From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtpo75.interia.pl (smtpo75.interia.pl [217.74.67.75]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3EA60489862 for ; Fri, 31 Jul 2026 08:36:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.74.67.75 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785486972; cv=none; b=CQoTEyRJmdvwuM/4/2uuRZ8WTW/WiI+EIbnu2mSOh9aoq3J/yQWZffwcZN7OUTGWJpuipldVdYOJ7rBvgtttXjlevSvIT548WEF3kt/YilgakIaV/iia5/3by52RMXDUjqy9/1mZLSSR90BgwiNsgWyFCdZqwYRKoOyrOxBcZSI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785486972; c=relaxed/simple; bh=Bl1nS70W/Gjrww2Mw95AiWzDR83LDhZSB/eEPRITF2s=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=N4DRn+zap3UG4lg8rLIsgdzfEpWKtt7bm6DoM6UZGAA+BTgUIO+h0IvPdS+DJsF7bf9Peeb6qP2auu+zDAwIxXac5dPdr7yvXF2l7kJSfAKs5Du0/cQaeIsrlFNIFxioAKRtMmrvMWP11KtTvx6npW0dtMR/s/NgIuntocnVvQE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=poczta.fm; spf=pass smtp.mailfrom=poczta.fm; dkim=pass (1024-bit key) header.d=poczta.fm header.i=@poczta.fm header.b=AnEXTdKh; arc=none smtp.client-ip=217.74.67.75 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=poczta.fm Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=poczta.fm Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=poczta.fm header.i=@poczta.fm header.b="AnEXTdKh" Received: from nr200 (unknown [80.68.231.31]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-256) server-digest SHA256) (No client certificate requested) by poczta.interia.pl (INTERIA.PL) with ESMTPSA; Fri, 31 Jul 2026 10:35:59 +0200 (CEST) Date: Fri, 31 Jul 2026 10:35:56 +0200 From: Slawomir Stepien To: syzbot Cc: syzkaller-upstream-moderation@googlegroups.com, syzbot@lists.linux.dev Subject: Re: [PATCH RFC] netdevsim: fix deadlock in nsim_bus_dev_max_vfs_write() Message-ID: References: Precedence: bulk X-Mailing-List: syzbot@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=poczta.fm; s=dk; t=1785486960; bh=sr+b8dafCtOjsxKxqYuWnaRElZkmtD7Yqhe/mb33XMs=; h=Date:From:To:Subject:Message-ID:MIME-Version:Content-Type; b=AnEXTdKhMcLL6ORARt+4wAzNswkiXMvdn6taN4sbnQxggifdF6CPzoKfoYTqB0u8y N1aAgOOPFTy0p3pDXOPjMcN8MgoS0+yoF1kxHUlZ/KIdyFVCBIddJHyQTWoJDAz91D DBpxtdedZJwf+Vm5Fnesy2CS5zjz5DQkfoOL+quw= On lip 01, 2026 13:15, syzbot wrote: > There is an ABBA deadlock between the devlink instance lock and the debugfs > active file reference mechanism. > > When a device is being removed (e.g., via nsim_drv_remove()), the driver > acquires the devlink instance lock and then calls > debugfs_remove_recursive(). This function blocks waiting for all active > file operations on the debugfs files to complete. > > Concurrently, if a user writes to the "max_vfs" debugfs file, the VFS layer > acquires an active reference to the file. The write handler, > nsim_bus_dev_max_vfs_write(), then attempts to acquire the devlink instance > lock, which is already held by the removal task. > > This creates a circular dependency resulting in a deadlock: > > INFO: task blocked for more than 143 seconds. > Call Trace: > wait_for_completion+0x2ca/0x5e0 kernel/sched/completion.c:153 > __debugfs_file_removed fs/debugfs/inode.c:751 [inline] > remove_one+0x2df/0x3b0 fs/debugfs/inode.c:758 > __simple_recursive_removal+0x215/0x520 fs/libfs.c:623 > debugfs_remove+0x5b/0x70 fs/debugfs/inode.c:781 > nsim_dev_debugfs_exit drivers/net/netdevsim/dev.c:372 [inline] > nsim_drv_remove+0xc0/0x170 drivers/net/netdevsim/dev.c:1803 > > INFO: task blocked for more than 143 seconds. > Call Trace: > __mutex_lock+0x7bf/0x1550 kernel/locking/mutex.c:821 > nsim_bus_dev_max_vfs_write+0x229/0x3d0 drivers/net/netdevsim/dev.c:276 > full_proxy_write+0x127/0x1f0 fs/debugfs/file.c:388 > vfs_write+0x296/0xba0 fs/read_write.c:685 > > To fix this, use devl_trylock() in nsim_bus_dev_max_vfs_write() instead of > devl_lock(). If the lock cannot be acquired, return -EBUSY. This aborts the > write operation, releases the debugfs active file reference, and allows the > pending debugfs_remove_recursive() to proceed. > > Fixes: 012ec02ae441 ("netdevsim: convert driver to use unlocked devlink API during init/fini") I think this Fixes commit isn't correct. The devl_lock() was there since aff3a925094633a5b77058b9a715efbb12fc2698 and there is a big change that before this change the mutex_lock that was there, was also deadlocking. I would remove this line since it is not correct in the 1st place. > Assisted-by: Gemini:gemini-3.1-pro-preview best-expensive syzbot > Reported-by: syzbot+3147c5de186107ffc7a1@syzkaller.appspotmail.com > Closes: https://syzkaller.appspot.com/bug?extid=3147c5de186107ffc7a1 > Link: https://syzkaller.appspot.com/ai_job?id=e3832a24-8c9f-416a-8161-a50e277bc628 > To: "Andrew Lunn" > To: "David S. Miller" > To: "Eric Dumazet" > To: "Jakub Kicinski" > To: > To: "Paolo Abeni" > Cc: > > --- > diff --git a/drivers/net/netdevsim/dev.c b/drivers/net/netdevsim/dev.c > index aed9ad5f1..421cd7327 100644 > --- a/drivers/net/netdevsim/dev.c > +++ b/drivers/net/netdevsim/dev.c > @@ -273,7 +273,11 @@ static ssize_t nsim_bus_dev_max_vfs_write(struct file *file, > return -ENOMEM; > > nsim_dev = file->private_data; > - devl_lock(priv_to_devlink(nsim_dev)); > + if (!devl_trylock(priv_to_devlink(nsim_dev))) { > + ret = -EBUSY; > + goto out; > + } > + > /* Reject if VFs are configured */ > if (nsim_dev_get_vfs(nsim_dev)) { > ret = -EBUSY; > @@ -285,6 +289,7 @@ static ssize_t nsim_bus_dev_max_vfs_write(struct file *file, > } > devl_unlock(priv_to_devlink(nsim_dev)); > > +out: > kfree(vfconfigs); > return ret; > } > > > base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482 -- Slawomir Stepien