On Fri, Dec 17, 2021 at 12:50:55PM -0800, dai.ngo@xxxxxxxxxx wrote:
>
> On 12/17/21 12:35 PM, Bruce Fields wrote:
> >On Tue, Dec 14, 2021 at 11:41:41PM +0000, Chuck Lever III wrote:
> >>
> >>>On Dec 13, 2021, at 12:24 PM, Dai Ngo <dai.ngo@xxxxxxxxxx> wrote:
> >>>
> >>>Add new callback, lm_expire_lock, to lock_manager_operations to allow
> >>>the lock manager to take appropriate action to resolve the lock conflict
> >>>if possible. The callback takes 2 arguments, file_lock of the blocker
> >>>and a testonly flag:
> >>>
> >>>testonly = 1 check and return lock manager's private data if lock conflict
> >>> can be resolved else return NULL.
> >>>testonly = 0 resolve the conflict if possible, return true if conflict
> >>> was resolved esle return false.
> >>>
> >>>Lock manager, such as NFSv4 courteous server, uses this callback to
> >>>resolve conflict by destroying lock owner, or the NFSv4 courtesy client
> >>>(client that has expired but allowed to maintains its states) that owns
> >>>the lock.
> >>>
> >>>Signed-off-by: Dai Ngo <dai.ngo@xxxxxxxxxx>
> >>>---
> >>>fs/locks.c | 40 +++++++++++++++++++++++++++++++++++++---
> >>>include/linux/fs.h | 1 +
> >>>2 files changed, 38 insertions(+), 3 deletions(-)
> >>>
> >>>diff --git a/fs/locks.c b/fs/locks.c
> >>>index 3d6fb4ae847b..5f3ea40ce2aa 100644
> >>>--- a/fs/locks.c
> >>>+++ b/fs/locks.c
> >>>@@ -952,8 +952,11 @@ void
> >>>posix_test_lock(struct file *filp, struct file_lock *fl)
> >>>{
> >>> struct file_lock *cfl;
> >>>+ struct file_lock *checked_cfl = NULL;
> >>> struct file_lock_context *ctx;
> >>> struct inode *inode = locks_inode(filp);
> >>>+ void *res_data;
> >>>+ void *(*func)(void *priv, bool testonly);
> >>>
> >>> ctx = smp_load_acquire(&inode->i_flctx);
> >>> if (!ctx || list_empty_careful(&ctx->flc_posix)) {
> >>>@@ -962,11 +965,24 @@ posix_test_lock(struct file *filp, struct file_lock *fl)
> >>> }
> >>>
> >>> spin_lock(&ctx->flc_lock);
> >>>+retry:
> >>> list_for_each_entry(cfl, &ctx->flc_posix, fl_list) {
> >>>- if (posix_locks_conflict(fl, cfl)) {
> >>>- locks_copy_conflock(fl, cfl);
> >>>- goto out;
> >>>+ if (!posix_locks_conflict(fl, cfl))
> >>>+ continue;
> >>>+ if (checked_cfl != cfl && cfl->fl_lmops &&
> >>>+ cfl->fl_lmops->lm_expire_lock) {
> >>>+ res_data = cfl->fl_lmops->lm_expire_lock(cfl, true);
> >>>+ if (res_data) {
> >>>+ func = cfl->fl_lmops->lm_expire_lock;
> >>>+ spin_unlock(&ctx->flc_lock);
> >>>+ func(res_data, false);
> >>>+ spin_lock(&ctx->flc_lock);
> >>>+ checked_cfl = cfl;
> >>>+ goto retry;
> >>>+ }
> >>> }
> >>Dai and I discussed this offline. Depending on a pointer to represent
> >>exactly the same struct file_lock across a dropped spinlock is racy.
> >Yes. There's also no need for that (checked_cfl != cfl) check, though.
> >By the time func() returns, that lock should be gone from the list
> >anyway.
