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

lib/libprocstat/libprocstat.c

@@ -460,6 +460,7 @@ procstat_getfiles_kvm(struct procstat *procstat, struct kinfo_proc *kp, int mmap
 	struct file file;
 	struct filedesc filed;
 	struct pwd pwd;
+	unsigned long pwd_addr;
 	struct vm_map_entry vmentry;
 	struct vm_object object;
 	struct vmspace vmspace;
@@ -488,10 +489,10 @@ procstat_getfiles_kvm(struct procstat *procstat, struct kinfo_proc *kp, int mmap
 		return (NULL);
 	}
 	haspwd = false;
-	if (filed.fd_pwd != NULL) {
-		if (!kvm_read_all(kd, (unsigned long)filed.fd_pwd, &pwd,
-		    sizeof(pwd))) {
-			warnx("can't read fd_pwd at %p", (void *)filed.fd_pwd);
+	pwd_addr = (unsigned long)(FILEDESC_KVM_LOAD_PWD(&filed));
+	if (pwd_addr != 0) {
+		if (!kvm_read_all(kd, pwd_addr, &pwd, sizeof(pwd))) {
+			warnx("can't read fd_pwd at %p", (void *)pwd_addr);
 			return (NULL);
 		}
 		haspwd = true;

sys/kern/kern_descrip.c

@@ -69,6 +69,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/sbuf.h>
 #include <sys/signalvar.h>
 #include <sys/kdb.h>
+#include <sys/smr.h>
 #include <sys/stat.h>
 #include <sys/sx.h>
 #include <sys/syscallsubr.h>
@@ -101,6 +102,8 @@ MALLOC_DECLARE(M_FADVISE);
 
 static __read_mostly uma_zone_t file_zone;
 static __read_mostly uma_zone_t filedesc0_zone;
+static __read_mostly uma_zone_t pwd_zone;
+static __read_mostly smr_t pwd_smr;
 
 static int	closefp(struct filedesc *fdp, int fd, struct file *fp,
 		    struct thread *td, int holdleaders);
@@ -1985,6 +1988,7 @@ fdinit(struct filedesc *fdp, bool prepfiles)
 {
 	struct filedesc0 *newfdp0;
 	struct filedesc *newfdp;
+	struct pwd *newpwd;
 
 	newfdp0 = uma_zalloc(filedesc0_zone, M_WAITOK | M_ZERO);
 	newfdp = &newfdp0->fd_fd;
@@ -2000,7 +2004,8 @@ fdinit(struct filedesc *fdp, bool prepfiles)
 	newfdp->fd_files->fdt_nfiles = NDFILE;
 
 	if (fdp == NULL) {
-		newfdp->fd_pwd = pwd_alloc();
+		newpwd = pwd_alloc();
+		smr_serialized_store(&newfdp->fd_pwd, newpwd, true);
 		return (newfdp);
 	}
 
@@ -2008,7 +2013,8 @@ fdinit(struct filedesc *fdp, bool prepfiles)
 		fdgrowtable(newfdp, fdp->fd_lastfile + 1);
 
 	FILEDESC_SLOCK(fdp);
-	newfdp->fd_pwd = pwd_hold_filedesc(fdp);
+	newpwd = pwd_hold_filedesc(fdp);
+	smr_serialized_store(&newfdp->fd_pwd, newpwd, true);
 
 	if (!prepfiles) {
 		FILEDESC_SUNLOCK(fdp);
@@ -2328,7 +2334,7 @@ fdescfree(struct thread *td)
 		return;
 
 	FILEDESC_XLOCK(fdp);
-	pwd = fdp->fd_pwd;
+	pwd = FILEDESC_XLOCKED_LOAD_PWD(fdp);
 	pwd_set(fdp, NULL);
 	FILEDESC_XUNLOCK(fdp);
 
@@ -2341,7 +2347,7 @@ void
 fdescfree_remapped(struct filedesc *fdp)
 {
 
-	pwd_drop(fdp->fd_pwd);
+	pwd_drop(smr_serialized_load(&fdp->fd_pwd, true));
 	fdescfree_fds(curthread, fdp, 0);
 }
 
@@ -3277,7 +3283,7 @@ pwd_hold_filedesc(struct filedesc *fdp)
 	struct pwd *pwd;
 
