linux-kernel - Re: [PATCH 0/3] v4 Improve task->comm locking situation

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Message-Id: <201105180316.p4I3G6f2094560@www262.sakura.ne.jp>
Date:	Wed, 18 May 2011 12:16:06 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	john.stultz@...aro.org
Cc:	tytso@....edu, mina86@...a86.com, jirislaby@...il.com,
	kosaki.motohiro@...fujitsu.com, rientjes@...gle.com,
	dave@...ux.vne, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] v4 Improve task->comm locking situation

What about replacing
	char comm[TASK_COMM_LEN];
with
	char *rcu_commname;
and switching it atomically using RCU?

Advantages:
  Readers can use RCU read lock rather than spinlock.
  Makes task_struct smaller.

Disadvantages:
  Need to allocate TASK_COMM_LEN bytes upon dup_task_struct() and set_task_comm().
  Need to rewrite all task_struct.comm readers (steps shown below).

Steps to rewrite task_struct.comm readers

(1) Introduce a temporary accessor (say, task_comm).

    #define task_comm(tsk) (tsk)->comm

(2) Rewrite all tsk->comm users to task_comm(tsk).

(3) Replace sizeof(tsk->comm) with TASK_COMM_LEN.

(4) Temporarily rename from

    char comm[TASK_COMM_LEN];

    to

    char comm_access_me_via_task_comm[TASK_COMM_LEN];

    for blocking new tsk->comm users.

(5) Convert to use RCU.

(6) Rename from

    char comm_access_me_via_task_comm[TASK_COMM_LEN];

    to

    char *rcu_commname;

    .

(7) Rewrite task_comm(tsk) to use %ptc .

(8) Remove the temporary accessor.
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