[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>

1 files changed, 258 insertions, 131 deletions

diff --git a/fs/file.c b/fs/file.c
index f5926ce73f37..2127a7b9dc3a 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 @@ void free_fd_array(struct file **array, int num)
                 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;
-        struct fdtable *fdt;
+        int fdset_size, fdarray_size;
 
-        
-        error = -EMFILE;
-        fdt = files_fdtable(files);
-        if (fdt->max_fds >= NR_OPEN || nr >= NR_OPEN)
-                goto out;
+        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);
+}
 
-        nfds = fdt->max_fds;
-        spin_unlock(&files->file_lock);
+static void fdtable_timer(unsigned long data)
+{
+        struct fdtable_defer *fddef = (struct fdtable_defer *)data;
 
-        /* 
-         * Expand to the max in easy steps, and keep expanding it until
-         * we have enough for the requested fd array size. 
+        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);
+}
 
-        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);
+static void free_fdtable_work(struct fdtable_defer *f)
+{
+        struct fdtable *fdt;
 
-        error = -ENOMEM;
-        new_fds = alloc_fd_array(nfds);
-        spin_lock(&files->file_lock);
-        if (!new_fds)
-                goto out;
+        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;
+        }
+}
 
-        /* Copy the existing array and install the new pointer */
-        fdt = files_fdtable(files);
+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;
 
-        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);
-                }
+        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 {
-                /* Somebody expanded the array while we slept ... */
-                spin_unlock(&files->file_lock);
-                free_fd_array(new_fds, nfds);
-                spin_lock(&files->file_lock);
+                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);
         }
-        error = 0;
-out:
-        return error;
+}
+
+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 @@ void free_fdset(fd_set *array, int num)
                 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;
-
-        error = -EMFILE;
-        fdt = files_fdtable(files);
-        if (fdt->max_fdset >= NR_OPEN || nr >= NR_OPEN)
-                goto out;
+        struct fdtable *fdt = NULL;
+        int nfds = 0;
+        fd_set *new_openset = NULL, *new_execset = NULL;
+        struct file **new_fds;
 
-        nfds = fdt->max_fdset;
-        spin_unlock(&files->file_lock);
+        fdt = kmalloc(sizeof(*fdt), GFP_KERNEL);
+        if (!fdt)
+                goto out;
+        memset(fdt, 0, sizeof(*fdt));
 
-        /* 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 @@ static int expand_fdset(struct files_struct *files, int nr)
                 }
         } 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;
+        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;
+}
 
-        error = 0;
-        
-        /* Copy the existing tables and install the new pointers */
+/*
+ * 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);
+        nfdt = alloc_fdtable(nr);
+        if (!nfdt) {
+                error = -ENOMEM;
+                spin_lock(&files->file_lock);
+                goto out;
+        }
+
+        spin_lock(&files->file_lock);
         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);
+        /*
+         * 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_fdset (new_openset, nfds);
-                free_fdset (new_execset, nfds);
+                __free_fdtable(nfdt);
                 spin_lock(&files->file_lock);
-                return 0;
-        } 
-        /* Somebody expanded the array while we slept ... */
-
+                goto out;
+        }
+        rcu_assign_pointer(files->fdt, nfdt);
+        free_fdtable(fdt);
 out:
-        spin_unlock(&files->file_lock);
-        if (new_openset)
-                free_fdset(new_openset, nfds);
-        if (new_execset)
-                free_fdset(new_execset, nfds);
-        spin_lock(&files->file_lock);
         return error;
 }
 
@@ -246,17 +354,36 @@ int expand_files(struct files_struct *files, int nr)
         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);
+}