From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 E598E1448E0 for ; Tue, 17 Mar 2026 23:39:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773790801; cv=none; b=Fo2UPIA3UBJdjl29WNSYdGOJyqvGEsuRrl10xNZkPyGLnjLZlrdN9X1tXFiiQWN9S6LGKcCBMyK4g5CM0iQmpFzgg4aiPWhtVdLH9M7RMSEHYWtY6vsa8jXyKmA0IY9NZjFL8mOC9YJ+see4Ni8fve7fEKzR2waefDmBYLAz2hI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773790801; c=relaxed/simple; bh=C5ugOR4n6xT/tyd6frXsDjFZroZfrBM7cUdz0iXW5vo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=paX7CYlRCIHok7H7rMUyugD4zre3REgPy2+uTJ1bw1ZtJM2Z8/rQ6EyFAYhbbNV1Xv+9leXhn2QfxiCqF4r8MEUWxpjyLjNfEvkZl2GNvHW9FNR0rz9IBE7fJXWXsgOssPzfLiYpHQ5R/ri5OTpF0nyz7f1OspjVoFWiF4kPdzY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=WpoLW9F9; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="WpoLW9F9" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1773790798; 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=t9UC3T1XnykHA86eAjVex7HUIzi/Vx02Pj4BXyT2DPY=; b=WpoLW9F992NSAQ5ZbQXjoRzfARhau33uIPM1B1HjRhKmpg+Amcce4tUrwVoDJLOMIH/ALj u9pUerW2+uxdQJ+oiOUA3Q3Jo+1/7/ZDhgM+k776N5BxZn2SA/685AwvBvT0A7wkqXTPpr LUDp10f1ZSgDx0cw3UTwzAltU706z4Y= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-644-1e6FaEfwPyWptQ4bMc3gjg-1; Tue, 17 Mar 2026 19:39:55 -0400 X-MC-Unique: 1e6FaEfwPyWptQ4bMc3gjg-1 X-Mimecast-MFC-AGG-ID: 1e6FaEfwPyWptQ4bMc3gjg_1773790794 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 61D26195609D; Tue, 17 Mar 2026 23:39:53 +0000 (UTC) Received: from [10.22.80.24] (unknown [10.22.80.24]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id DB3D81800351; Tue, 17 Mar 2026 23:39:50 +0000 (UTC) Message-ID: <1d3e9ff7-cdd7-4f75-baf0-198ec0ecbf2d@redhat.com> Date: Tue, 17 Mar 2026 19:39:50 -0400 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] locking/rwsem: Fix logic error in rwsem_del_waiter() To: Andrei Vagin Cc: Andrei Vagin , "Matthew Wilcox (Oracle)" , Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng , linux-kernel@vger.kernel.org, syzbot+3d2ff92c67127d337463@syzkaller.appspotmail.com References: <20260314182607.3343346-1-avagin@google.com> <025cdbad-99e1-4342-9f37-2564c555a8d1@redhat.com> <5927a9df-3e06-4b92-bc84-75188290967d@redhat.com> <913ba029-c4ac-4842-b294-31a4174476d3@redhat.com> Content-Language: en-US From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 On 3/17/26 5:23 PM, Andrei Vagin wrote: > On Mon, Mar 16, 2026 at 12:10 PM Waiman Long wrote: >> On 3/16/26 1:49 PM, Waiman Long wrote: >>> On 3/16/26 1:34 PM, Waiman Long wrote: >>>> On 3/14/26 2:26 PM, Andrei Vagin wrote: >>>>> Commit 1ea4b473504b ("locking/rwsem: Remove the list_head from struct >>>>> rw_semaphore") introduced a logic error in rwsem_del_waiter(). >>>>> >>>>> The root cause of this issue is an inconsistency in the return >>>>> values of >>>>> __rwsem_del_waiter() and rwsem_del_waiter(). Specifically, >>>>> __rwsem_del_waiter() returns true when the wait list becomes empty, >>>>> whereas rwsem_del_waiter() is supposed to return true if the wait list >>>>> is NOT empty. >>>>> >>>>> This caused a null pointer dereference in rwsem_mark_wake() because it >>>>> was being called when sem->first_waiter was NULL. >>>>> >>>>> Cc: Matthew Wilcox (Oracle) >>>>> Reported-by:syzbot+3d2ff92c67127d337463@syzkaller.appspotmail.com >>>>> Tested-by:syzbot+3d2ff92c67127d337463@syzkaller.appspotmail.com >>>>> Fixes: 1ea4b473504b ("locking/rwsem: Remove the list_head from >>>>> struct rw_semaphore") >>>>> Signed-off-by: Andrei Vagin >>>>> --- >>>>> kernel/locking/rwsem.c | 4 ++-- >>>>> 1 file changed, 2 insertions(+), 2 deletions(-) >>>>> >>>>> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c >>>>> index ba4cb74de064..bf647097369c 100644 >>>>> --- a/kernel/locking/rwsem.c >>>>> +++ b/kernel/locking/rwsem.c >>>>> @@ -370,7 +370,7 @@ bool __rwsem_del_waiter(struct rw_semaphore >>>>> *sem, struct rwsem_waiter *waiter) >>>>> { >>>>> if (list_empty(&waiter->list)) { >>>>> sem->first_waiter = NULL; >>>>> - return true; >>>>> + return false; >>>>> } >>>>> if (sem->first_waiter == waiter) { >>>>> @@ -379,7 +379,7 @@ bool __rwsem_del_waiter(struct rw_semaphore >>>>> *sem, struct rwsem_waiter *waiter) >>>>> } >>>>> list_del(&waiter->list); >>>>> - return false; >>>>> + return true; >>>>> } >>>>> /* >>>> It will be better if we also document what does the return value of >>>> __rwsem_del_waiter() means as the we can't guess from the function >>>> name itself. Other that that, >>>> >>>> Reviewed-by: Waiman Long >>>> >>> Thinking a bit more about it. I think it will be better to not return >>> a value in __rwsem_del_waiter() at all. Something like >>> >> Sorry, my mailer screwed up the diff. It should be as follows. >> >> Cheers, >> Longman >> >> =================================[ Cut here ]============================ >> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c >> index ba4cb74de064..ce57ad3c1120 100644 >> --- a/kernel/locking/rwsem.c >> +++ b/kernel/locking/rwsem.c >> @@ -365,12 +365,11 @@ enum rwsem_wake_type { >> #define MAX_READERS_WAKEUP 0x100 >> >> static inline >> -bool __rwsem_del_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter) >> +void __rwsem_del_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter) >> __must_hold(&sem->wait_lock) >> { >> if (list_empty(&waiter->list)) { >> sem->first_waiter = NULL; >> - return true; > We still need `return` here. > >> } >> >> if (sem->first_waiter == waiter) { >> @@ -378,8 +377,6 @@ bool __rwsem_del_waiter(struct rw_semaphore *sem, struct rw> >> struct rwsem_waiter, list); >> } >> list_del(&waiter->list); >> - >> - return false; >> } >> >> /* >> @@ -394,7 +391,8 @@ static inline bool >> rwsem_del_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter) >> { >> lockdep_assert_held(&sem->wait_lock); >> - if (__rwsem_del_waiter(sem, waiter)) >> + __rwsem_del_waiter(sem, waiter); >> + if (rwsem_is_contended(sem)) > I am ok with this approach too. Do you want to send this patch? Sure. I will send out later tonight or tomorrow morning. Cheers, Longman