Date: Mon, 13 Jul 2015 14:28:40 +0200 From: Luigi Rizzo <rizzo@iet.unipi.it> To: freebsd-current <freebsd-current@freebsd.org>, Adrian Chadd <adrian@freebsd.org>, John Baldwin <jhb@freebsd.org> Cc: Luigi Rizzo <rizzo@iet.unipi.it> Subject: protection against module unloading ? Message-ID: <CA%2BhQ2%2BiNWHzH=L6YC8TQvsWsz3EhMcLTv9VOP5nXtmDWD4TD0g@mail.gmail.com>
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Hi, I am trying to understand how to protect efficiently against module removals when a device driver is in use. This issue came up some time ago when trying netmap loaded as a module. --- Summary of the current mode of operation (as i understand it): --- When a device is compiled as a module, all devfs calls in sys/fs/devfs/devfs_vnops.c are protected by setting dev->si_threadcount (within dev*_refthread()) for the duration of the call. This is implemented using devfs_fp_check()/dev_relthread() which in turn, for the module case, updates the dev->si_threadcount under dev_lock() which is a single shared mutex, devmtx . The above is problematic for driver modules that do a lot of I/O using poll, ioctl, read or write, because of the system-wide contention on devmtx To mitigate the contention, statically compiled modules have the SI_ETERNAL attribute that prevents the lock (major contention point) and si_threadcount update. --- Alternative ? I was hoping to make the protection mechanism cheaper by only increasing dev->si_threadcount on devfs_open() without calling dev_relthread() ), and then decreasing si_threadcount on defvs_close() . This way the reference is active for the lifetime of the file descriptor without needing to track individual high-frequency calls. This is probably not enough because there could be mmap handlers, which remain active even after the device is closed. However I do not think the current devfs_fp_check() suffices either, and so drivers that have their own mmap code in a module that can be unloaded must already implement their own mechanism to protect against unloading. In fact, this could be achieved easily by making these drivers use dev_refthread() to hold a refcount while active. Would the above work ? Anything missing ? cheers luigi -- -----------------------------------------+------------------------------- Prof. Luigi RIZZO, rizzo@iet.unipi.it . Dip. di Ing. dell'Informazione http://www.iet.unipi.it/~luigi/ . Universita` di Pisa TEL +39-050-2217533 . via Diotisalvi 2 Mobile +39-338-6809875 . 56122 PISA (Italy) -----------------------------------------+-------------------------------
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