Arnd Bergmann wrote:
The block layer is one of the remaining users
of the Big Kernel Lock. In there, it is used
mainly for serializing the blkdev_get and
blkdev_put functions and some ioctl implementations
as well as some less common open functions of
related character devices following a previous
pushdown and parts of the blktrace code.
The only one that seems to be a bit nasty is the
blkdev_get function which is actually recursive
and may try to get the BKL twice.
All users except the one in blkdev_get seem to
be outermost locks, meaning we don't rely on
the release-on-sleep semantics to avoid deadlocks.
The ctl_mutex (pktcdvd.ko), raw_mutex (raw.ko),
state_mutex (dasd.ko), reconfig_mutex (md.ko),
and jfs_log_mutex (jfs.ko) may be held when
blkdev_get is called, but as far as I can tell,
these mutexes are never acquired from any of the
functions that get converted in this patch.
In order to get rid of the BKL, this introduces
a new global mutex called blkdev_mutex, which
replaces the BKL in all drivers that directly
interact with the block layer. In case of blkdev_get,
the mutex is moved outside of the function itself
in order to avoid the recursive taking of blkdev_mutex.
Testing so far has shown no problems whatsoever
from this patch, but the usage in blkdev_get
may introduce extra latencies, and I may have
missed corner cases where an block device ioctl
function sleeps for a significant amount of time,
which may be harmful to the performance of other
threads.
<snip>
diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index dee1c96..ec5b43e 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -229,7 +229,7 @@ sg_open(struct inode *inode, struct file *filp)
int res;
int retval;
- lock_kernel();
+ mutex_lock(&blkdev_mutex);
nonseekable_open(inode, filp);
SCSI_LOG_TIMEOUT(3, printk("sg_open: dev=%d, flags=0x%x\n", dev, flags));
sdp = sg_get_dev(dev);
@@ -307,7 +307,7 @@ error_out:
sg_put:
if (sdp)
sg_put_dev(sdp);
- unlock_kernel();
+ mutex_unlock(&blkdev_mutex);
return retval;
}
@@ -757,9 +757,7 @@ sg_common_write(Sg_fd * sfp, Sg_request * srp,
return 0;
}
-static int
-sg_ioctl(struct inode *inode, struct file *filp,
- unsigned int cmd_in, unsigned long arg)
+static long sg_ioctl(struct file *filp, unsigned int cmd_in, unsigned long arg)
{
void __user *p = (void __user *)arg;
int __user *ip = p;
@@ -1078,6 +1076,17 @@ sg_ioctl(struct inode *inode, struct file *filp,
}
}
+static long sg_unlocked_ioctl(struct file *filp, unsigned int cmd_in, unsigned long arg)
+{
+ int ret;
+
+ mutex_lock(&blkdev_mutex);
+ ret = sg_ioctl(filp, cmd_in, arg);
+ mutex_unlock(&blkdev_mutex);
+
+ return ret;
+}
+
#ifdef CONFIG_COMPAT
static long sg_compat_ioctl(struct file *filp, unsigned int cmd_in, unsigned long arg)
{
@@ -1322,7 +1331,8 @@ static const struct file_operations sg_fops = {
.read = sg_read,
.write = sg_write,
.poll = sg_poll,
- .ioctl = sg_ioctl,
+ .llseek = generic_file_llseek,
The sg driver has no seek semantics on its read() and
write() calls. And sg_open() calls nonseekable_open(). So
.llseek = no_llseek,
seems more appropriate.
+ .unlocked_ioctl = sg_unlocked_ioctl,
#ifdef CONFIG_COMPAT
.compat_ioctl = sg_compat_ioctl,
#endif
And I just checked st.c (SCSI tape driver) and it calls
lock_kernel() .
Doug Gilbert
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