 	FILEDESC_LOCK_ASSERT(fdp);
-	pwd = fdp->fd_pwd;
+	pwd = FILEDESC_LOCKED_LOAD_PWD(fdp);
 	if (pwd != NULL)
 		refcount_acquire(&pwd->pwd_refcount);
 	return (pwd);
@@ -3291,11 +3297,14 @@ pwd_hold(struct thread *td)
 
 	fdp = td->td_proc->p_fd;
 
-	FILEDESC_SLOCK(fdp);
-	pwd = fdp->fd_pwd;
+	smr_enter(pwd_smr);
+	for (;;) {
+		pwd = smr_entered_load(&fdp->fd_pwd, pwd_smr);
 		MPASS(pwd != NULL);
-	refcount_acquire(&pwd->pwd_refcount);
-	FILEDESC_SUNLOCK(fdp);
+		if (refcount_acquire_if_not_zero(&pwd->pwd_refcount))
+			break;
+	}
+	smr_exit(pwd_smr);
 	return (pwd);
 }
 
@@ -3304,7 +3313,8 @@ pwd_alloc(void)
 {
 	struct pwd *pwd;
 
-	pwd = malloc(sizeof(*pwd), M_PWD, M_WAITOK | M_ZERO);
+	pwd = uma_zalloc_smr(pwd_zone, M_WAITOK);
+	bzero(pwd, sizeof(*pwd));
 	refcount_init(&pwd->pwd_refcount, 1);
 	return (pwd);
 }
@@ -3322,7 +3332,7 @@ pwd_drop(struct pwd *pwd)
 		vrele(pwd->pwd_rdir);
 	if (pwd->pwd_jdir != NULL)
 		vrele(pwd->pwd_jdir);
-	free(pwd, M_PWD);
+	uma_zfree_smr(pwd_zone, pwd);
 }
 
 /*
@@ -3340,7 +3350,7 @@ pwd_chroot(struct thread *td, struct vnode *vp)
 	fdp = td->td_proc->p_fd;
 	newpwd = pwd_alloc();
 	FILEDESC_XLOCK(fdp);
-	oldpwd = fdp->fd_pwd;
+	oldpwd = FILEDESC_XLOCKED_LOAD_PWD(fdp);
 	if (chroot_allow_open_directories == 0 ||
 	    (chroot_allow_open_directories == 1 &&
 	    oldpwd->pwd_rdir != rootvnode)) {
@@ -3376,7 +3386,7 @@ pwd_chdir(struct thread *td, struct vnode *vp)
 	newpwd = pwd_alloc();
 	fdp = td->td_proc->p_fd;
 	FILEDESC_XLOCK(fdp);
-	oldpwd = fdp->fd_pwd;
+	oldpwd = FILEDESC_XLOCKED_LOAD_PWD(fdp);
 	newpwd->pwd_cdir = vp;
 	pwd_fill(oldpwd, newpwd);
 	pwd_set(fdp, newpwd);
@@ -3392,7 +3402,7 @@ pwd_ensure_dirs(void)
 
 	fdp = curproc->p_fd;
 	FILEDESC_XLOCK(fdp);
-	oldpwd = fdp->fd_pwd;
+	oldpwd = FILEDESC_XLOCKED_LOAD_PWD(fdp);
 	if (oldpwd->pwd_cdir != NULL && oldpwd->pwd_rdir != NULL) {
 		FILEDESC_XUNLOCK(fdp);
 		return;
@@ -3401,7 +3411,7 @@ pwd_ensure_dirs(void)
 
 	newpwd = pwd_alloc();
 	FILEDESC_XLOCK(fdp);
-	oldpwd = fdp->fd_pwd;
+	oldpwd = FILEDESC_XLOCKED_LOAD_PWD(fdp);
 	pwd_fill(oldpwd, newpwd);
 	if (newpwd->pwd_cdir == NULL) {
 		vrefact(rootvnode);
@@ -3441,7 +3451,7 @@ mountcheckdirs(struct vnode *olddp, struct vnode *newdp)
 		if (fdp == NULL)
 			continue;
 		FILEDESC_XLOCK(fdp);
-		oldpwd = fdp->fd_pwd;
+		oldpwd = FILEDESC_XLOCKED_LOAD_PWD(fdp);
 		if (oldpwd == NULL ||
 		    (oldpwd->pwd_cdir != olddp &&
 		    oldpwd->pwd_rdir != olddp &&
@@ -4074,6 +4084,7 @@ int
 kern_proc_cwd_out(struct proc *p,  struct sbuf *sb, ssize_t maxlen)
 {
 	struct filedesc *fdp;
+	struct pwd *pwd;
 	struct export_fd_buf *efbuf;
 	struct vnode *cdir;
 	int error;
@@ -4091,7 +4102,8 @@ kern_proc_cwd_out(struct proc *p,  struct sbuf *sb, ssize_t maxlen)
 	efbuf->remainder = maxlen;
 
