linux-kernel - qrwlock && read-after-read

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Date:	Tue, 4 Aug 2015 15:00:53 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <Waiman.Long@...com>
Cc:	linux-kernel@...r.kernel.org
Subject: qrwlock && read-after-read

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Unless I am totally confused read-after-read is no longer valid,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?

Oleg.

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