[PATCH 1/2] devtmpfs: fix placement of complete() call

[PATCH 1/2] devtmpfs: fix placement of complete() call

[Rasmus Villemoes]

Calling complete() from within the __init function is wrong -
theoretically, the init process could proceed all the way to freeing
the init mem before the devtmpfsd thread gets to execute the return
instruction in devtmpfs_setup().

In practice, it seems to be harmless as gcc inlines devtmpfs_setup()
into devtmpfsd(). So the calls of the __init functions init_chdir()
etc. actually happen from devtmpfs_setup(), but the __ref on that one
silences modpost (it's all right, because those calls happen before
the complete()). But it does make the __init annotation of the setup
function moot, which we'll fix in a subsequent patch.

Fixes: bcbacc4909f1 ("devtmpfs: refactor devtmpfsd()")
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 drivers/base/devtmpfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c
index 653c8c6ac7a7..aedeb2dc1a18 100644
--- a/drivers/base/devtmpfs.c
+++ b/drivers/base/devtmpfs.c
@@ -419,7 +419,6 @@ static int __init devtmpfs_setup(void *p)
 	init_chroot(".");
 out:
 	*(int *)p = err;
-	complete(&setup_done);
 	return err;
 }
 
@@ -432,6 +431,7 @@ static int __ref devtmpfsd(void *p)
 {
 	int err = devtmpfs_setup(p);
 
+	complete(&setup_done);
 	if (err)
 		return err;
 	devtmpfs_work_loop();
-- 
2.29.2

[PATCH 2/2] devtmpfs: actually reclaim some init memory

[Rasmus Villemoes]

Currently gcc seems to inline devtmpfs_setup() into devtmpfsd(), so
its memory footprint isn't reclaimed as intended. Mark it noinline to
make sure it gets put in .init.text.

While here, setup_done can also be put in .init.data: After complete()
releases the internal spinlock, the completion object is never touched
again by that thread, and the waiting thread doesn't proceed until it
observes ->done while holding that spinlock.

This is now the same pattern as for kthreadd_done in init/main.c:
complete() is done in a __ref function, while the corresponding
wait_for_completion() is in an __init function.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 drivers/base/devtmpfs.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c
index aedeb2dc1a18..8be352ab4ddb 100644
--- a/drivers/base/devtmpfs.c
+++ b/drivers/base/devtmpfs.c
@@ -371,7 +371,7 @@ int __init devtmpfs_mount(void)
 	return err;
 }
 
-static DECLARE_COMPLETION(setup_done);
+static __initdata DECLARE_COMPLETION(setup_done);
 
 static int handle(const char *name, umode_t mode, kuid_t uid, kgid_t gid,
 		  struct device *dev)
@@ -405,7 +405,7 @@ static void __noreturn devtmpfs_work_loop(void)
 	}
 }
 
-static int __init devtmpfs_setup(void *p)
+static noinline int __init devtmpfs_setup(void *p)
 {
 	int err;
 
-- 
2.29.2

Re: [PATCH 1/2] devtmpfs: fix placement of complete() call

[Christoph Hellwig]

Looks fine:

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH 2/2] devtmpfs: actually reclaim some init memory

[Christoph Hellwig]

On Fri, Mar 12, 2021 at 11:30:27AM +0100, Rasmus Villemoes wrote:
> Currently gcc seems to inline devtmpfs_setup() into devtmpfsd(), so
> its memory footprint isn't reclaimed as intended. Mark it noinline to
> make sure it gets put in .init.text.
> 
> While here, setup_done can also be put in .init.data: After complete()
> releases the internal spinlock, the completion object is never touched
> again by that thread, and the waiting thread doesn't proceed until it
> observes ->done while holding that spinlock.
> 
> This is now the same pattern as for kthreadd_done in init/main.c:
> complete() is done in a __ref function, while the corresponding
> wait_for_completion() is in an __init function.

I'm not sure if this matters in any way, but it does look fine to me:

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH 1/2] devtmpfs: fix placement of complete() call

[Rasmus Villemoes]

On 12/03/2021 11.30, Rasmus Villemoes wrote:
> Calling complete() from within the __init function is wrong -
> theoretically, the init process could proceed all the way to freeing
> the init mem before the devtmpfsd thread gets to execute the return
> instruction in devtmpfs_setup().

Greg, ping? Also for the other one.

Re: [PATCH 1/2] devtmpfs: fix placement of complete() call

[Greg Kroah-Hartman]

On Thu, Mar 18, 2021 at 01:44:04PM +0100, Rasmus Villemoes wrote:
> On 12/03/2021 11.30, Rasmus Villemoes wrote:
> > Calling complete() from within the __init function is wrong -
> > theoretically, the init process could proceed all the way to freeing
> > the init mem before the devtmpfsd thread gets to execute the return
> > instruction in devtmpfs_setup().
> 
> Greg, ping? Also for the other one.

I'll pick this up for my tree, give me a chance to catch up, there's no
rush with this one :)

thanks,

greg k-h