linux-kernel - Re: qrwlock && read-after-read

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: Windows password security audit tool. GUI, reports in PDF.

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

Message-ID: <20150804131036.GQ25159@twins.programming.kicks-ass.net>
Date:	Tue, 4 Aug 2015 15:10:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <Waiman.Long@...com>, linux-kernel@...r.kernel.org
Subject: Re: qrwlock && read-after-read

On Tue, Aug 04, 2015 at 03:00:53PM +0200, Oleg Nesterov wrote:
> I am working on the (off-topic) bug report which motivated me to
> look at locking/qrwlock.c and it seems to me there is a problem
> with the queued rwlocks.
> 
> Unless I am totally confused read-after-read is no longer valid,
> write_lock() stops the new readers. And lockdep doesn't know this,
> read_lock()->rwlock_acquire_read() doesn't match the reality. The
> code doing
> 
> 	read_lock(X);
> 	read_lock(X);
> 
> can deadlock if another CPU does write_lock(X) in between. This
> was fine before rwlock_t was changed to use qrwlock.
> 
> A nested read_lock() in interrupt should be fine though, and this
> is because queue_read_lock_slowpath() "ignores" _QW_WAITING if
> in_interrupt().
> 
> This means that rwlock_t has the really strange semantics imho,
> and again, it is not lockdep-friendly.
> 
> What do you think we can/should do? Or did I misread this code?

Fix lockdep, although that's non trivial from what I remember.

These (new) semantics were very much on purpose and suggested by Linus
IIRC.
--
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/