linux-kernel - Re: [EDT][PATCH 1/1] msgsnd use freezable blocking call

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Date:	Wed, 6 May 2015 15:22:09 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	maninder1.s@...sung.com
Cc:	linux-kernel@...r.kernel.org,
	"v.narang@...sung.com" <v.narang@...sung.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [EDT][PATCH 1/1] msgsnd use freezable blocking call

On Wed, 06 May 2015 11:29:57 +0000 (GMT) Maninder Singh <maninder1.s@...sung.com> wrote:

> EP-F6AA0618C49C4AEDA73BFF1B39950BAB
> 
> Hi ,
> 
> Recently shared a patch for using freezable_schedule instead of schedule in msgrcv,
> and after analysing message queuie implemntation we have realized even msgsnd can also block, if queue is full,
> So in this scenerio msgsnd sets task state as TASK_INTERRUPTIBLE and can schedule function, similar behaviour as msgrcv.
> This change is applicable for msgsnd as well.
> 
> we have created patch for remotes/linux-next/akpm, because msgrcv patch is already applied at this branch.
> so we didnt include  <linux/freezer.h>
> 

This changelog is quite poor.  It doesn't explain why the change is
being made, it doesn't explain the user-visible effects which are being
fixed, etc.

So I threw it away and copied text from "ipc/msg.c: use freezable
blocking call":

One thing which isn't clear to me: *why* do we want to "Avoid waking up
every thread sleeping in XXX during suspend and resume"?  Suspend and
resume are rare operations.  Why do we care if a few threads wake up
then go to sleep again?

From: Maninder Singh <maninder1.s@...sung.com>
Subject: ipc/msg.c: msgsnd: use freezable blocking call

Avoid waking up every thread sleeping in a msgsnd call during suspend and
resume by calling a freezable blocking call.  Previous patches modified
the freezer to avoid sending wakeups to threads that are blocked in
freezable blocking calls.

This call was selected to be converted to a freezable call because it
doesn't hold any locks or release any resources when interrupted that
might be needed by another freezing task or a kernel driver during
suspend, and is a common site where idle userspace tasks are blocked.

Signed-off-by: Vaneet narang <v.narang@...sung.com>
Signed-off-by: Maninder Singh <maninder1.s@...sung.com>
Cc: Yogesh Gaur <yn.gaur@...sung.com>
Cc: Manjeet Pawar <manjeet.p@...sung.com>
Cc: Ajeet Yadav <ajeet.y@...sung.com>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Tejun Heo <tj@...nel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 ipc/msg.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff -puN ipc/msg.c~ipc-msgc-msgsnd-use-freezable-blocking-call ipc/msg.c
--- a/ipc/msg.c~ipc-msgc-msgsnd-use-freezable-blocking-call
+++ a/ipc/msg.c
@@ -673,7 +673,7 @@ long do_msgsnd(int msqid, long mtype, vo

 		ipc_unlock_object(&msq->q_perm);
 		rcu_read_unlock();
-		schedule();
+		freezable_schedule();

 		rcu_read_lock();
 		ipc_lock_object(&msq->q_perm);
_

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