aio: document, clarify aio_read_events() and shadow_tail

Signed-off-by: Kent Overstreet <koverstreet@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/aio.c b/fs/aio.c
index 66ebd56fb968aa531d95ffd0d4a5b3739bd3246b..36c1944f6ea87790d32e24733b01812efee1c0ce 100644 (file)
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -103,6 +103,19 @@ struct kioctx {
        struct {
                struct mutex    ring_lock;
                wait_queue_head_t wait;
+
+               /*
+                * Copy of the real tail, that aio_complete uses - to reduce
+                * cacheline bouncing. The real tail will tend to be much more
+                * contended - since typically events are delivered one at a
+                * time, and then aio_read_events() slurps them up a bunch at a
+                * time - so it's helpful if aio_read_events() isn't also
+                * contending for the tail. So, aio_complete() updates
+                * shadow_tail whenever it updates tail.
+                *
+                * Also needed because tail is used as a hacky lock and isn't
+                * always the real tail.
+                */
                unsigned        shadow_tail;
        } ____cacheline_aligned_in_smp;
 
@@ -860,10 +873,7 @@ static long aio_read_events_ring(struct kioctx *ctx,
        long ret = 0;
        int copy_ret;
 
-       if (!mutex_trylock(&ctx->ring_lock)) {
-               __set_current_state(TASK_RUNNING);
-               mutex_lock(&ctx->ring_lock);
-       }
+       mutex_lock(&ctx->ring_lock);
 
        ring = kmap_atomic(ctx->ring_pages[0]);
        head = ring->head;
@@ -874,8 +884,6 @@ static long aio_read_events_ring(struct kioctx *ctx,
        if (head == ctx->shadow_tail)
                goto out;
 
-       __set_current_state(TASK_RUNNING);
-
        while (ret < nr) {
                long avail = (head < ctx->shadow_tail
                              ? ctx->shadow_tail : ctx->nr) - head;
@@ -954,6 +962,20 @@ static long read_events(struct kioctx *ctx, long min_nr, long nr,
                until = timespec_to_ktime(ts);
        }
 
+       /*
+        * Note that aio_read_events() is being called as the conditional - i.e.
+        * we're calling it after prepare_to_wait() has set task state to
+        * TASK_INTERRUPTIBLE.
+        *
+        * But aio_read_events() can block, and if it blocks it's going to flip
+        * the task state back to TASK_RUNNING.
+        *
+        * This should be ok, provided it doesn't flip the state back to
+        * TASK_RUNNING and return 0 too much - that causes us to spin. That
+        * will only happen if the mutex_lock() call blocks, and we then find
+        * the ringbuffer empty. So in practice we should be ok, but it's
+        * something to be aware of when touching this code.
+        */
        wait_event_interruptible_hrtimeout(ctx->wait,
                        aio_read_events(ctx, min_nr, nr, event, &ret), until);