[PATCH] scstadmin: Handle -EAGAIN when reading from sysfs

Reading from /sys/kernel/scst_tgt/devices/*/filename can (at
least in theory) fail with -EAGAIN. Make sure that scstadmin
retries reading from a sysfs variable if that happens. This
patch has been tested by adding the following error injection
code in scst_sysfs.c:

 --------------------------- scst/src/scst_sysfs.c ----------------------------
index 0509703..69efd80 100644
@@ -523,6 +523,22 @@ int scst_sysfs_queue_wait_work(struct scst_sysfs_work_item *work)
 			"failed: %d", atomic_read(&uid_thread_name),
 			(int)PTR_ERR(t));

+#if 1
+	{
+		static int cnt;
+		if (!work->read_only_action || cnt++ % 4 < 3) {
+			/*
+			 * Helps testing user space code that writes to or
+			 * reads from SCST sysfs variables.
+			 */
+			timeout = 0;
+			rc = 0;
+			res = -EAGAIN;
+			goto out_put;
+		}
+	}
+#endif
+
 	while (1) {
 		rc = wait_for_completion_interruptible_timeout(
 			&work->sysfs_work_done, timeout);
 ------------------------------------------------------------------------------

Signed-off-by: Bart Van Assche <bvanassche@acm.org>


git-svn-id: http://svn.code.sf.net/p/scst/svn/trunk@4498 d57e44dd-8a1f-0410-8b47-8ef2f437770f

scstadmin/scstadmin.sysfs/scst-0.9.10/lib/SCST/SCST.pm

@@ -2614,10 +2614,10 @@ sub deviceAttributes {
 					return undef;
 				}
 
-				my $value = <$io>;
+				my ($value, $is_key) = split("\n", _sysread($io) , 2);
+
 				chomp $value;
 
-				my $is_key = <$io>;
 				$is_key = new_sysfs_interface() &&
 				    (!$is_static || defined($$dca{$attribute}))
 				    || ($is_key =~ /\[key\]/) ? TRUE : FALSE;
@@ -4855,6 +4855,22 @@ sub errorString {
 	return $string;
 }
 
+# Read from the SCST sysfs file $1. Return either the data read or undef if
+# reading failed.
+sub _sysread {
+	my $io = shift;
+	my $deadline = time() + $TIMEOUT;
+	my $result;
+
+	while (time() < $deadline) {
+		my $bytes = sysread($io, $result, 4096);
+		last if (defined($bytes) || $! != EAGAIN);
+		sleep 1;
+	}
+
+	return $result;
+}
+
 # Write the first $3 bytes of $2 into the SCST sysfs file $1. Return either
 # the number of bytes written or undef if writing failed.
 sub _syswrite {