[PATCH] files: files struct with RCU
Patch to eliminate struct files_struct.file_lock spinlock on the reader side
and use rcu refcounting rcuref_xxx api for the f_count refcounter. The
updates to the fdtable are done by allocating a new fdtable structure and
setting files->fdt to point to the new structure. The fdtable structure is
protected by RCU thereby allowing lock-free lookup. For fd arrays/sets that
are vmalloced, we use keventd to free them since RCU callbacks can't sleep. A
global list of fdtable to be freed is not scalable, so we use a per-cpu list.
If keventd is already handling the current cpu's work, we use a timer to defer
queueing of that work.
Since the last publication, this patch has been re-written to avoid using
explicit memory barriers and use rcu_assign_pointer(), rcu_dereference()
premitives instead. This required that the fd information is kept in a
separate structure (fdtable) and updated atomically.
Signed-off-by: Dipankar Sarma <dipankar@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/fs/file.c b/fs/file.c
index f5926ce..2127a7b 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -13,6 +13,25 @@
#include <linux/vmalloc.h>
#include <linux/file.h>
#include <linux/bitops.h>
+#include <linux/interrupt.h>
+#include <linux/spinlock.h>
+#include <linux/rcupdate.h>
+#include <linux/workqueue.h>
+
+struct fdtable_defer {
+ spinlock_t lock;
+ struct work_struct wq;
+ struct timer_list timer;
+ struct fdtable *next;
+};
+
+/*
+ * We use this list to defer free fdtables that have vmalloced
+ * sets/arrays. By keeping a per-cpu list, we avoid having to embed
+ * the work_struct in fdtable itself which avoids a 64 byte (i386) increase in
+ * this per-task structure.
+ */
+static DEFINE_PER_CPU(struct fdtable_defer, fdtable_defer_list);
/*
@@ -48,85 +67,143 @@
vfree(array);
}
-/*
- * Expand the fd array in the files_struct. Called with the files
- * spinlock held for write.
- */
-
-static int expand_fd_array(struct files_struct *files, int nr)
- __releases(files->file_lock)
- __acquires(files->file_lock)
+static void __free_fdtable(struct fdtable *fdt)
{
- struct file **new_fds;
- int error, nfds;
+ int fdset_size, fdarray_size;
+
+ fdset_size = fdt->max_fdset / 8;
+ fdarray_size = fdt->max_fds * sizeof(struct file *);
+ free_fdset(fdt->open_fds, fdset_size);
+ free_fdset(fdt->close_on_exec, fdset_size);
+ free_fd_array(fdt->fd, fdarray_size);
+ kfree(fdt);
+}
+
+static void fdtable_timer(unsigned long data)
+{
+ struct fdtable_defer *fddef = (struct fdtable_defer *)data;
+
+ spin_lock(&fddef->lock);
+ /*
+ * If someone already emptied the queue return.
+ */
+ if (!fddef->next)
+ goto out;
+ if (!schedule_work(&fddef->wq))
+ mod_timer(&fddef->timer, 5);
+out:
+ spin_unlock(&fddef->lock);
+}
+
+static void free_fdtable_work(struct fdtable_defer *f)
+{
struct fdtable *fdt;
-
- error = -EMFILE;
- fdt = files_fdtable(files);
- if (fdt->max_fds >= NR_OPEN || nr >= NR_OPEN)
- goto out;
-
- nfds = fdt->max_fds;
- spin_unlock(&files->file_lock);
-
- /*
- * Expand to the max in easy steps, and keep expanding it until
- * we have enough for the requested fd array size.
