linux-kernel - Re: [PATCH -mm -next] ipc,sem: fix lockdep false positive

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1364308023.5053.40.camel@laptop>
Date:	Tue, 26 Mar 2013 15:27:03 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Rik van Riel <riel@...riel.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	torvalds@...ux-foundation.org, davidlohr.bueso@...com,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	hhuang@...hat.com, jason.low2@...com, lwoodman@...hat.com,
	chegu_vinod@...com, Dave Jones <davej@...hat.com>,
	benisty.e@...il.com, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH -mm -next] ipc,sem: fix lockdep false positive

On Tue, 2013-03-26 at 06:40 -0700, Michel Lespinasse wrote:

> sem_nsems is user provided as the array size in some semget system
> call. It's the size of an ipc semaphore array.

So we're basically adding a random (big) number to preempt_count
(obviously while preemption is disabled), seems rather costly and
undesirable.

> complex semop operations take the array's lock plus every semaphore
> locks; simple semop operations (operating on a single semaphore) only
> take that one semaphore's lock.

Right, standard global/local lock like stuff. Is there a way we can add
a r/o test to the 'local' lock operation and avoid doing the above?

Maybe something like:

void sma_lock(struct sem_array *sma) /* global */
{
	int i;

	sma->global_locked = 1;
	smp_wmb(); /* can we merge with the LOCK ? */
	spin_lock(&sma->global_lock);

	/* wait for all local locks to go away */
	for (i = 0; i < sma->sem_nsems; i++)
		spin_unlock_wait(&sem->sem_base[i]->lock);	
}

void sma_lock_one(struct sem_array *sma, int nr) /* local */
{
	smp_rmb(); /* pairs with wmb in sma_lock() */
	if (unlikely(sma->global_locked)) { /* wait for global lock */
		while (sma->global_locked)
			spin_unlock_wait(&sma->global_lock);
	}
	spin_lock(&sma->sem_base[nr]->lock);
}

This still has the problem of a non-preemptible section of O(sem_nsems) 
(with the avg wait-time on the local lock). Could we make the global 
lock a sleeping lock?



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/