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 0A93D33C198 for ; Mon, 16 Mar 2026 19:04:58 +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=1773687900; cv=none; b=gJr/DdemT89IL8SG/+Tn2pexIeHj2FNE/n2SYkR+NuuaJ/2T6t3jT0F2fKJnZeNTA+1ANVIeL0suLBffsZHyAgrklKFDnUzv5dl07ldWGxKlLV64DrAaAPIqagCJedyDA9mg9S3K2Wdk7kF4xxCNtVDD7OXhnZGBlaXYZWDgqdE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773687900; c=relaxed/simple; bh=yZMVqybSP9ogXcwE3dEQmdm6TtVlw3EN1Curss20e20=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=SYsCngaXENxnLGrCqLJtPnDeKyLkIbPVjFMEILrbIm1uRk2ekNpeRg2qcb26BBG+t7shDTzIEMiBuveO1U27IQ8S5MAGnEPESM5XUDetrUZ4jPe7IIueP97j3zJba2Xy8ccY7cs33qSJ3nTQeOg8fjawd9bZf4zzbudHoUTiVyU= 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=FkG1e+nC; 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="FkG1e+nC" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1773687898; 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=/UOL6/6t+s7iDY6pHAtZKu/X0YkbiD5s2TBFFA7omv0=; b=FkG1e+nChwkY2fB7IG3kwJXNUYIgriqJhxQUVAnHGAtqxshtijaX5wT6K5PkIyQKadS2wE e5zviGyrjtyO/zAxyfBP3K4I7uGhUD8rX2QhmC7gSqWOMX2rYtFh3U+H+/3OmksZJu4fCv 8RNEyGozQrkqxQeZeGPcKpqNXJuUjtk= Received: from mx-prod-mc-03.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-662-47XIYNPPM12ocPju_uCARg-1; Mon, 16 Mar 2026 15:04:52 -0400 X-MC-Unique: 47XIYNPPM12ocPju_uCARg-1 X-Mimecast-MFC-AGG-ID: 47XIYNPPM12ocPju_uCARg_1773687891 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (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-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id C500319560BB; Mon, 16 Mar 2026 19:04:50 +0000 (UTC) Received: from [10.22.89.195] (unknown [10.22.89.195]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 4C15919560B1; Mon, 16 Mar 2026 19:04:48 +0000 (UTC) Message-ID: <913ba029-c4ac-4842-b294-31a4174476d3@redhat.com> Date: Mon, 16 Mar 2026 15:04:47 -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() From: Waiman Long To: Andrei Vagin , "Matthew Wilcox (Oracle)" , Peter Zijlstra Cc: 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> Content-Language: en-US In-Reply-To: <5927a9df-3e06-4b92-bc84-75188290967d@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 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; } 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)) return true; atomic_long_andnot(RWSEM_FLAG_HANDOFF | RWSEM_FLAG_WAITERS, &sem->count> return false;