linux-kernel - Re: [PATCH v4 1/2] locking/qrwlock: Better optimization for interrupt context readers

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Date:	Thu, 18 Jun 2015 13:41:10 +0100
From:	Will Deacon <will.deacon@....com>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v4 1/2] locking/qrwlock: Better optimization for
 interrupt context readers

On Thu, Jun 18, 2015 at 03:20:22AM +0100, Waiman Long wrote:
> The qrwlock is fair in the process context, but becoming unfair when
> in the interrupt context to support use cases like the tasklist_lock.
> 
> The current code isn't that well-documented on what happens when
> in the interrupt context. The rspin_until_writer_unlock() will only
> spin if the writer has gotten the lock. If the writer is still in the
> waiting state, the increment in the reader count will cause the writer
> to remain in the waiting state and the new interrupt context reader
> will get the lock and return immediately. The current code, however,
> do an additional read of the lock value which is not necessary as
> the information have already been there in the fast path. This may
> sometime cause an additional cacheline transfer when the lock is
> highly contended.
> 
> This patch passes the lock value information gotten in the fast path
> to the slow path to eliminate the additional read. It also document
> the action for the interrupt context readers more clearly.
> 
> Signed-off-by: Waiman Long <Waiman.Long@...com>
> ---
>  include/asm-generic/qrwlock.h |    4 ++--
>  kernel/locking/qrwlock.c      |   13 +++++++------
>  2 files changed, 9 insertions(+), 8 deletions(-)

LGTM:

  Reviewed-by: Will Deacon <will.deacon@....com>

Will
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