linux-kernel - Re: [PATCH RFC 2/3] locking/rwsem: clear reader-owner on unlock to reduce false positives

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Message-Id: <20250624092620.3346ac39e882434aafb0b93d@kernel.org>
Date: Tue, 24 Jun 2025 09:26:20 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, zi.li@...ux.dev, anna.schumaker@...cle.com,
 boqun.feng@...il.com, joel.granados@...nel.org, jstultz@...gle.com,
 kent.overstreet@...ux.dev, leonylgao@...cent.com,
 linux-kernel@...r.kernel.org, longman@...hat.com, mhiramat@...nel.org,
 mingo@...hat.com, mingzhe.yang@...com, peterz@...radead.org,
 rostedt@...dmis.org, senozhatsky@...omium.org, tfiga@...omium.org,
 will@...nel.org, Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH RFC 2/3] locking/rwsem: clear reader-owner on unlock to
 reduce false positives

On Thu, 12 Jun 2025 12:19:25 +0800
Lance Yang <ioworker0@...il.com> wrote:

> From: Lance Yang <lance.yang@...ux.dev>
> 
> When CONFIG_DETECT_HUNG_TASK_BLOCKER is enabled, a stale owner pointer in a
> reader-owned rwsem can lead to false positives in blocker tracking.
> 
> To mitigate this, let’s try to clear the owner field on unlock, as a NULL
> owner is better than a stale one for diagnostics.

Can we merge this to [PATCH 1/3]? It seems that you removed #ifdef and
remove it. This means in anyway we need the feature enabled by DEBUG_RWSEMS.

Thanks,

> 
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---
>  kernel/locking/rwsem.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 6cb29442d4fc..a310eb9896de 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -205,14 +205,12 @@ bool is_rwsem_reader_owned(struct rw_semaphore *sem)
>  		return false;
>  	return rwsem_test_oflags(sem, RWSEM_READER_OWNED);
>  }
> -#endif
>  
> -#ifdef CONFIG_DEBUG_RWSEMS
>  /*
> - * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
> - * is a task pointer in owner of a reader-owned rwsem, it will be the
> - * real owner or one of the real owners. The only exception is when the
> - * unlock is done by up_read_non_owner().
> + * With CONFIG_DEBUG_RWSEMS or CONFIG_DETECT_HUNG_TASK_BLOCKER configured,
> + * it will make sure that the owner field of a reader-owned rwsem either
> + * points to a real reader-owner(s) or gets cleared. The only exception is
> + * when the unlock is done by up_read_non_owner().
>   */
>  static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
>  {
> -- 
> 2.49.0
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>