aio: reqs_active -> reqs_available

The number of outstanding kiocbs is one of the few shared things left that
has to be touched for every kiocb - it'd be nice to make it percpu.

We can make it per cpu by treating it like an allocation problem: we have
a maximum number of kiocbs that can be outstanding (i.e.  slots) - then we
just allocate and free slots, and we know how to write per cpu allocators.

So as prep work for that, we convert reqs_active to reqs_available.

Signed-off-by: Kent Overstreet <koverstreet@google.com>
Cc: Zach Brown <zab@redhat.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Asai Thambi S P <asamymuthupa@micron.com>
Cc: Selvan Mani <smani@micron.com>
Cc: Sam Bradshaw <sbradshaw@micron.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Reviewed-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>

1 files changed, 22 insertions, 16 deletions

diff --git a/fs/aio.c b/fs/aio.c
index dedeea01e4e4..0e23dfa77b0e 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -94,7 +94,13 @@ struct kioctx {
         struct work_struct      rcu_work;
 
         struct {
-                atomic_t        reqs_active;
+                /*
+                 * This counts the number of available slots in the ringbuffer,
+                 * so we avoid overflowing it: it's decremented (if positive)
+                 * when allocating a kiocb and incremented when the resulting
+                 * io_event is pulled off the ringbuffer.
+                 */
+                atomic_t        reqs_available;
         } ____cacheline_aligned_in_smp;
 
         struct {
@@ -404,19 +410,20 @@ static void free_ioctx(struct kioctx *ctx)
         head = ring->head;
         kunmap_atomic(ring);
 
-        while (atomic_read(&ctx->reqs_active) > 0) {
+        while (atomic_read(&ctx->reqs_available) < ctx->nr_events - 1) {
                 wait_event(ctx->wait,
-                                head != ctx->tail ||
-                                atomic_read(&ctx->reqs_active) <= 0);
+                           (head != ctx->tail) ||
+                           (atomic_read(&ctx->reqs_available) >=
+                            ctx->nr_events - 1));
 
                 avail = (head <= ctx->tail ? ctx->tail : ctx->nr_events) - head;
 
-                atomic_sub(avail, &ctx->reqs_active);
+                atomic_add(avail, &ctx->reqs_available);
                 head += avail;
                 head %= ctx->nr_events;
         }
 
-        WARN_ON(atomic_read(&ctx->reqs_active) < 0);
+        WARN_ON(atomic_read(&ctx->reqs_available) > ctx->nr_events - 1);
 
         aio_free_ring(ctx);
 
@@ -475,6 +482,8 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)
         if (aio_setup_ring(ctx) < 0)
                 goto out_freectx;
 
+        atomic_set(&ctx->reqs_available, ctx->nr_events - 1);
+
         /* limit the number of system wide aios */
         spin_lock(&aio_nr_lock);
         if (aio_nr + nr_events > aio_max_nr ||
@@ -586,7 +595,7 @@ void exit_aio(struct mm_struct *mm)
                                 "exit_aio:ioctx still alive: %d %d %d\n",
                                 atomic_read(&ctx->users),
                                 atomic_read(&ctx->dead),
-                                atomic_read(&ctx->reqs_active));
+                                atomic_read(&ctx->reqs_available));
                 /*
                  * We don't need to bother with munmap() here -
                  * exit_mmap(mm) is coming and it'll unmap everything.
@@ -615,12 +624,9 @@ static inline struct kiocb *aio_get_req(struct kioctx *ctx)
 {
         struct kiocb *req;
 
-        if (atomic_read(&ctx->reqs_active) >= ctx->nr_events)
+        if (atomic_dec_if_positive(&ctx->reqs_available) <= 0)
                 return NULL;
 
-        if (atomic_inc_return(&ctx->reqs_active) > ctx->nr_events - 1)
-                goto out_put;
-
         req = kmem_cache_alloc(kiocb_cachep, GFP_KERNEL|__GFP_ZERO);
         if (unlikely(!req))
                 goto out_put;
@@ -630,7 +636,7 @@ static inline struct kiocb *aio_get_req(struct kioctx *ctx)
 
         return req;
 out_put:
-        atomic_dec(&ctx->reqs_active);
+        atomic_inc(&ctx->reqs_available);
         return NULL;
 }
 
@@ -701,7 +707,7 @@ void aio_complete(struct kiocb *iocb, long res, long res2)
 
         /*
          * Take rcu_read_lock() in case the kioctx is being destroyed, as we
-         * need to issue a wakeup after decrementing reqs_active.
+         * need to issue a wakeup after incrementing reqs_available.
          */
         rcu_read_lock();
 
@@ -719,7 +725,7 @@ void aio_complete(struct kiocb *iocb, long res, long res2)
          */
         if (unlikely(xchg(&iocb->ki_cancel,
                           KIOCB_CANCELLED) == KIOCB_CANCELLED)) {
-                atomic_dec(&ctx->reqs_active);
+                atomic_inc(&ctx->reqs_available);
                 /* Still need the wake_up in case free_ioctx is waiting */
                 goto put_rq;
         }
@@ -857,7 +863,7 @@ static long aio_read_events_ring(struct kioctx *ctx,
 
         pr_debug("%li  h%u t%u\n", ret, head, ctx->tail);
 
-        atomic_sub(ret, &ctx->reqs_active);
+        atomic_add(ret, &ctx->reqs_available);
 out:
         mutex_unlock(&ctx->ring_lock);
 
@@ -1241,7 +1247,7 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
         aio_put_req(req);       /* drop extra ref to req */
         return 0;
 out_put_req:
-        atomic_dec(&ctx->reqs_active);
+        atomic_inc(&ctx->reqs_available);
         aio_put_req(req);       /* drop extra ref to req */
         aio_put_req(req);       /* drop i/o ref to req */
         return ret;