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fsnotify: Provide framework for dropping SRCU lock in ->handle_event

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fanotify wants to drop fsnotify_mark_srcu lock when waiting for response
from userspace so that the whole notification subsystem is not blocked
during that time. This patch provides a framework for safely getting
mark reference for a mark found in the object list which pins the mark
in that list. We can then drop fsnotify_mark_srcu, wait for userspace
response and then safely continue iteration of the object list once we
reaquire fsnotify_mark_srcu.

Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
This commit is contained in:
Jan Kara
2016-11-10 16:02:11 +01:00
parent f09b04a03e
commit abc77577a6
4 changed files with 94 additions and 0 deletions
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82
fs/notify/mark.c
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@@ -109,6 +109,16 @@ void fsnotify_get_mark(struct fsnotify_mark *mark)
atomic_inc(&mark->refcnt);
}
/*
* Get mark reference when we found the mark via lockless traversal of object
* list. Mark can be already removed from the list by now and on its way to be
* destroyed once SRCU period ends.
*/
static bool fsnotify_get_mark_safe(struct fsnotify_mark *mark)
{
return atomic_inc_not_zero(&mark->refcnt);
}
static void __fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
{
u32 new_mask = 0;
@@ -243,6 +253,72 @@ void fsnotify_put_mark(struct fsnotify_mark *mark)
FSNOTIFY_REAPER_DELAY);
}
bool fsnotify_prepare_user_wait(struct fsnotify_iter_info *iter_info)
{
struct fsnotify_group *group;
if (WARN_ON_ONCE(!iter_info->inode_mark && !iter_info->vfsmount_mark))
return false;
if (iter_info->inode_mark)
group = iter_info->inode_mark->group;
else
group = iter_info->vfsmount_mark->group;
/*
* Since acquisition of mark reference is an atomic op as well, we can
* be sure this inc is seen before any effect of refcount increment.
*/
atomic_inc(&group->user_waits);
if (iter_info->inode_mark) {
/* This can fail if mark is being removed */
if (!fsnotify_get_mark_safe(iter_info->inode_mark))
goto out_wait;
}
if (iter_info->vfsmount_mark) {
if (!fsnotify_get_mark_safe(iter_info->vfsmount_mark))
goto out_inode;
}
/*
* Now that both marks are pinned by refcount in the inode / vfsmount
* lists, we can drop SRCU lock, and safely resume the list iteration
* once userspace returns.
*/
srcu_read_unlock(&fsnotify_mark_srcu, iter_info->srcu_idx);
return true;
out_inode:
if (iter_info->inode_mark)
fsnotify_put_mark(iter_info->inode_mark);
out_wait:
if (atomic_dec_and_test(&group->user_waits) && group->shutdown)
wake_up(&group->notification_waitq);
return false;
}
void fsnotify_finish_user_wait(struct fsnotify_iter_info *iter_info)
{
struct fsnotify_group *group = NULL;
iter_info->srcu_idx = srcu_read_lock(&fsnotify_mark_srcu);
if (iter_info->inode_mark) {
group = iter_info->inode_mark->group;
fsnotify_put_mark(iter_info->inode_mark);
}
if (iter_info->vfsmount_mark) {
group = iter_info->vfsmount_mark->group;
fsnotify_put_mark(iter_info->vfsmount_mark);
}
/*
* We abuse notification_waitq on group shutdown for waiting for all
* marks pinned when waiting for userspace.
*/
if (atomic_dec_and_test(&group->user_waits) && group->shutdown)
wake_up(&group->notification_waitq);
}
/*
* Mark mark as detached, remove it from group list. Mark still stays in object
* list until its last reference is dropped. Note that we rely on mark being
@@ -647,6 +723,12 @@ void fsnotify_detach_group_marks(struct fsnotify_group *group)
fsnotify_free_mark(mark);
fsnotify_put_mark(mark);
}
/*
* Some marks can still be pinned when waiting for response from
* userspace. Wait for those now. fsnotify_prepare_user_wait() will
* not succeed now so this wait is race-free.
*/
wait_event(group->notification_waitq, !atomic_read(&group->user_waits));
}
/* Destroy all marks attached to inode / vfsmount */