Subject: Re: [PATCH] rwsem-spinlock: Fix EINTR branch in __down_write_common()
To: Kirill Tkhai <ktkhai@virtuozzo.com>, <peterz@infradead.org>,
        <linux-kernel@vger.kernel.org>, <mingo@kernel.org>
References: <149762063282.19811.9129615532201147826.stgit@localhost.localdomain>
From: Niklas Cassel <niklas.cassel@axis.com>
Message-ID: <76aba539-a6c7-66e8-2088-d0f5938535dc@axis.com>
Date: Wed, 28 Jun 2017 15:20:20 +0200
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Good catch!

This should go to -stable as well.


Perhaps

if (!list_empty(&sem->wait_list) && sem->count > 0)
	__rwsem_do_wake(sem, 0);

Rather than

if (!list_empty(&sem->wait_list) && sem->count >= 0)
	__rwsem_do_wake(sem, 0);

Since we have the spinlock, and since we just checked
if sem->count == 0, we still know that it can't be 0.

Either way:

Acked-by: Niklas Cassel <niklas.cassel@axis.com>

On 06/16/2017 03:44 PM, Kirill Tkhai wrote:
> If a writer could been woken up, the above branch
> 
> 	if (sem->count == 0)
> 		break;
> 
> would have moved us to taking the sem. So, it's
> not the time to wake a writer now, and only readers
> are allowed now. Thus, 0 must be passed to __rwsem_do_wake().
> 
> Next, __rwsem_do_wake() wakes readers unconditionally.
> But we mustn't do that if the sem is owned by writer
> in the moment. Otherwise, writer and reader own the sem
> the same time, which leads to memory corruption in
> callers.
> 
> rwsem-xadd.c does not need that, as:
> 1)the similar check is made lockless there,
> 2)in __rwsem_mark_wake::try_reader_grant we test,
> that sem is not owned by writer.
> 
> Fixes: 17fcbd590d0c "locking/rwsem: Fix down_write_killable() for CONFIG_RWSEM_GENERIC_SPINLOCK=y"
> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> CC: Niklas Cassel <niklas.cassel@axis.com>
> CC: Peter Zijlstra (Intel) <peterz@infradead.org>
> CC: Ingo Molnar <mingo@kernel.org>
> ---
>  kernel/locking/rwsem-spinlock.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/rwsem-spinlock.c b/kernel/locking/rwsem-spinlock.c
> index c65f7989f850..20819df98125 100644
> --- a/kernel/locking/rwsem-spinlock.c
> +++ b/kernel/locking/rwsem-spinlock.c
> @@ -231,8 +231,8 @@ int __sched __down_write_common(struct rw_semaphore *sem, int state)
>  
>  out_nolock:
>  	list_del(&waiter.list);
> -	if (!list_empty(&sem->wait_list))
> -		__rwsem_do_wake(sem, 1);
> +	if (!list_empty(&sem->wait_list) && sem->count >= 0)
> +		__rwsem_do_wake(sem, 0);
>  	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
>  
>  	return -EINTR;
>