- */
-
- do {
-#if NR_OPEN_DEFAULT < 256
- if (nfds < 256)
- nfds = 256;
- else
-#endif
- if (nfds < (PAGE_SIZE / sizeof(struct file *)))
- nfds = PAGE_SIZE / sizeof(struct file *);
- else {
- nfds = nfds * 2;
- if (nfds > NR_OPEN)
- nfds = NR_OPEN;
- }
- } while (nfds <= nr);
-
- error = -ENOMEM;
- new_fds = alloc_fd_array(nfds);
- spin_lock(&files->file_lock);
- if (!new_fds)
- goto out;
-
- /* Copy the existing array and install the new pointer */
- fdt = files_fdtable(files);
-
- if (nfds > fdt->max_fds) {
- struct file **old_fds;
- int i;
-
- old_fds = xchg(&fdt->fd, new_fds);
- i = xchg(&fdt->max_fds, nfds);
-
- /* Don't copy/clear the array if we are creating a new
- fd array for fork() */
- if (i) {
- memcpy(new_fds, old_fds, i * sizeof(struct file *));
- /* clear the remainder of the array */
- memset(&new_fds[i], 0,
- (nfds-i) * sizeof(struct file *));
-
- spin_unlock(&files->file_lock);
- free_fd_array(old_fds, i);
- spin_lock(&files->file_lock);
- }
- } else {
- /* Somebody expanded the array while we slept ... */
- spin_unlock(&files->file_lock);
- free_fd_array(new_fds, nfds);
- spin_lock(&files->file_lock);
+ spin_lock_bh(&f->lock);
+ fdt = f->next;
+ f->next = NULL;
+ spin_unlock_bh(&f->lock);
+ while(fdt) {
+ struct fdtable *next = fdt->next;
+ __free_fdtable(fdt);
+ fdt = next;
}
- error = 0;
-out:
- return error;
+}
+
+static void free_fdtable_rcu(struct rcu_head *rcu)
+{
+ struct fdtable *fdt = container_of(rcu, struct fdtable, rcu);
+ int fdset_size, fdarray_size;
+ struct fdtable_defer *fddef;
+
+ BUG_ON(!fdt);
+ fdset_size = fdt->max_fdset / 8;
+ fdarray_size = fdt->max_fds * sizeof(struct file *);
+
+ if (fdt->free_files) {
+ /*
+ * The this fdtable was embedded in the files structure
+ * and the files structure itself was getting destroyed.
+ * It is now safe to free the files structure.
+ */
+ kmem_cache_free(files_cachep, fdt->free_files);
+ return;
+ }
+ if (fdt->max_fdset <= __FD_SETSIZE && fdt->max_fds <= NR_OPEN_DEFAULT) {
+ /*
+ * The fdtable was embedded
+ */
+ return;
+ }
+ if (fdset_size <= PAGE_SIZE && fdarray_size <= PAGE_SIZE) {
+ kfree(fdt->open_fds);
+ kfree(fdt->close_on_exec);
+ kfree(fdt->fd);
+ kfree(fdt);
+ } else {
+ fddef = &get_cpu_var(fdtable_defer_list);
+ spin_lock(&fddef->lock);
+ fdt->next = fddef->next;
+ fddef->next = fdt;
+ /*
+ * vmallocs are handled from the workqueue context.
+ * If the per-cpu workqueue is running, then we
+ * defer work scheduling through a timer.
+ */
+ if (!schedule_work(&fddef->wq))
+ mod_timer(&fddef->timer, 5);
+ spin_unlock(&fddef->lock);
+ put_cpu_var(fdtable_defer_list);
+ }
+}
+
+void free_fdtable(struct fdtable *fdt)
+{
+ if (fdt->free_files || fdt->max_fdset > __FD_SETSIZE ||
+ fdt->max_fds > NR_OPEN_DEFAULT)
+ call_rcu(&fdt->rcu, free_fdtable_rcu);
+}
+
+/*
+ * Expand the fdset in the files_struct. Called with the files spinlock
+ * held for write.
+ */
+static void copy_fdtable(struct fdtable *nfdt, struct fdtable *fdt)
+{
+ int i;
+ int count;
+
+ BUG_ON(nfdt->max_fdset < fdt->max_fdset);
+ BUG_ON(nfdt->max_fds < fdt->max_fds);
+ /* Copy the existing tables and install the new pointers */
+
+ i = fdt->max_fdset / (sizeof(unsigned long) * 8);
+ count = (nfdt->max_fdset - fdt->max_fdset) / 8;
+
+ /*
+ * Don't copy the entire array if the current fdset is
+ * not yet initialised.
