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

From: Will Deacon
Date: Thu Jun 18 2015 - 08:41:24 EST

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@xxxxxx>
> ---
> include/asm-generic/qrwlock.h | 4 ++--
> kernel/locking/qrwlock.c | 13 +++++++------
> 2 files changed, 9 insertions(+), 8 deletions(-)


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

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