From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-179.mta1.migadu.com (out-179.mta1.migadu.com [95.215.58.179]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B283530E84D for ; Tue, 24 Jun 2025 05:02:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.179 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750741372; cv=none; b=qK+Abxqj6B+TWtz4OdPGyqVSfOfQeQsbUtOPCfpYJmqOp6fh2xN+uahukrZ+L3cwmLa9Qggn7RiRKONY1U6H+YToCuuT3s/bzliPy+Fk5kOFipnrOVxXWIf18l5X2OBERxTBqBkAqefzb7hI7BKYNpfPyEQb0X4q0+WEU8h992c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750741372; c=relaxed/simple; bh=z7WvZIb7MTSzMY5RwquCP4zNBDdWoXgFStMYS17iWEQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=tl5lfZhRnW4/Z213RsBxOliJ0Ppa8jVoYQ4eQAvZNvsAgSJ0980/1y6lj8MXffhG8SHO77OGbpKAgF0nJe+SGrjpc0xKEK49oP/1vKntGWcI1XKRKNa9wRyBbGS0vFFlcnWvKIiz/9ER33RfPjehwOmGQoLQAlLkaLbd3z2uzZg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=IjYuLYak; arc=none smtp.client-ip=95.215.58.179 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="IjYuLYak" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1750741367; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=/8PfEQ9j7G4E1vXnhssI7YdCA55VfUahO4Yb8WPZi3I=; b=IjYuLYaksml1UJr8wPZCa40Vpr8kxcWugUD2rBkIE1j5mnPkFJgjLcAMSpkeF7G5AyertJ oO7Zx9NX0popiPjDYFuZf3lJJ1FNU9PKAMNtNiX+URJmINOtqHTbMbZE/aBPnBLVBLPLD9 Drw0FQGc6hVgOA894U95jiDVt7BQQ+g= Date: Tue, 24 Jun 2025 13:02:31 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH RFC 2/3] locking/rwsem: clear reader-owner on unlock to reduce false positives Content-Language: en-US To: "Masami Hiramatsu (Google)" Cc: akpm@linux-foundation.org, zi.li@linux.dev, anna.schumaker@oracle.com, boqun.feng@gmail.com, joel.granados@kernel.org, jstultz@google.com, kent.overstreet@linux.dev, leonylgao@tencent.com, linux-kernel@vger.kernel.org, longman@redhat.com, mingo@redhat.com, mingzhe.yang@ly.com, peterz@infradead.org, rostedt@goodmis.org, senozhatsky@chromium.org, tfiga@chromium.org, will@kernel.org, Lance Yang References: <20250612042005.99602-1-lance.yang@linux.dev> <20250612042005.99602-3-lance.yang@linux.dev> <20250624092620.3346ac39e882434aafb0b93d@kernel.org> <21ef5892-afdf-491e-937f-7821cac63d16@linux.dev> <20250624125358.25a7d4cd5ea02ea0bbe373a6@kernel.org> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: <20250624125358.25a7d4cd5ea02ea0bbe373a6@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2025/6/24 11:53, Masami Hiramatsu (Google) wrote: > On Tue, 24 Jun 2025 09:44:55 +0800 > Lance Yang wrote: > >> >> >> On 2025/6/24 08:26, Masami Hiramatsu (Google) wrote: >>> On Thu, 12 Jun 2025 12:19:25 +0800 >>> Lance Yang wrote: >>> >>>> From: Lance Yang >>>> >>>> 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 for the feedback! I see your point about the dependency ;) >> >> Personlly, I'd perfer to keep them separate. The reasoning is that >> they addreess two distinct things, and I think splitting them makes >> this series clearer and easier to review ;) >> >> Patch #1 focuses on "ownership tracking": Its only job is to make >> the existing owner-related helpers (rwsem_owner(), is_rwsem_reader_owned()) >> globally available when blocker tracking is enabled. >> >> Patch #2, on the other hand, is about "reader-owner cleanup": It >> introduces a functional change to the unlock path, trying to clear >> the owner field for reader-owned rwsems. > > But without clearing the owner, the owner information can be > broken, right? Since CONFIG_DEBUG_RWSEMS is working as it is, You're right, the owner info would be broken without the cleanup logic in patch #2. But ... > I think those cannot be decoupled. For example, comparing the > result of both DETECT_HUNG_TASK_BLOCKER and DEBUG_RWSEMS are > enabled and only DETECT_HUNG_TASK_BLOCKER is enabled, the > result is different. The actual blocker tracking for rwsems is only turned on in patch #3. So, there's no case where the feature is active without the cleanup logic already being in place. > >> >> Does this reasoning make sense to you? > > Sorry, no. I think "reader-owner cleanup" is a part of "ownership > tracking" as DEBUG_RWSEMS does (and that keeps consistency of > the ownership tracking behavior same as DEBUG_RWSEM). I thought this step-by-step approach was a bit cleaner, since there are currently only two users for these owner helpers (DEBUG_RWSEMS and DETECT_HUNG_TASK_BLOCKER). Anyway, if you still feel strongly that they should be merged, I'm happy to rework the series as you suggested ;p Thanks, Lance > > Thank you, > >> >> Thanks, >> Lance >> >>> >>> Thanks, >>> >>>> >>>> Signed-off-by: Lance Yang >>>> --- >>>> 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 >>>> >>> >>> >> > >