 	FILEDESC_SLOCK(fdp);
-	cdir = fdp->fd_pwd->pwd_cdir;
+	pwd = FILEDESC_LOCKED_LOAD_PWD(fdp);
+	cdir = pwd->pwd_cdir;
 	if (cdir == NULL) {
 		error = EINVAL;
 	} else {
@@ -4279,6 +4291,9 @@ filelistinit(void *dummy)
 	    NULL, NULL, UMA_ALIGN_PTR, UMA_ZONE_NOFREE);
 	filedesc0_zone = uma_zcreate("filedesc0", sizeof(struct filedesc0),
 	    NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
+	pwd_zone = uma_zcreate("PWD", sizeof(struct pwd), NULL, NULL,
+	    NULL, NULL, UMA_ALIGN_PTR, UMA_ZONE_SMR);
+	pwd_smr = uma_zone_get_smr(pwd_zone);
 	mtx_init(&sigio_lock, "sigio lock", NULL, MTX_DEF);
 }
 SYSINIT(select, SI_SUB_LOCK, SI_ORDER_FIRST, filelistinit, NULL);

sys/kern/kern_linker.c

@@ -2063,6 +2063,22 @@ linker_hwpmc_list_objects(void)
 }
 #endif
 
+/* check if root file system is not mounted */
+static bool
+linker_root_mounted(void)
+{
+	struct pwd *pwd;
+	bool ret;
+
+	if (rootvnode == NULL)
+		return (false);
+
+	pwd = pwd_hold(curthread);
+	ret = pwd->pwd_rdir != NULL;
+	pwd_drop(pwd);
+	return (ret);
+}
+
 /*
  * Find a file which contains given module and load it, if "parent" is not
  * NULL, register a reference to it.
@@ -2084,15 +2100,13 @@ linker_load_module(const char *kldname, const char *modname,
  		 */
 		KASSERT(verinfo == NULL, ("linker_load_module: verinfo"
 		    " is not NULL"));
-		/* check if root file system is not mounted */
-		if (rootvnode == NULL || curproc->p_fd->fd_pwd->pwd_rdir == NULL)
+		if (!linker_root_mounted())
 			return (ENXIO);
 		pathname = linker_search_kld(kldname);
 	} else {
 		if (modlist_lookup2(modname, verinfo) != NULL)
 			return (EEXIST);
-		/* check if root file system is not mounted */
-		if (rootvnode == NULL || curproc->p_fd->fd_pwd->pwd_rdir == NULL)
+		if (!linker_root_mounted())
 			return (ENXIO);
 		if (kldname != NULL)
 			pathname = strdup(kldname, M_LINKER);

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 */

sys/ufs/ffs/ffs_alloc.c

@@ -3590,6 +3590,7 @@ buffered_write(fp, uio, active_cred, flags, td)
 	int flags;
 	struct thread *td;
 {
+	struct pwd *pwd;
 	struct vnode *devvp, *vp;
 	struct inode *ip;
 	struct buf *bp;
@@ -3610,7 +3611,8 @@ buffered_write(fp, uio, active_cred, flags, td)
 		return (EINVAL);
 	fdp = td->td_proc->p_fd;
 	FILEDESC_SLOCK(fdp);
-	vp = fdp->fd_pwd->pwd_cdir;
+	pwd = FILEDESC_LOCKED_LOAD_PWD(fdp);
+	vp = pwd->pwd_cdir;
 	vref(vp);
 	FILEDESC_SUNLOCK(fdp);
 	vn_lock(vp, LK_SHARED | LK_RETRY);