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Q: On restoring terminal with hinted index

PATCH Cyrill Gorcunov 2 participants 3 messages Aug 15, 2012

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Cyrill Gorcunov

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#0 lore raw

Hi Peter,

that happened that in a sake of restoring ptys after checkpoint we need to create them with predefined indices, as they were at moment of dumping. So we have two options -- 1) Open terminals in sequence until needed index reached 2) Use some other way to say the kernel that some index is preferred.

So I thought, would it be acceptible to provide such hint via sysctl, as in patch below (note I've not even compiled it yet, but would like to know your opinion early).

(Maybe even make it one shot, thus once ida_get_new_above called
the pty_next reset to zero).

---
 fs/devpts/inode.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)
fs/devpts/inode.c +8 -1
Index: linux-2.6.git/fs/devpts/inode.c
===================================================================
@@ -45,6 +45,7 @@ static int pty_reserve = NR_UNIX98_PTY_R
 static int pty_limit_min;
 static int pty_limit_max = INT_MAX;
 static int pty_count;
+static int pty_next;
 
 static struct ctl_table pty_table[] = {
 	{

@@ -69,6 +70,12 @@ static struct ctl_table pty_table[] = {
 		.mode		= 0444,
 		.data		= &pty_count,
 		.proc_handler	= proc_dointvec,
+	}, {
+		.procname	= "next",
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.data		= &pty_next,
+		.proc_handler	= proc_dointvec,
 	},
 	{}
 };

@@ -516,7 +523,7 @@ retry:
 		return -ENOSPC;
 	}
 
-	ida_ret = ida_get_new(&fsi->allocated_ptys, &index);
+	ida_ret = ida_get_new_above(&fsi->allocated_ptys, pty_next, &index);
 	if (ida_ret < 0) {
 		mutex_unlock(&allocated_ptys_lock);
 		if (ida_ret == -EAGAIN)
#1 lore raw
Quoted 8 lines · Cyrill Gorcunov

Hi Peter,

that happened that in a sake of restoring ptys after checkpoint we need to create them with predefined indices, as they were at moment of dumping. So we have two options -- 1) Open terminals in sequence until needed index reached 2) Use some other way to say the kernel that some index is preferred.

Quoted 7 lines

So I thought, would it be acceptible to provide such hint via sysctl, as in patch below (note I've not even compiled it yet, but would like to know your opinion early).

(Maybe even make it one shot, thus once ida_get_new_above called
the pty_next reset to zero).

Pardon me while I vomit... an interaction between sysctl and open? Talk about a race condition from hell. It is stateful, and not connected to the filesystem instance that it is supposed to be affecting. So no, please don't. I understand your objective but it really needs to be something attached to the pts filesystem instance in question.

-hpa

author
#2 lore raw
Quoted 17 lines · H. Peter Anvin
Quoted 8 lines · Cyrill Gorcunov

Hi Peter,

that happened that in a sake of restoring ptys after checkpoint we need to create them with predefined indices, as they were at moment of dumping. So we have two options -- 1) Open terminals in sequence until needed index reached 2) Use some other way to say the kernel that some index is preferred.

Quoted 7 lines

So I thought, would it be acceptible to provide such hint via sysctl, as in patch below (note I've not even compiled it yet, but would like to know your opinion early).

(Maybe even make it one shot, thus once ida_get_new_above called
the pty_next reset to zero).

Quoted 5 lines

Pardon me while I vomit... an interaction between sysctl and open? Talk about a race condition from hell. It is stateful, and not connected to the filesystem instance that it is supposed to be affecting. So no, please don't. I understand your objective but it really needs to be something attached to the pts filesystem instance in question.

Sure, i'll think on finding some other way to provide such hint to the kernel.

Cyrill