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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