+ */
+ if (i) {
+ memcpy (nfdt->open_fds, fdt->open_fds,
+ fdt->max_fdset/8);
+ memcpy (nfdt->close_on_exec, fdt->close_on_exec,
+ fdt->max_fdset/8);
+ memset (&nfdt->open_fds->fds_bits[i], 0, count);
+ memset (&nfdt->close_on_exec->fds_bits[i], 0, count);
+ }
+
+ /* Don't copy/clear the array if we are creating a new
+ fd array for fork() */
+ if (fdt->max_fds) {
+ memcpy(nfdt->fd, fdt->fd,
+ fdt->max_fds * sizeof(struct file *));
+ /* clear the remainder of the array */
+ memset(&nfdt->fd[fdt->max_fds], 0,
+ (nfdt->max_fds - fdt->max_fds) *
+ sizeof(struct file *));
+ }
+ nfdt->next_fd = fdt->next_fd;
}
/*
@@ -157,28 +234,21 @@
vfree(array);
}
-/*
- * Expand the fdset in the files_struct. Called with the files spinlock
- * held for write.
- */
-static int expand_fdset(struct files_struct *files, int nr)
- __releases(file->file_lock)
- __acquires(file->file_lock)
+static struct fdtable *alloc_fdtable(int nr)
{
- fd_set *new_openset = NULL, *new_execset = NULL;
- int error, nfds = 0;
- struct fdtable *fdt;
+ struct fdtable *fdt = NULL;
+ int nfds = 0;
+ fd_set *new_openset = NULL, *new_execset = NULL;
+ struct file **new_fds;
- error = -EMFILE;
- fdt = files_fdtable(files);
- if (fdt->max_fdset >= NR_OPEN || nr >= NR_OPEN)
- goto out;
+ fdt = kmalloc(sizeof(*fdt), GFP_KERNEL);
+ if (!fdt)
+ goto out;
+ memset(fdt, 0, sizeof(*fdt));
- nfds = fdt->max_fdset;
- spin_unlock(&files->file_lock);
-
- /* Expand to the max in easy steps */
- do {
+ nfds = __FD_SETSIZE;
+ /* Expand to the max in easy steps */
+ do {
if (nfds < (PAGE_SIZE * 8))
nfds = PAGE_SIZE * 8;
else {
@@ -188,50 +258,88 @@
}
} while (nfds <= nr);
- error = -ENOMEM;
- new_openset = alloc_fdset(nfds);
- new_execset = alloc_fdset(nfds);
- spin_lock(&files->file_lock);
- if (!new_openset || !new_execset)
+ new_openset = alloc_fdset(nfds);
+ new_execset = alloc_fdset(nfds);
+ if (!new_openset || !new_execset)
+ goto out;
+ fdt->open_fds = new_openset;
+ fdt->close_on_exec = new_execset;
+ fdt->max_fdset = nfds;
+
+ nfds = NR_OPEN_DEFAULT;
+ /*
+ * Expand to the max in easy steps, and keep expanding it until
+ * we have enough for the requested fd array size.
+ */
+ do {
+#if NR_OPEN_DEFAULT < 256
+ if (nfds < 256)
+ nfds = 256;
+ else
+#endif
+ if (nfds < (PAGE_SIZE / sizeof(struct file *)))
+ nfds = PAGE_SIZE / sizeof(struct file *);
+ else {
+ nfds = nfds * 2;
+ if (nfds > NR_OPEN)
+ nfds = NR_OPEN;
+ }
+ } while (nfds <= nr);
+ new_fds = alloc_fd_array(nfds);
+ if (!new_fds)
goto out;
-
- error = 0;
-
- /* Copy the existing tables and install the new pointers */
- fdt = files_fdtable(files);
- if (nfds > fdt->max_fdset) {
- int i = fdt->max_fdset / (sizeof(unsigned long) * 8);
- int count = (nfds - fdt->max_fdset) / 8;
-
- /*
- * Don't copy the entire array if the current fdset is
- * not yet initialised.
