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limits in the code which is deep in the call stack, and owns several critical system resources, like vnode locks. Attempt to wait while the per-mount softupdate thread cleans up the backlog may deadlock, because the thread might need to lock the same vnode which is owned by the waiting thread. Instead of synchronously waiting for the worker, perform the worker' tickle and pause until the backlog is cleaned, at the safe point during return from kernel to usermode. A new ast request to call softdep_ast_cleanup() is created, the SU code now only checks the size of queue and schedules ast. There is no ast delivery for the kernel threads, so they are exempted from the mechanism, except NFS daemon threads. NFS server loop explicitely checks for the request, and informs the schedule_cleanup() that it is capable of handling the requests by the process P2_AST_SU flag. This is needed because nfsd may be the sole cause of the SU workqueue overflow. But, to not cause nsfd to spawn additional threads just because we slow down existing workers, only tickle su threads, without waiting for the backlog cleanup. Reviewed by: jhb, mckusick Tested by: pho Sponsored by: The FreeBSD Foundation MFC after: 2 weeks
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