linux-kernel - Re: [PATCH -next] locking/percpu-rwsem: fix a task

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Date:   Fri, 27 Mar 2020 10:37:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qian Cai <cai@....pw>
Cc:     mingo@...hat.com, will@...nel.org, dbueso@...e.de,
        juri.lelli@...hat.com, longman@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] locking/percpu-rwsem: fix a task_struct refcount

On Thu, Mar 26, 2020 at 11:10:57PM -0400, Qian Cai wrote:
> There are some memory leaks due to a missing put_task_struct().

This is an absolutely inadequate changelog. There is no explaning what
the actual race is and why this patch is correct.

> Fixes: 7f26482a872c ("locking/percpu-rwsem: Remove the embedded rwsem")
> Signed-off-by: Qian Cai <cai@....pw>
> ---
>  kernel/locking/percpu-rwsem.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
> index a008a1ba21a7..6f487e5d923f 100644
> --- a/kernel/locking/percpu-rwsem.c
> +++ b/kernel/locking/percpu-rwsem.c
> @@ -123,8 +123,10 @@ static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
>  	struct percpu_rw_semaphore *sem = key;
>  
>  	/* concurrent against percpu_down_write(), can get stolen */
> -	if (!__percpu_rwsem_trylock(sem, reader))
> +	if (!__percpu_rwsem_trylock(sem, reader)) {
> +		put_task_struct(p);
>  		return 1;
> +	}


If the trylock fails, someone else got the lock and we remain on the
waitqueue. It seems like a very bad idea to put the task while it
remains on the waitqueue, no?

>  
>  	list_del_init(&wq_entry->entry);
>  	smp_store_release(&wq_entry->private, NULL);
> -- 
> 2.21.0 (Apple Git-122.2)
>