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SubjectRe: fsnotify_mark_srcu wtf?
On Thu, Nov 10, 2016 at 9:46 PM, Jan Kara <jack@suse.cz> wrote:

...

>
> Well but how would you like to protect the mark list hanging off the inode
> / mountpoint with two SRCUs? You'd need two lists hanging off the inode &
> mountpoint (for different priorities) and that's too big cost to pay to
> accomodate broken userspace...
>

This is the plan.
I have a rough implementation, which still requires some testing:

/*
* fsnotify_perm_mark[1] protects reads of the first half of inode/vfsmount
* mark lists, which consist of the high priority (>0) marks, whose masks
* may contain permission events.
*
* fsnotify_perm_mark[0] protects reads of the second half of inode/vfsmount
* mark lists, which consist of the low priority (0) marks, whose masks
* do not contain permission events.
*
* High priority marks and low priority marks are added to different
* destroy lists and freed by different reapers, who synchronize only
* the relevant srcu.
*
* Before traversing to first mark on the list we must hold both srcu.
* While traversing first half, we may temporary drop fsnotify_perm_mark[0]
* to handle a blocking permission event.
* When we reach the first low level mark, we may drop fsnotify_perm_mark[1]
* for good.
*/

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