Commit 021a160a authored by Linus Torvalds's avatar Linus Torvalds Committed by Christian Brauner
Browse files

fs: use __fput_sync in close(2) 



close(2) is a special case which guarantees a shallow kernel stack,
making delegation to task_work machinery unnecessary. Said delegation is
problematic as it involves atomic ops and interrupt masking trips, none
of which are cheap on x86-64. Forcing close(2) to do it looks like an
oversight in the original work.

Moreover presence of CONFIG_RSEQ adds an additional overhead as fput()
-> task_work_add(..., TWA_RESUME) -> set_notify_resume() makes the
thread returning to userspace land in resume_user_mode_work(), where
rseq_handle_notify_resume takes a SMAP round-trip if rseq is enabled for
the thread (and it is by default with contemporary glibc).

Sample result when benchmarking open1_processes -t 1 from will-it-scale
(that's an open + close loop) + tmpfs on /tmp, running on the Sapphire
Rapid CPU (ops/s):
stock+RSEQ:     1329857
stock-RSEQ:     1421667 (+7%)
patched:        1523521 (+14.5% / +7%) (with / without rseq)

Patched result is the same regardless of rseq as the codepath is avoided.

Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent ed192c59

fs/file_table.c

+1 −4
Original line number Diff line number Diff line
@@ -461,12 +461,9 @@ void fput(struct file *file) 
 */

void __fput_sync(struct file *file)

{

	if (atomic_long_dec_and_test(&file->f_count)) {

		struct task_struct *task = current;

		BUG_ON(!(task->flags & PF_KTHREAD));

	if (atomic_long_dec_and_test(&file->f_count))

		__fput(file);

}

}



EXPORT_SYMBOL(fput);

EXPORT_SYMBOL(__fput_sync);

fs/open.c

+24 −3
Original line number Diff line number Diff line
@@ -1503,7 +1503,7 @@ SYSCALL_DEFINE2(creat, const char __user *, pathname, umode_t, mode) 
 * "id" is the POSIX thread ID. We use the

 * files pointer for this..

 */

int filp_close(struct file *filp, fl_owner_t id)

static int filp_flush(struct file *filp, fl_owner_t id)

{

	int retval = 0;
@@ -1520,10 +1520,18 @@ int filp_close(struct file *filp, fl_owner_t id) 
		dnotify_flush(filp, id);

		locks_remove_posix(filp, id);

	}

	fput(filp);

	return retval;

}



int filp_close(struct file *filp, fl_owner_t id)

{

	int retval;



	retval = filp_flush(filp, id);

	fput(filp);



	return retval;

}

EXPORT_SYMBOL(filp_close);



/*
@@ -1533,7 +1541,20 @@ EXPORT_SYMBOL(filp_close); 
 */

SYSCALL_DEFINE1(close, unsigned int, fd)

{

	int retval = close_fd(fd);

	int retval;

	struct file *file;



	file = close_fd_get_file(fd);

	if (!file)

		return -EBADF;



	retval = filp_flush(file, current->files);



	/*

	 * We're returning to user space. Don't bother

	 * with any delayed fput() cases.

	 */

	__fput_sync(file);



	/* can't restart close syscall because file table entry was cleared */

	if (unlikely(retval == -ERESTARTSYS ||