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From: Al Viro <viro@zeniv.linux.org.uk>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2] fput: Allow calling __fput_sync() from !PF_KTHREAD thread.
Date: Thu, 10 Sep 2020 04:57:50 +0100	[thread overview]
Message-ID: <20200910035750.GX1236603@ZenIV.linux.org.uk> (raw)
In-Reply-To: <1596027885-4730-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>

On Wed, Jul 29, 2020 at 10:04:45PM +0900, Tetsuo Handa wrote:
> __fput_sync() was introduced by commit 4a9d4b024a3102fc ("switch fput to
> task_work_add") with BUG_ON(!(current->flags & PF_KTHREAD)) check, and
> the only user of __fput_sync() was introduced by commit 17c0a5aaffa63da6
> ("make acct_kill() wait for file closing."). However, the latter commit is
> effectively calling __fput_sync() from !PF_KTHREAD thread because of
> schedule_work() call followed by immediate wait_for_completion() call.
> That is, there is no need to defer close_work() to a WQ context. I guess
> that the reason to defer was nothing but to bypass this BUG_ON() check.
> While we need to remain careful about calling __fput_sync(), we can remove
> bypassable BUG_ON() check from __fput_sync().
> 
> If this change is accepted, racy fput()+flush_delayed_fput() introduced
> by commit e2dc9bf3f5275ca3 ("umd: Transform fork_usermode_blob into
> fork_usermode_driver") will be replaced by this raceless __fput_sync().

NAK.  The reason to defer is *NOT* to bypass that BUG_ON() - we really do not
want that thing done on anything other than extremely shallow stack.
Incidentally, why is that thing ever done _not_ in a kernel thread context?

  parent reply	other threads:[~2020-09-10  3:58 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-08 14:24 [PATCH] fput: Allow calling __fput_sync() from !PF_KTHREAD thread Tetsuo Handa
2020-07-29 13:04 ` [PATCH v2] " Tetsuo Handa
2020-08-19 12:42   ` [PATCH v2 (resend)] " Tetsuo Handa
2020-09-09 21:59     ` Tetsuo Handa
2020-09-10  3:57   ` Al Viro [this message]
2020-09-10  5:26     ` [PATCH v2] " Tetsuo Handa
2020-09-10 11:25       ` Al Viro
2020-09-10 20:06         ` Eric W. Biederman

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