linux-kernel - Re: [PATCH] rwlock_t unfairness and tasklist

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Message-ID: <20130112174607.GB22338@redhat.com>
Date:	Sat, 12 Jan 2013 18:46:07 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	David Howells <dhowells@...hat.com>,
	Salman Qazi <sqazi@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rwlock_t unfairness and tasklist_lock

On 01/11, Michel Lespinasse wrote:
>
> So I looked again at getpriority() since that's what I had used for my
> DOS test code, and it looks like everything there is already protected
> by RCU or smaller granularity locks and refcounts. Patch attached to
> remove this tasklist_lock usage.

And probably the change in getpriority() is fine, but ...

> @@ -189,7 +189,6 @@ SYSCALL_DEFINE3(setpriority, int, which, int, who, int, niceval)
>  		niceval = 19;
>
>  	rcu_read_lock();
> -	read_lock(&tasklist_lock);
>  	switch (which) {
>  		case PRIO_PROCESS:
>  			if (who)
> @@ -226,7 +225,6 @@ SYSCALL_DEFINE3(setpriority, int, which, int, who, int, niceval)
>  			break;
>  	}
>  out_unlock:
> -	read_unlock(&tasklist_lock);

you also changed setpriority(), this should be documented at least ;)

OK. Even without this change, say, sys_setpriority(PRIO_PGRP) can obviously
race with fork(), so this change probably is not bad.

Oleg.

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