ANDROID: dma-buf: Hoist files_fdtable()->max_fds

The READ_ONCE inside files_fdtable depends on volatile to prevent
caching the result in a register, resulting in 2 loads every loop
iteration. Where max_fds is used (entirely from within locked sections)
max_fds cannot change, so avoid the overhead by reading once into a
local variable.

Bug: 424648392
Change-Id: If53534445752a50c3b4dffee186606dc16b0614f
Suggested-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: T.J. Mercier <tjmercier@google.com>

drivers/dma-buf/dma-buf.c

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index c3ae181..bcb645e 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -664,7 +664,8 @@ void put_dmabuf_info(struct task_dma_buf_info *dmabuf_info)
 
 #define COUNT_DMABUF_FDS(file_lookup_func) ({ \
 	size_t count = 0; \
-	for (unsigned int n = 0; n < files_fdtable(current->files)->max_fds; ++n) { \
+	unsigned int max_fds = files_fdtable(current->files)->max_fds; \
+	for (unsigned int n = 0; n < max_fds; ++n) { \
 		struct file *file = file_lookup_func(current->files, n); \
 		if (file && is_dma_buf_file(file)) \
 			++count; \
@@ -697,6 +698,7 @@ int dma_buf_begin_new_exec(struct files_struct *old_files)
 	if (my_files) {
 		size_t num_dmabuf_fds, num_dmabuf_fds_check;
 		unsigned int retries = 0;
+		unsigned int max_fds;
 
 		/* Attempt to count dmabuf FDs locklessly before allocating */
 		rcu_read_lock();
@@ -725,7 +727,8 @@ int dma_buf_begin_new_exec(struct files_struct *old_files)
 			goto retry;
 		}
 
-		for (unsigned int n = 0; n < files_fdtable(my_files)->max_fds; n++) {
+		max_fds = files_fdtable(my_files)->max_fds;
+		for (unsigned int n = 0; n < max_fds; n++) {
 			struct file *file = files_lookup_fd_locked(my_files, n);
 			int err;