.brennan@oracle.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>,
	James Morris <jmorris@namei.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	linux-security-module@vger.kernel.org,
	Paul Moore <paul@paul-moore.com>,
	Stephen Smalley <stephen.smalley.work@gmail.com>,
	Eric Paris <eparis@parisplace.org>,
	selinux@vger.kernel.org, Casey Schaufler <casey@schaufler-ca.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU
Date: Mon, 14 Dec 2020 09:19:02 -0800	[thread overview]
Message-ID: <87sg88tiex.fsf@stepbren-lnx.us.oracle.com> (raw)
In-Reply-To: <87tusplqwf.fsf@x220.int.ebiederm.org>

ebiederm@xmission.com (Eric W. Biederman) writes:

> Stephen Brennan <stephen.s.brennan@oracle.com> writes:
>
>> The pid_revalidate() function requires dropping from RCU into REF lookup
>> mode. When many threads are resolving paths within /proc in parallel,
>> this can result in heavy spinlock contention as each thread tries to
>> grab a reference to the /proc dentry lock (and drop it shortly
>> thereafter).
>
> I am feeling dense at the moment.  Which lock specifically are you
> referring to?  The only locks I can thinking of are sleeping locks,
> not spinlocks.

The lock in question is the d_lockref field (aliased as d_lock) of
struct dentry. It is contended in this code path while processing the
"/proc" dentry, switching from RCU to REF mode.

    walk_component()
      lookup_fast()
        d_revalidate()
          pid_revalidate() // returns -ECHILD
        unlazy_child()
          lockref_get_not_dead(&nd->path.dentry->d_lockref)

>
>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>> index ebea9501afb8..833d55a59e20 100644
>> --- a/fs/proc/base.c
>> +++ b/fs/proc/base.c
>> @@ -1830,19 +1846,22 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
>>  {
>>  	struct inode *inode;
>>  	struct task_struct *task;
>> +	int rv = 0;
>>  
>> -	if (flags & LOOKUP_RCU)
>> -		return -ECHILD;
>> -
>> -	inode = d_inode(dentry);
>> -	task = get_proc_task(inode);
>> -
>> -	if (task) {
>> -		pid_update_inode(task, inode);
>> -		put_task_struct(task);
>> -		return 1;
>> +	if (flags & LOOKUP_RCU) {
>
> Why do we need to test flags here at all?
> Why can't the code simply take an rcu_read_lock unconditionally and just
> pass flags into do_pid_update_inode?
>

I don't have any good reason. If it is safe to update the inode without
holding a reference to the task struct (or holding any other lock) then
I can consolidate the whole conditional.

>
>> +		inode = d_inode_rcu(dentry);
>> +		task = pid_task(proc_pid(inode), PIDTYPE_PID);
>> +		if (task)
>> +			rv = do_pid_update_inode(task, inode, flags);
>> +	} else {
>> +		inode = d_inode(dentry);
>> +		task = get_proc_task(inode);
>> +		if (task) {
>> +			rv = do_pid_update_inode(task, inode, flags);
>> +			put_task_struct(task);
>> +		}
>
>>  	}
>> -	return 0;
>> +	return rv;
>>  }
>>  
>>  static inline bool proc_inode_is_dead(struct inode *inode)
>
> Eric

  parent reply	other threads:[~2020-12-14 17:20 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-04  0:02 [PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU Stephen Brennan
2020-12-12 20:55 ` Matthew Wilcox
2020-12-13 14:22   ` Eric W. Biederman
2020-12-13 16:29     ` Matthew Wilcox
2020-12-13 23:00       ` Paul Moore
2020-12-15 18:09         ` Casey Schaufler
2020-12-15 22:04           ` Eric W. Biederman
2020-12-15 22:53             ` Casey Schaufler
2020-12-16  1:05               ` Stephen Brennan
2020-12-14 18:45       ` Casey Schaufler
2020-12-14 18:15     ` Stephen Brennan
2020-12-13 14:30 ` Eric W. Biederman
2020-12-13 16:32   ` Matthew Wilcox
2020-12-14 17:19   ` Stephen Brennan [this message]
2020-12-15 21:45     ` Eric W. Biederman

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