Messages in this thread | | | Date | Fri, 08 Oct 1999 17:38:32 +0200 | From | Manfred Spraul <> | Subject | Re: how to write get_block? |
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Alexander Viro wrote: >All stuff with truncate()/write() is racey right now, > AFAICS. The next thing I'm going to do is a (dumb) implementation of > blocking rwlocks. write() being reader and truncate() - writer.
IMHO that's the wrong approach:
1) overlapping concurrent write operations violate Nfsv2.
2) the pipe code needs O_NONBLOCK characteristics, ie no synchonization is allowed. The 2.2 code uses an ugly hack: sys_write() acquires i_sem, and inside the pipe code the semaphore is dropped.
3) do not forget f_pos when you implement a synchonization: even concurrent reads are not allowed if both use the same file pointer.
What about these ideas: a) rename i_sem (*), and add public functions such as inode_lock_exclusive(), inode_lock_truncate(), inode_lock_write() etc.
Currently, everyone directly accesses the VFS synchonization (nfsd, drivers/char/tty_io.c; arch/*/kernel/sys*.c).
b) the requirements of the VFS users differ completely, add flags: 0: synchonize nothing, eg pipes. I_FPOS_SYNC: synchonize f_pos access, nothing else I_FILE_SYNC: multiple read, non-overlapping write, truncate.
Since it will be hidden by inode_lock_??(), it will be easy to optimize the implementation as needed.
Eg I'm pretty sure that Stephen will not like it if raw-io is slowed down by a multiple reader-single writer synchonization.
-- Manfred
(*) I prefer compiler errors instead of missing synchonization. You could rename it back to i_sem later.
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