>
> func() eventually calls expire_client. But we do not know if expire_client
> succeeds.
expire_client always succeeds, maybe you're thinking of
mark_client_expired_locked or something?
If there's a chance something might fail here, the only reason should be
that the client is no longer a courtesy client because it's come back to
life. But in that case the correct behavior would be to just honor the
lock conflict and return -EAGAIN.
--b.
> One simple way to know if the conflict client was successfully
> expired is to check the list again. If the client was successfully expired
> then its locks were removed from the list. Otherwise we get the same 'cfl'
> from the list again on the next get.
>
> -Dai
>
> >
> >It's a little inefficient to have to restart the list every time--but
> >that theoretical n^2 behavior won't matter much compared to the time
> >spent waiting for clients to expire. And this approach has the benefit
> >of being simple.
> >
> >--b.
> >
> >>Dai plans to investigate other mechanisms to perform this check
> >>reliably.
> >>
> >>
> >>>+ locks_copy_conflock(fl, cfl);
> >>>+ goto out;
> >>> }
> >>> fl->fl_type = F_UNLCK;
> >>>out:
> >>>@@ -1136,10 +1152,13 @@ static int posix_lock_inode(struct inode *inode, struct file_lock *request,
> >>> struct file_lock *new_fl2 = NULL;
> >>> struct file_lock *left = NULL;
> >>> struct file_lock *right = NULL;
> >>>+ struct file_lock *checked_fl = NULL;
> >>> struct file_lock_context *ctx;
> >>> int error;
> >>> bool added = false;
> >>> LIST_HEAD(dispose);
> >>>+ void *res_data;
> >>>+ void *(*func)(void *priv, bool testonly);
> >>>
> >>> ctx = locks_get_lock_context(inode, request->fl_type);
> >>> if (!ctx)
> >>>@@ -1166,9 +1185,24 @@ static int posix_lock_inode(struct inode *inode, struct file_lock *request,
> >>> * blocker's list of waiters and the global blocked_hash.
> >>> */
> >>> if (request->fl_type != F_UNLCK) {
> >>>+retry:
> >>> list_for_each_entry(fl, &ctx->flc_posix, fl_list) {
> >>> if (!posix_locks_conflict(request, fl))
> >>> continue;
> >>>+ if (checked_fl != fl && fl->fl_lmops &&
> >>>+ fl->fl_lmops->lm_expire_lock) {
> >>>+ res_data = fl->fl_lmops->lm_expire_lock(fl, true);
> >>>+ if (res_data) {
> >>>+ func = fl->fl_lmops->lm_expire_lock;
> >>>+ spin_unlock(&ctx->flc_lock);
> >>>+ percpu_up_read(&file_rwsem);
> >>>+ func(res_data, false);
> >>>+ percpu_down_read(&file_rwsem);
> >>>+ spin_lock(&ctx->flc_lock);
> >>>+ checked_fl = fl;
> >>>+ goto retry;
> >>>+ }
> >>>+ }
> >>> if (conflock)
> >>> locks_copy_conflock(conflock, fl);
> >>> error = -EAGAIN;
> >>>diff --git a/include/linux/fs.h b/include/linux/fs.h
> >>>index e7a633353fd2..8cb910c3a394 100644
> >>>--- a/include/linux/fs.h
> >>>+++ b/include/linux/fs.h
> >>>@@ -1071,6 +1071,7 @@ struct lock_manager_operations {
> >>> int (*lm_change)(struct file_lock *, int, struct list_head *);
> >>> void (*lm_setup)(struct file_lock *, void **);
> >>> bool (*lm_breaker_owns_lease)(struct file_lock *);
> >>>+ void *(*lm_expire_lock)(void *priv, bool testonly);
> >>>};
> >>>
> >>>struct lock_manager {
> >>>--
> >>>2.9.5
> >>>
> >>--
> >>Chuck Lever
> >>
> >>
