LKML Archive on lore.kernel.org
 help / color / Atom feed
From: Prateek Sood <prsood@codeaurora.org>
To: mingo@kernel.org, longman@redhat.com, peterz@infradead.org,
	parri.andrea@gmail.com, dave@stgolabs.net
Cc: linux-kernel@vger.kernel.org, sramana@codeaurora.org
Subject: Re: [PATCH] rwsem: fix missed wakeup due to reordering of load
Date: Wed, 27 Sep 2017 00:07:38 +0530
Message-ID: <a2d18a0a-6bf8-689e-38d8-65d69eb44ffe@codeaurora.org> (raw)
In-Reply-To: <1504794658-15397-1-git-send-email-prsood@codeaurora.org>

On 09/07/2017 08:00 PM, Prateek Sood wrote:
> If a spinner is present, there is a chance that the load of
> rwsem_has_spinner() in rwsem_wake() can be reordered with
> respect to decrement of rwsem count in __up_write() leading
> to wakeup being missed.
> 
>  spinning writer                  up_write caller
>  ---------------                  -----------------------
>  [S] osq_unlock()                 [L] osq
>   spin_lock(wait_lock)
>   sem->count=0xFFFFFFFF00000001
>             +0xFFFFFFFF00000000
>   count=sem->count
>   MB
>                                    sem->count=0xFFFFFFFE00000001
>                                              -0xFFFFFFFF00000001
>                                    spin_trylock(wait_lock)
>                                    return
>  rwsem_try_write_lock(count)
>  spin_unlock(wait_lock)
>  schedule()
> 
> Reordering of atomic_long_sub_return_release() in __up_write()
> and rwsem_has_spinner() in rwsem_wake() can cause missing of
> wakeup in up_write() context. In spinning writer, sem->count
> and local variable count is 0XFFFFFFFE00000001. It would result
> in rwsem_try_write_lock() failing to acquire rwsem and spinning
> writer going to sleep in rwsem_down_write_failed().
> 
> The smp_rmb() will make sure that the spinner state is
> consulted after sem->count is updated in up_write context.
> 
> Signed-off-by: Prateek Sood <prsood@codeaurora.org>
> ---
>  kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 02f6606..1fefe6d 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -613,6 +613,33 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
>  	DEFINE_WAKE_Q(wake_q);
>  
>  	/*
> +	* __rwsem_down_write_failed_common(sem)
> +	*   rwsem_optimistic_spin(sem)
> +	*     osq_unlock(sem->osq)
> +	*   ...
> +	*   atomic_long_add_return(&sem->count)
> +	*
> +	*      - VS -
> +	*
> +	*              __up_write()
> +	*                if (atomic_long_sub_return_release(&sem->count) < 0)
> +	*                  rwsem_wake(sem)
> +	*                    osq_is_locked(&sem->osq)
> +	*
> +	* And __up_write() must observe !osq_is_locked() when it observes the
> +	* atomic_long_add_return() in order to not miss a wakeup.
> +	*
> +	* This boils down to:
> +	*
> +	* [S.rel] X = 1                [RmW] r0 = (Y += 0)
> +	*         MB                         RMB
> +	* [RmW]   Y += 1               [L]   r1 = X
> +	*
> +	* exists (r0=1 /\ r1=0)
> +	*/
> +	smp_rmb();
> +
> +	/*
>  	 * If a spinner is present, it is not necessary to do the wakeup.
>  	 * Try to do wakeup only if the trylock succeeds to minimize
>  	 * spinlock contention which may introduce too much delay in the
> 

Hi Folks,

Do you have any more suggestion/feedback on this patch.


Regards
Prateek

-- 
Qualcomm India Private Limited, on behalf of Qualcomm Innovation
Center, Inc., is a member of Code Aurora Forum, a Linux Foundation
Collaborative Project

  parent reply index

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-09-07 14:30 Prateek Sood
2017-09-19 14:05 ` Andrea Parri
2017-09-20 14:52 ` Davidlohr Bueso
2017-09-20 21:17   ` Andrea Parri
2017-09-27 21:20     ` Davidlohr Bueso
2017-09-26 18:37 ` Prateek Sood [this message]
2017-09-29  9:30 ` [tip:locking/urgent] locking/rwsem-xadd: Fix " tip-bot for Prateek Sood
  -- strict thread matches above, loose matches on Subject: below --
2017-08-23 11:28 [PATCH] rwsem: fix " Prateek Sood
2017-08-24 11:29 ` Peter Zijlstra
2017-08-24 12:33   ` Peter Zijlstra
2017-08-24 12:52     ` Peter Zijlstra
2017-09-07 14:08       ` Prateek Sood
2017-07-26 20:17 Prateek Sood
2017-07-27 15:48 ` Waiman Long
2017-07-27 16:59   ` Peter Zijlstra
2017-08-10  8:32   ` Andrea Parri
2017-08-10 10:41     ` Peter Zijlstra
2017-08-10 10:44 ` Peter Zijlstra

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=a2d18a0a-6bf8-689e-38d8-65d69eb44ffe@codeaurora.org \
    --to=prsood@codeaurora.org \
    --cc=dave@stgolabs.net \
    --cc=linux-kernel@vger.kernel.org \
    --cc=longman@redhat.com \
    --cc=mingo@kernel.org \
    --cc=parri.andrea@gmail.com \
    --cc=peterz@infradead.org \
    --cc=sramana@codeaurora.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git