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From: Conrad Meyer <cem@freebsd.org>
Date: Fri, 13 Nov 2020 08:24:30 -0800
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Subject: Re: svn commit: r367631 - in head/sys: kern sys
To: Konstantin Belousov <kib@freebsd.org>
Cc: src-committers <src-committers@freebsd.org>,
svn-src-all <svn-src-all@freebsd.org>,
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Hi Konstantin,
On Fri, Nov 13, 2020 at 1:32 AM Konstantin Belousov <kib@freebsd.org> wrote:
>
> Author: kib
> Date: Fri Nov 13 09:31:57 2020
> New Revision: 367631
> URL: https://svnweb.freebsd.org/changeset/base/367631
>
> Log:
> Implement vn_lock_pair().
>
> Modified: head/sys/kern/vfs_vnops.c
> ==============================================================================
> --- head/sys/kern/vfs_vnops.c Fri Nov 13 02:05:45 2020 (r367630)
> +++ head/sys/kern/vfs_vnops.c Fri Nov 13 09:31:57 2020 (r367631)
> @@ -3317,4 +3325,92 @@ vn_fallocate(struct file *fp, off_t offset, off_t len,
> ...
> +
> +static void
> +vn_lock_pair_pause(const char *wmesg)
> +{
> + atomic_add_long(&vn_lock_pair_pause_cnt, 1);
> + pause(wmesg, prng32_bounded(hz / 10));
> +}
This function is called when the try-lock of the second lock in the
pair (either order) fails. The back-off period is up to 100ms,
expected average 50ms. That seems really high?
Separately: prng32_bounded() may return 0, which is transparently
converted to the equivalent of 1 by pause_sbt(9). This means a 1 tick
pause is marginally more likely than any other possible duration. It
probably doesn't matter.
Thanks,
Conrad
> +
> +/*
> + * Lock pair of vnodes vp1, vp2, avoiding lock order reversal.
> + * vp1_locked indicates whether vp1 is exclusively locked; if not, vp1
> + * must be unlocked. Same for vp2 and vp2_locked. One of the vnodes
> + * can be NULL.
> + *
> + * The function returns with both vnodes exclusively locked, and
> + * guarantees that it does not create lock order reversal with other
> + * threads during its execution. Both vnodes could be unlocked
> + * temporary (and reclaimed).
> + */
> +void
> +vn_lock_pair(struct vnode *vp1, bool vp1_locked, struct vnode *vp2,
> + bool vp2_locked)
> +{
> + int error;
> +
> + if (vp1 == NULL && vp2 == NULL)
> + return;
> + if (vp1 != NULL) {
> + if (vp1_locked)
> + ASSERT_VOP_ELOCKED(vp1, "vp1");
> + else
> + ASSERT_VOP_UNLOCKED(vp1, "vp1");
> + } else {
> + vp1_locked = true;
> + }
> + if (vp2 != NULL) {
> + if (vp2_locked)
> + ASSERT_VOP_ELOCKED(vp2, "vp2");
> + else
> + ASSERT_VOP_UNLOCKED(vp2, "vp2");
> + } else {
> + vp2_locked = true;
> + }
> + if (!vp1_locked && !vp2_locked) {
> + vn_lock(vp1, LK_EXCLUSIVE | LK_RETRY);
> + vp1_locked = true;
> + }
> +
> + for (;;) {
> + if (vp1_locked && vp2_locked)
> + break;
> + if (vp1_locked && vp2 != NULL) {
> + if (vp1 != NULL) {
> + error = VOP_LOCK1(vp2, LK_EXCLUSIVE | LK_NOWAIT,
> + __FILE__, __LINE__);
> + if (error == 0)
> + break;
> + VOP_UNLOCK(vp1);
> + vp1_locked = false;
> + vn_lock_pair_pause("vlp1");
(Pause called here and in similar elided case for vp2 -> vp1 below.)
> + }
> + vn_lock(vp2, LK_EXCLUSIVE | LK_RETRY);
> + vp2_locked = true;
> + }