From: Vitaliy Makkoveev Subject: Re: Use foffset() to push the KERNEL_LOCK() To: Mark Kettenis , anton@openbsd.org, visa@openbsd.org, tech@openbsd.org Date: Fri, 14 Nov 2025 21:58:05 +0300 On Thu, Nov 13, 2025 at 09:38:53AM +0000, Martin Pieuchot wrote: > On 12/11/25(Wed) 22:05, Vitaliy Makkoveev wrote: > > [...] > > I like the idea to push kernel lock deeper and keep it around > > VOP_READ()/VOP_WRITE(). We don't need to move vn_lock()/VOP_LOCK(), so > > the race for `f_offset' will be excluded. > > > > I can make this diff by myself if mpi@ has no interest in this > > direction. > > Sure, I don't mind. > > Note that there are currently two independent bottlenecks when it comes > to FS syscalls. One is related to VOP_READ() and VOP_WRITE() and I have > a plan to address it. > > The other is related to namei(9) (doopenat(), dofstatat(), and > sys___realpath()) and will certainly reduce the KERNREL_LOCK() > contention with fewer efforts. > > So if you're interested in reducing the KERNEL_LOCK() contention in this > area, may I suggest you look at vn_open() and all others parts of the > kernel namei(9) is not yet surrounded by a KERNEL_LOCK() dance? Then we > can start pushing it further into namei(9). > > Thanks, > Martin > > This diff is based on mpi@'s, but lefts vn_lock() untouched. So we have races `f_offset'. Index: sys/dev/dt/dt_dev.c =================================================================== RCS file: /cvs/src/sys/dev/dt/dt_dev.c,v retrieving revision 1.46 diff -u -p -r1.46 dt_dev.c --- sys/dev/dt/dt_dev.c 22 Sep 2025 07:49:43 -0000 1.46 +++ sys/dev/dt/dt_dev.c 14 Nov 2025 18:51:57 -0000 @@ -711,7 +711,9 @@ dt_ioctl_rd_vnode(struct dt_softc *sc, s fp->f_type = DTYPE_VNODE; fp->f_ops = &vnops; fp->f_data = vn; + mtx_enter(&fp->f_mtx); fp->f_offset = 0; + mtx_leave(&fp->f_mtx); dtrv->dtrv_fd = fd; fdplock(p->p_fd); fdinsert(p->p_fd, fd, UF_EXCLOSE, fp); Index: sys/kern/syscalls.master =================================================================== RCS file: /cvs/src/sys/kern/syscalls.master,v retrieving revision 1.270 diff -u -p -r1.270 syscalls.master --- sys/kern/syscalls.master 24 May 2025 06:49:16 -0000 1.270 +++ sys/kern/syscalls.master 14 Nov 2025 18:52:00 -0000 @@ -213,7 +213,7 @@ 97 STD NOLOCK { int sys_socket(int domain, int type, int protocol); } 98 STD NOLOCK { int sys_connect(int s, const struct sockaddr *name, \ socklen_t namelen); } -99 STD { int sys_getdents(int fd, void *buf, size_t buflen); } +99 STD NOLOCK { int sys_getdents(int fd, void *buf, size_t buflen); } 100 STD { int sys_getpriority(int which, id_t who); } 101 STD NOLOCK { int sys_pipe2(int *fdp, int flags); } 102 STD NOLOCK { int sys_dup3(int from, int to, int flags); } Index: sys/kern/vfs_syscalls.c =================================================================== RCS file: /cvs/src/sys/kern/vfs_syscalls.c,v retrieving revision 1.378 diff -u -p -r1.378 vfs_syscalls.c --- sys/kern/vfs_syscalls.c 20 Sep 2025 13:53:36 -0000 1.378 +++ sys/kern/vfs_syscalls.c 14 Nov 2025 18:52:02 -0000 @@ -3146,6 +3146,7 @@ sys_getdents(struct proc *p, void *v, re struct iovec aiov; size_t buflen; int error, eofflag; + off_t offset; buflen = SCARG(uap, buflen); @@ -3165,7 +3166,8 @@ sys_getdents(struct proc *p, void *v, re vn_lock(vp, LK_EXCLUSIVE | LK_RETRY); - if (fp->f_offset < 0) { + offset = foffset(fp); + if (offset < 0) { VOP_UNLOCK(vp); error = EINVAL; goto bad; @@ -3179,8 +3181,10 @@ sys_getdents(struct proc *p, void *v, re auio.uio_segflg = UIO_USERSPACE; auio.uio_procp = p; auio.uio_resid = buflen; - auio.uio_offset = fp->f_offset; + auio.uio_offset = offset; + KERNEL_LOCK(); error = VOP_READDIR(vp, &auio, fp->f_cred, &eofflag); + KERNEL_UNLOCK(); mtx_enter(&fp->f_mtx); fp->f_offset = auio.uio_offset; mtx_leave(&fp->f_mtx); Index: sys/kern/vfs_vnops.c =================================================================== RCS file: /cvs/src/sys/kern/vfs_vnops.c,v retrieving revision 1.125 diff -u -p -r1.125 vfs_vnops.c --- sys/kern/vfs_vnops.c 6 Jan 2025 08:57:23 -0000 1.125 +++ sys/kern/vfs_vnops.c 14 Nov 2025 18:52:02 -0000 @@ -346,12 +346,10 @@ vn_read(struct file *fp, struct uio *uio off_t offset; int error; - KERNEL_LOCK(); - vn_lock(vp, LK_EXCLUSIVE | LK_RETRY); if ((fflags & FO_POSITION) == 0) - offset = uio->uio_offset = fp->f_offset; + offset = uio->uio_offset = foffset(fp); else offset = uio->uio_offset; @@ -366,8 +364,10 @@ vn_read(struct file *fp, struct uio *uio goto done; } + KERNEL_LOCK(); error = VOP_READ(vp, uio, (fp->f_flag & FNONBLOCK) ? IO_NDELAY : 0, cred); + KERNEL_UNLOCK(); if ((fflags & FO_POSITION) == 0) { mtx_enter(&fp->f_mtx); fp->f_offset += count - uio->uio_resid; @@ -375,7 +375,6 @@ vn_read(struct file *fp, struct uio *uio } done: VOP_UNLOCK(vp); - KERNEL_UNLOCK(); return (error); }