From 253ca8678d30bcf94410b54476fc1e0f1627a137 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sun, 26 Nov 2023 12:24:38 -0800 Subject: Improve __fget_files_rcu() code generation (and thus __fget_light()) Commit 0ede61d8589c ("file: convert to SLAB_TYPESAFE_BY_RCU") caused a performance regression as reported by the kernel test robot. The __fget_light() function is one of those critical ones for some loads, and the code generation was unnecessarily impacted. Let's just write that function to better. Reported-by: kernel test robot Cc: Christian Brauner Cc: Jann Horn Cc: Mateusz Guzik Closes: https://lore.kernel.org/oe-lkp/202311201406.2022ca3f-oliver.sang@intel.com Signed-off-by: Linus Torvalds Link: https://lore.kernel.org/r/CAHk-=wiCJtLbFWNURB34b9a_R_unaH3CiMRXfkR0-iihB_z68A@mail.gmail.com Signed-off-by: Christian Brauner --- fs/file.c | 51 +++++++++++++++++++++++++++++++++------------------ 1 file changed, 33 insertions(+), 18 deletions(-) (limited to 'fs/file.c') diff --git a/fs/file.c b/fs/file.c index 5fb0b146e79e..50df31e104a5 100644 --- a/fs/file.c +++ b/fs/file.c @@ -959,31 +959,45 @@ static inline struct file *__fget_files_rcu(struct files_struct *files, struct file *file; struct fdtable *fdt = rcu_dereference_raw(files->fdt); struct file __rcu **fdentry; + unsigned long nospec_mask; - if (unlikely(fd >= fdt->max_fds)) - return NULL; - - fdentry = fdt->fd + array_index_nospec(fd, fdt->max_fds); + /* Mask is a 0 for invalid fd's, ~0 for valid ones */ + nospec_mask = array_index_mask_nospec(fd, fdt->max_fds); /* - * Ok, we have a file pointer. However, because we do - * this all locklessly under RCU, we may be racing with - * that file being closed. - * - * Such a race can take two forms: - * - * (a) the file ref already went down to zero and the - * file hasn't been reused yet or the file count - * isn't zero but the file has already been reused. + * fdentry points to the 'fd' offset, or fdt->fd[0]. + * Loading from fdt->fd[0] is always safe, because the + * array always exists. */ - file = __get_file_rcu(fdentry); + fdentry = fdt->fd + (fd & nospec_mask); + + /* Do the load, then mask any invalid result */ + file = rcu_dereference_raw(*fdentry); + file = (void *)(nospec_mask & (unsigned long)file); if (unlikely(!file)) return NULL; - if (unlikely(IS_ERR(file))) + /* + * Ok, we have a file pointer that was valid at + * some point, but it might have become stale since. + * + * We need to confirm it by incrementing the refcount + * and then check the lookup again. + * + * atomic_long_inc_not_zero() gives us a full memory + * barrier. We only really need an 'acquire' one to + * protect the loads below, but we don't have that. + */ + if (unlikely(!atomic_long_inc_not_zero(&file->f_count))) continue; /* + * Such a race can take two forms: + * + * (a) the file ref already went down to zero and the + * file hasn't been reused yet or the file count + * isn't zero but the file has already been reused. + * * (b) the file table entry has changed under us. * Note that we don't need to re-check the 'fdt->fd' * pointer having changed, because it always goes @@ -991,7 +1005,8 @@ static inline struct file *__fget_files_rcu(struct files_struct *files, * * If so, we need to put our ref and try again. */ - if (unlikely(rcu_dereference_raw(files->fdt) != fdt)) { + if (unlikely(file != rcu_dereference_raw(*fdentry)) || + unlikely(rcu_dereference_raw(files->fdt) != fdt)) { fput(file); continue; } @@ -1128,13 +1143,13 @@ static unsigned long __fget_light(unsigned int fd, fmode_t mask) * atomic_read_acquire() pairs with atomic_dec_and_test() in * put_files_struct(). */ - if (atomic_read_acquire(&files->count) == 1) { + if (likely(atomic_read_acquire(&files->count) == 1)) { file = files_lookup_fd_raw(files, fd); if (!file || unlikely(file->f_mode & mask)) return 0; return (unsigned long)file; } else { - file = __fget(fd, mask); + file = __fget_files(files, fd, mask); if (!file) return 0; return FDPUT_FPUT | (unsigned long)file; -- cgit v1.2.3