[PATCH v1 net 07/16] sysctl: Add proc_doulongvec_minmax_lockless().

[Kuniyuki Iwashima]

A sysctl variable is accessed concurrently, and there is always a chance of
data-race.  So, all readers and writers need some basic protection to avoid
load/store-tearing.

This patch changes proc_doulongvec_minmax() to use READ_ONCE()/WRITE_ONCE()
internally to fix a data-race on the sysctl side.  For now,
proc_doulongvec_minmax() itself is tolerant to a data-race, but we still
need to add annotations on the other subsystem's side.

In case we miss such fixes, this patch converts proc_doulongvec_minmax() to
a wrapper of proc_doulongvec_minmax_lockless().  When we fix a data-race in
the other subsystem, we can explicitly set it as a handler.

Also, this patch removes proc_doulongvec_minmax()'s document and adds
proc_doulongvec_minmax_lockless()'s one so that no one will use
proc_doulongvec_minmax() anymore.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
---
 include/linux/sysctl.h |  1 +
 kernel/sysctl.c        | 21 +++++++++++++++++----
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 830d1a8f21d4..c23b6beef748 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -88,6 +88,7 @@ PROC_HANDLER(proc_dointvec_lockless);
 PROC_HANDLER(proc_douintvec_lockless);
 PROC_HANDLER(proc_dointvec_minmax_lockless);
 PROC_HANDLER(proc_douintvec_minmax_lockless);
+PROC_HANDLER(proc_doulongvec_minmax_lockless);
 
 /*
  * Register a set of sysctl names by calling register_sysctl_table
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 8ff57b8d1212..931ab58985f2 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1127,9 +1127,11 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table,
 				err = -EINVAL;
 				break;
 			}
-			*i = val;
+
+			WRITE_ONCE(*i, val);
 		} else {
-			val = convdiv * (*i) / convmul;
+			val = convdiv * READ_ONCE(*i) / convmul;
+
 			if (!first)
 				proc_put_char(&buffer, &left, '\t');
 			proc_put_long(&buffer, &left, val, false);
@@ -1157,7 +1159,8 @@ static int do_proc_doulongvec_minmax(struct ctl_table *table, int write,
 }
 
 /**
- * proc_doulongvec_minmax - read a vector of long integers with min/max values
+ * proc_doulongvec_minmax_lockless - read/write a vector of long integers
+ * with min/max values locklessly
  * @table: the sysctl table
  * @write: %TRUE if this is a write to the sysctl file
  * @buffer: the user buffer
@@ -1172,10 +1175,18 @@ static int do_proc_doulongvec_minmax(struct ctl_table *table, int write,
  *
  * Returns 0 on success.
  */
+int proc_doulongvec_minmax_lockless(struct ctl_table *table, int write,
+				    void *buffer, size_t *lenp, loff_t *ppos)
+{
+	return do_proc_doulongvec_minmax(table, write, buffer, lenp, ppos,
+					 1l, 1l);
+}
+
 int proc_doulongvec_minmax(struct ctl_table *table, int write,
 			   void *buffer, size_t *lenp, loff_t *ppos)
 {
-    return do_proc_doulongvec_minmax(table, write, buffer, lenp, ppos, 1l, 1l);
+	return proc_doulongvec_minmax_lockless(table, write, buffer,
+					       lenp, ppos);
 }
 
 /**
@@ -1540,6 +1551,7 @@ PROC_HANDLER_ENOSYS(proc_dointvec_lockless);
 PROC_HANDLER_ENOSYS(proc_douintvec_lockless);
 PROC_HANDLER_ENOSYS(proc_dointvec_minmax_lockless);
 PROC_HANDLER_ENOSYS(proc_douintvec_minmax_lockless);
+PROC_HANDLER_ENOSYS(proc_doulongvec_minmax_lockless);
 
 #endif /* CONFIG_PROC_SYSCTL */
 
@@ -2455,3 +2467,4 @@ EXPORT_SYMBOL(proc_dointvec_lockless);
 EXPORT_SYMBOL(proc_douintvec_lockless);
 EXPORT_SYMBOL(proc_dointvec_minmax_lockless);
 EXPORT_SYMBOL_GPL(proc_douintvec_minmax_lockless);
+EXPORT_SYMBOL(proc_doulongvec_minmax_lockless);
-- 
2.30.2