Messages in this thread Patch in this message | | | Date | Tue, 1 Nov 2005 01:28:46 +0100 | From | Kay Sievers <> | Subject | Re: Race between "mount" uevent and /proc/mounts? |
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On Wed, Oct 26, 2005 at 08:28:59PM +0100, Al Viro wrote: > On Wed, Oct 26, 2005 at 04:34:17PM +0200, Kay Sievers wrote: > > > Semantics for events depends on which objects you are interested in. > > > Existing ones do not match _any_ of the real objects and I have no > > > idea what exactly had been intended for them. I've asked gregkh, but > > > he didn't remember that either. Apparently they are used by different > > > people as (bad) approximations to different things. Which doesn't work > > > well. And until somebody cares to describe what exactly are they trying > > > to watch the situation obviously won't improve. > > > > They are actually events for claim/release of a block device. As uevents > > are bound to kobjects we needed to send these events from an existing device > > which is the blockdev itself. > > > > Sure, the event itself, has nothing to do with a filesystem. The names are > > like this for historical reasons and "CLAIM/RELEASE" may be less confusing. > > The events are used as a trigger to rescan /proc/mounts instead of polling > > it constantly. > > But that makes no sense. /proc/*/mounts changes when mount tree changes. > Which is obviously not an event happening to block devices. Moreover, > changes of mount tree may involve no changes in the set of active filesystems > or be separated in time from such changes by arbitrary intervals. > > Looks like seriously wrong assumptions in userland code working with these > events... _IF_ you want to keep track of /proc/*/mounts changes, the obvious > solution would be to implement ->poll() for them.
Ok, makes sense. The attached seems to work for me. If we can get something like this, we can remove the superblock claim/release events completely and just read the events from the /proc/mounts file itself.
Thanks, Kay
diff --git a/fs/namespace.c b/fs/namespace.c --- a/fs/namespace.c +++ b/fs/namespace.c @@ -26,6 +26,7 @@ #include <asm/unistd.h> extern int __init init_rootfs(void); +DECLARE_WAIT_QUEUE_HEAD(mounts_wait); #ifdef CONFIG_SYSFS extern int __init sysfs_init(void); @@ -120,6 +121,8 @@ static void detach_mnt(struct vfsmount * list_del_init(&mnt->mnt_child); list_del_init(&mnt->mnt_hash); old_nd->dentry->d_mounted--; + current->namespace->mounts_poll_pending = 1; + wake_up_interruptible(&mounts_wait); } static void attach_mnt(struct vfsmount *mnt, struct nameidata *nd) @@ -129,6 +132,8 @@ static void attach_mnt(struct vfsmount * list_add(&mnt->mnt_hash, mount_hashtable+hash(nd->mnt, nd->dentry)); list_add_tail(&mnt->mnt_child, &nd->mnt->mnt_mounts); nd->dentry->d_mounted++; + current->namespace->mounts_poll_pending = 1; + wake_up_interruptible(&mounts_wait); } static struct vfsmount *next_mnt(struct vfsmount *p, struct vfsmount *root) @@ -1395,6 +1400,7 @@ static void __init init_mount_tree(void) list_add(&mnt->mnt_list, &namespace->list); namespace->root = mnt; mnt->mnt_namespace = namespace; + namespace->mounts_poll_pending = 0; init_task.namespace = namespace; read_lock(&tasklist_lock); diff --git a/fs/proc/base.c b/fs/proc/base.c --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -57,6 +57,7 @@ #include <linux/init.h> #include <linux/file.h> #include <linux/string.h> +#include <linux/poll.h> #include <linux/seq_file.h> #include <linux/namei.h> #include <linux/namespace.h> @@ -660,6 +661,8 @@ static struct file_operations proc_smaps #endif extern struct seq_operations mounts_op; +extern wait_queue_head_t mounts_wait; + static int mounts_open(struct inode *inode, struct file *file) { struct task_struct *task = proc_task(inode); @@ -692,10 +695,36 @@ static int mounts_release(struct inode * return seq_release(inode, file); } +static unsigned int mounts_poll(struct file *file, poll_table *wait) +{ + struct task_struct *task = proc_task(file->f_dentry->d_inode); + struct namespace *namespace; + int ret = 0; + + task_lock(task); + namespace = task->namespace; + if (namespace) + get_namespace(namespace); + task_unlock(task); + + if (!namespace) + return -EINVAL; + + poll_wait(file, &mounts_wait, wait); + if (namespace->mounts_poll_pending) { + namespace->mounts_poll_pending = 0; + ret = POLLIN | POLLRDNORM; + } + put_namespace(namespace); + + return ret; +} + static struct file_operations proc_mounts_operations = { .open = mounts_open, .read = seq_read, .llseek = seq_lseek, + .poll = mounts_poll, .release = mounts_release, }; diff --git a/include/linux/namespace.h b/include/linux/namespace.h --- a/include/linux/namespace.h +++ b/include/linux/namespace.h @@ -10,6 +10,7 @@ struct namespace { struct vfsmount * root; struct list_head list; struct rw_semaphore sem; + int mounts_poll_pending; }; extern int copy_namespace(int, struct task_struct *); - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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