lib/vfscore: Reorder locks in *at syscalls

*at syscalls are implemented in Unikraft in two stages: (1) build the
full target path relative to the supplied dirfd, and (2) delegate to the
regular syscall. Previously these syscalls held the lock on the dirfd
vnode throughout steps (1) and (2); however, the sub-syscalls often need
to acquire the same lock (along with perhaps others), causing
double-lock asserts to trigger.
This commit changes this logic to only hold the vnode lock during step
(1) and release it immediately before (2). While this relaxes the
previous atomicity guarantees somewhat, holding the vnode lock on dirfd
only prevented concurrent changes to first-generation descendents, and
unorthogonal and arguably unexpected behavior. The useful purpose of
holding the dirfd lock is to ensure we build a full path that was (at
one point in time) valid, with the sub-syscalls implementing their own
atomicity guarantees as per usual.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #951

lib/vfscore/main.c

@@ -205,11 +205,11 @@ UK_LLSYSCALL_R_DEFINE(int, openat, int, dirfd, const char *, pathname,
 	strlcat(p, "/", PATH_MAX);
 	strlcat(p, pathname, PATH_MAX);
 
-	error = uk_syscall_r_open((long int)p, flags, mode);
-
 	vn_unlock(vp);
 	fdrop(fp);
 
+	error = uk_syscall_r_open((long int)p, flags, mode);
+
 	return error;
 }
 
@@ -1295,11 +1295,11 @@ UK_SYSCALL_R_DEFINE(int, mkdirat, int, dirfd,
 	strlcat(p, "/", PATH_MAX);
 	strlcat(p, pathname, PATH_MAX);
 
-	error = uk_syscall_r_mkdir((long) p, (long) mode);
-
 	vn_unlock(vp);
 	fdrop(fp);
 
+	error = uk_syscall_r_mkdir((long) p, (long) mode);
+
 	return error;
 }
 
@@ -1745,14 +1745,14 @@ static int __fxstatat_helper(int ver __unused, int dirfd, const char *pathname,
 	strlcat(p, "/", PATH_MAX);
 	strlcat(p, pathname, PATH_MAX);
 
+	vn_unlock(vp);
+	fdrop(fp);
+
 	if (flags & AT_SYMLINK_NOFOLLOW)
 		error = uk_syscall_r_lstat((long) p, (long) st);
 	else
 		error = uk_syscall_r_stat((long) p, (long) st);
 
-	vn_unlock(vp);
-	fdrop(fp);
-
 	return error;
 }
 
@@ -2235,11 +2235,11 @@ UK_SYSCALL_R_DEFINE(int, faccessat, int, dirfd, const char*, pathname, int, mode
 	strlcat(p, "/", PATH_MAX);
 	strlcat(p, pathname, PATH_MAX);
 
-	error = uk_syscall_r_access((long) p, (long) mode);
-
 	vn_unlock(vp);
 	fdrop(fp);
 
+	error = uk_syscall_r_access((long) p, (long) mode);
+
 	out_error:
 	return error;
 }