- */
- if (i) {
- memcpy (new_openset, fdt->open_fds, fdt->max_fdset/8);
- memcpy (new_execset, fdt->close_on_exec, fdt->max_fdset/8);
- memset (&new_openset->fds_bits[i], 0, count);
- memset (&new_execset->fds_bits[i], 0, count);
- }
-
- nfds = xchg(&fdt->max_fdset, nfds);
- new_openset = xchg(&fdt->open_fds, new_openset);
- new_execset = xchg(&fdt->close_on_exec, new_execset);
- spin_unlock(&files->file_lock);
- free_fdset (new_openset, nfds);
- free_fdset (new_execset, nfds);
- spin_lock(&files->file_lock);
- return 0;
- }
- /* Somebody expanded the array while we slept ... */
-
+ fdt->fd = new_fds;
+ fdt->max_fds = nfds;
+ fdt->free_files = NULL;
+ return fdt;
out:
+ if (new_openset)
+ free_fdset(new_openset, nfds);
+ if (new_execset)
+ free_fdset(new_execset, nfds);
+ kfree(fdt);
+ return NULL;
+}
+
+/*
+ * Expands the file descriptor table - it will allocate a new fdtable and
+ * both fd array and fdset. It is expected to be called with the
+ * files_lock held.
+ */
+static int expand_fdtable(struct files_struct *files, int nr)
+ __releases(files->file_lock)
+ __acquires(files->file_lock)
+{
+ int error = 0;
+ struct fdtable *fdt;
+ struct fdtable *nfdt = NULL;
+
spin_unlock(&files->file_lock);
- if (new_openset)
- free_fdset(new_openset, nfds);
- if (new_execset)
- free_fdset(new_execset, nfds);
+ nfdt = alloc_fdtable(nr);
+ if (!nfdt) {
+ error = -ENOMEM;
+ spin_lock(&files->file_lock);
+ goto out;
+ }
+
spin_lock(&files->file_lock);
+ fdt = files_fdtable(files);
+ /*
+ * Check again since another task may have expanded the
+ * fd table while we dropped the lock
+ */
+ if (nr >= fdt->max_fds || nr >= fdt->max_fdset) {
+ copy_fdtable(nfdt, fdt);
+ } else {
+ /* Somebody expanded while we dropped file_lock */
+ spin_unlock(&files->file_lock);
+ __free_fdtable(nfdt);
+ spin_lock(&files->file_lock);
+ goto out;
+ }
+ rcu_assign_pointer(files->fdt, nfdt);
+ free_fdtable(fdt);
+out:
return error;
}
@@ -246,17 +354,36 @@
struct fdtable *fdt;
fdt = files_fdtable(files);
- if (nr >= fdt->max_fdset) {
- expand = 1;
- if ((err = expand_fdset(files, nr)))
+ if (nr >= fdt->max_fdset || nr >= fdt->max_fds) {
+ if (fdt->max_fdset >= NR_OPEN ||
+ fdt->max_fds >= NR_OPEN || nr >= NR_OPEN) {
+ err = -EMFILE;
goto out;
- }
- if (nr >= fdt->max_fds) {
+ }
expand = 1;
- if ((err = expand_fd_array(files, nr)))
+ if ((err = expand_fdtable(files, nr)))
goto out;
}
err = expand;
out:
return err;
}
+
+static void __devinit fdtable_defer_list_init(int cpu)
+{
+ struct fdtable_defer *fddef = &per_cpu(fdtable_defer_list, cpu);
+ spin_lock_init(&fddef->lock);
+ INIT_WORK(&fddef->wq, (void (*)(void *))free_fdtable_work, fddef);
+ init_timer(&fddef->timer);
+ fddef->timer.data = (unsigned long)fddef;
+ fddef->timer.function = fdtable_timer;
+ fddef->next = NULL;
+}
+
+void __init files_defer_init(void)
+{
+ int i;
+ /* Really early - can't use for_each_cpu */
+ for (i = 0; i < NR_CPUS; i++)
+ fdtable_defer_list_init(i);
+}