fd: use smr for managing struct pwd

This has a side effect of eliminating filedesc slock/sunlock during path
lookup, which in turn removes contention vs concurrent modifications to the fd
table.

Reviewed by:	markj, kib
Differential Revision:	https://reviews.freebsd.org/D23889

1 files changed, 44 insertions, 3 deletions

diff --git a/sys/sys/filedesc.h b/sys/sys/filedesc.h
index 116649757de4..aa0a5770630f 100644
--- a/sys/sys/filedesc.h
+++ b/sys/sys/filedesc.h
@@ -42,6 +42,8 @@
 #include <sys/priority.h>
 #include <sys/seqc.h>
 #include <sys/sx.h>
+#include <sys/_smr.h>
+#include <sys/smr_types.h>
 
 #include <machine/_limits.h>
 
@@ -76,16 +78,23 @@ struct fdescenttbl {
  */
 #define NDSLOTTYPE	u_long
 
+/*
+ * This struct is copy-on-write and allocated from an SMR zone.
+ * All fields are constant after initialization apart from the reference count.
+ *
+ * Check pwd_* routines for usage.
+ */
 struct pwd {
 	volatile u_int pwd_refcount;
 	struct	vnode *pwd_cdir;		/* current directory */
 	struct	vnode *pwd_rdir;		/* root directory */
 	struct	vnode *pwd_jdir;		/* jail root directory */
 };
+typedef SMR_POINTER(struct pwd *) smrpwd_t;
 
 struct filedesc {
 	struct	fdescenttbl *fd_files;	/* open files table */
-	struct	pwd *fd_pwd;		/* directories */
+	smrpwd_t fd_pwd;		/* directories */
 	NDSLOTTYPE *fd_map;		/* bitmap of free fds */
 	int	fd_lastfile;		/* high-water mark of fd_ofiles */
 	int	fd_freefile;		/* approx. next free file */
@@ -141,6 +150,38 @@ struct filedesc_to_leader {
 					    SX_NOTRECURSED)
 #define	FILEDESC_UNLOCK_ASSERT(fdp)	sx_assert(&(fdp)->fd_sx, SX_UNLOCKED)
 
+#define	FILEDESC_LOCKED_LOAD_PWD(fdp)	({					\
+	struct filedesc *_fdp = (fdp);						\
+	struct pwd *_pwd;							\
+	_pwd = smr_serialized_load(&(_fdp)->fd_pwd,				\
+	    (FILEDESC_LOCK_ASSERT(_fdp), true));				\
+	_pwd;									\
+})
+
+#define	FILEDESC_XLOCKED_LOAD_PWD(fdp)	({					\
+	struct filedesc *_fdp = (fdp);						\
+	struct pwd *_pwd;							\
+	_pwd = smr_serialized_load(&(_fdp)->fd_pwd,				\
+	    (FILEDESC_XLOCK_ASSERT(_fdp), true));				\
+	_pwd;									\
+})
+
+#else
+
+/*
+ * Accessor for libkvm et al.
+ */
+#define	FILEDESC_KVM_LOAD_PWD(fdp)	({					\
+	struct filedesc *_fdp = (fdp);						\
+	struct pwd *_pwd;							\
+	_pwd = smr_kvm_load(&(_fdp)->fd_pwd);					\
+	_pwd;									\
+})
+
+#endif
+
+#ifdef _KERNEL
+
 /* Operation types for kern_dup(). */
 enum {
 	FDDUP_NORMAL,		/* dup() behavior. */
@@ -265,8 +306,8 @@ static inline void
 pwd_set(struct filedesc *fdp, struct pwd *newpwd)
 {
 
-	FILEDESC_XLOCK_ASSERT(fdp);
-	fdp->fd_pwd = newpwd;
+	smr_serialized_store(&fdp->fd_pwd, newpwd,
+	    (FILEDESC_XLOCK_ASSERT(fdp), true));
 }
 
 #endif /* _KERNEL */