linux-kernel - Re: [PATCH v9] kernel/fork: beware of __put_task

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Date:   Tue, 16 May 2023 15:50:13 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Wander Lairson Costa <wander@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>,
        Brian Cain <bcain@...cinc.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Stafford Horne <shorne@...il.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrei Vagin <avagin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...nel.org>,
        Yu Zhao <yuzhao@...gle.com>,
        Alexey Gladkov <legion@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Yang Shi <shy828301@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        Hu Chunyu <chuhu@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Luis Goncalves <lgoncalv@...hat.com>
Subject: Re: [PATCH v9] kernel/fork: beware of __put_task_struct calling
 context

On Tue, 16 May 2023 22:41:18 +0100 Matthew Wilcox <willy@...radead.org> wrote:

> 
> Oh, I missed that put_task_struct() was still inlined.  Should it be?
> It seems quite large now.

It's not significantly worse because of this patch.  In fact, it's
unchanged for non-RT kernels.

Possibly put_task_struct() *should* be uninlined, because it made the
mistake of using the dang refcount stuff, which never saw a byte which
it couldn't consume :(


I mean...

--- a/fs/open.c~a
+++ a/fs/open.c
@@ -1572,3 +1572,9 @@ int stream_open(struct inode *inode, str
 }
 
 EXPORT_SYMBOL(stream_open);
+
+#include <linux/refcount.h>
+bool foo(refcount_t *r)
+{
+	return refcount_dec_and_test(r);
+}
_

is worth

339 bytes of text for an allmodconfig build 
67 bytes of text for an allnoconfig build 
77 bytes of text for a defconfig build