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authorAnatol Pomozov <anatol.pomozov@gmail.com>2014-04-15 11:31:33 -0700
committerBenjamin LaHaise <bcrl@kvack.org>2014-04-16 13:38:04 -0400
commite02ba72aabfade4c9cd6e3263e9b57bf890ad25c (patch)
tree153f7763591a0cb4497d3b72d9ca644d2f3d4aae /fs
parent10ec34fcb100412ab186c141a9c3557d1270effd (diff)
downloadlinux-stericsson-e02ba72aabfade4c9cd6e3263e9b57bf890ad25c.tar.gz
aio: block io_destroy() until all context requests are completed
deletes aio context and all resources related to. It makes sense that no IO operations connected to the context should be running after the context is destroyed. As we removed io_context we have no chance to get requests status or call io_getevents(). man page for io_destroy says that this function may block until all context's requests are completed. Before kernel 3.11 io_destroy() blocked indeed, but since aio refactoring in 3.11 it is not true anymore. Here is a pseudo-code that shows a testcase for a race condition discovered in 3.11: initialize io_context io_submit(read to buffer) io_destroy() // context is destroyed so we can free the resources free(buffers); // if the buffer is allocated by some other user he'll be surprised // to learn that the buffer still filled by an outstanding operation // from the destroyed io_context The fix is straight-forward - add a completion struct and wait on it in io_destroy, complete() should be called when number of in-fligh requests reaches zero. If two or more io_destroy() called for the same context simultaneously then only the first one waits for IO completion, other calls behaviour is undefined. Tested: ran http://pastebin.com/LrPsQ4RL testcase for several hours and do not see the race condition anymore. Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com> Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Diffstat (limited to 'fs')
-rw-r--r--fs/aio.c36
1 files changed, 32 insertions, 4 deletions
diff --git a/fs/aio.c b/fs/aio.c
index 12a3de0ee6da..2adbb0398ab9 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -112,6 +112,11 @@ struct kioctx {
struct work_struct free_work;
+ /*
+ * signals when all in-flight requests are done
+ */
+ struct completion *requests_done;
+
struct {
/*
* This counts the number of available slots in the ringbuffer,
@@ -508,6 +513,10 @@ static void free_ioctx_reqs(struct percpu_ref *ref)
{
struct kioctx *ctx = container_of(ref, struct kioctx, reqs);
+ /* At this point we know that there are no any in-flight requests */
+ if (ctx->requests_done)
+ complete(ctx->requests_done);
+
INIT_WORK(&ctx->free_work, free_ioctx);
schedule_work(&ctx->free_work);
}
@@ -718,7 +727,8 @@ err:
* when the processes owning a context have all exited to encourage
* the rapid destruction of the kioctx.
*/
-static void kill_ioctx(struct mm_struct *mm, struct kioctx *ctx)
+static void kill_ioctx(struct mm_struct *mm, struct kioctx *ctx,
+ struct completion *requests_done)
{
if (!atomic_xchg(&ctx->dead, 1)) {
struct kioctx_table *table;
@@ -747,7 +757,11 @@ static void kill_ioctx(struct mm_struct *mm, struct kioctx *ctx)
if (ctx->mmap_size)
vm_munmap(ctx->mmap_base, ctx->mmap_size);
+ ctx->requests_done = requests_done;
percpu_ref_kill(&ctx->users);
+ } else {
+ if (requests_done)
+ complete(requests_done);
}
}
@@ -809,7 +823,7 @@ void exit_aio(struct mm_struct *mm)
*/
ctx->mmap_size = 0;
- kill_ioctx(mm, ctx);
+ kill_ioctx(mm, ctx, NULL);
}
}
@@ -1185,7 +1199,7 @@ SYSCALL_DEFINE2(io_setup, unsigned, nr_events, aio_context_t __user *, ctxp)
if (!IS_ERR(ioctx)) {
ret = put_user(ioctx->user_id, ctxp);
if (ret)
- kill_ioctx(current->mm, ioctx);
+ kill_ioctx(current->mm, ioctx, NULL);
percpu_ref_put(&ioctx->users);
}
@@ -1203,8 +1217,22 @@ SYSCALL_DEFINE1(io_destroy, aio_context_t, ctx)
{
struct kioctx *ioctx = lookup_ioctx(ctx);
if (likely(NULL != ioctx)) {
- kill_ioctx(current->mm, ioctx);
+ struct completion requests_done =
+ COMPLETION_INITIALIZER_ONSTACK(requests_done);
+
+ /* Pass requests_done to kill_ioctx() where it can be set
+ * in a thread-safe way. If we try to set it here then we have
+ * a race condition if two io_destroy() called simultaneously.
+ */
+ kill_ioctx(current->mm, ioctx, &requests_done);
percpu_ref_put(&ioctx->users);
+
+ /* Wait until all IO for the context are done. Otherwise kernel
+ * keep using user-space buffers even if user thinks the context
+ * is destroyed.
+ */
+ wait_for_completion(&requests_done);
+
return 0;
}
pr_debug("EINVAL: io_destroy: invalid context id\n");