linux-kernel - Re: [patch V3a 17/18] posix-timers: Provide a mechanism to allocate a given timer ID

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Message-ID: <87bju6ze2h.ffs@tglx>
Date: Wed, 12 Mar 2025 12:24:54 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Cyrill Gorcunov <gorcunov@...il.com>, Frederic Weisbecker
 <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Anna-Maria Behnsen
 <anna-maria@...utronix.de>, Benjamin Segall <bsegall@...gle.com>, Eric
 Dumazet <edumazet@...gle.com>, Andrey Vagin <avagin@...nvz.org>, Pavel
 Tikhomirov <ptikhomirov@...tuozzo.com>, Peter Zijlstra
 <peterz@...radead.org>
Subject: Re: [patch V3a 17/18] posix-timers: Provide a mechanism to allocate
 a given timer ID

On Wed, Mar 12 2025 at 10:56, Cyrill Gorcunov wrote:
> On Tue, Mar 11, 2025 at 11:32:58PM +0100, Frederic Weisbecker wrote:
> ...
>> > 
>> > Recreating two timers with IDs 1000000 and 2000000 takes 1.5 seconds with
>> > the create/delete method. With the prctl() it takes 3 microseconds.
>> > 
>> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>> 
>> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
>
> One thing which just popped up in my head -- this interface may be used not
> only by criu but any application which wants to create timer with specified
> id (hell know why, but whatever). As far as I understand we don't provide

Sure. Application developers are creative :)

> an interface to _read_ this property, don't we? Thus criu will restore such
> application which already has this bit set incorrectly.

Delta patch below.

Thanks,

        tglx
---
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -362,5 +362,6 @@ struct prctl_mm_map {
 #define PR_TIMER_CREATE_RESTORE_IDS		77
 # define PR_TIMER_CREATE_RESTORE_IDS_OFF	0
 # define PR_TIMER_CREATE_RESTORE_IDS_ON		1
+# define PR_TIMER_CREATE_RESTORE_IDS_GET	2
 
 #endif /* _LINUX_PRCTL_H */
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -391,11 +391,17 @@ static enum hrtimer_restart posix_timer_
 
 long posixtimer_create_prctl(unsigned long ctrl)
 {
-	if (ctrl > PR_TIMER_CREATE_RESTORE_IDS_ON)
-		return -EINVAL;
-
-	current->signal->timer_create_restore_ids = ctrl == PR_TIMER_CREATE_RESTORE_IDS_ON;
-	return 0;
+	switch (ctrl) {
+	case PR_TIMER_CREATE_RESTORE_IDS_OFF:
+		current->signal->timer_create_restore_ids = 0;
+		return 0;
+	case PR_TIMER_CREATE_RESTORE_IDS_ON:
+		current->signal->timer_create_restore_ids = 0;
+		return 0;
+	case PR_TIMER_CREATE_RESTORE_IDS_GET:
+		return current->signal->timer_create_restore_ids;
+	}
+	return -EINVAL;
 }
 
 static struct pid *good_sigevent(sigevent_t * event)