linux kernel may guarantee thread-safety - but this only apply to serializing data at the per-syscall level. Ie, every read() will complete, before being intercepted by another read() from another thread. But at the file level u still may get file corruption/file datastructure mangled if u mixed write/read without properly serialization at the userspace level. thus, kernel locking + userspace locking are needed - for different purpose.
below discussion is useful (first answer esp):
http://stackoverflow.com/questions/5268307/thread-safety-of-read-pread-system-calls
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Regards,
Peter Teoh
in the kernel for each file descriptor, there is only one single offset value to indicate the current file pointer position. so at the userspace level, different read/write combination will affect the file pointer value - which explained also why userspace locking (for logical reasons) are needed.
On Thu, Feb 7, 2013 at 6:23 PM, Peter Teoh <htmldeveloper@xxxxxxxxx> wrote:
Multiple concurrent write() by different thread is possible, as they all can share the same file descriptor in a single similar process, and this is not allowed. So nevertheless, the problem you posed is not allowed/acceptable by the kernel, so Linus himself fixed it:See here:And Linus patch:but my present version (3.2.0) has rcu lock over it (higher performance):INIT_LIST_HEAD(&f->f_u.fu_list);atomic_long_set(&f->f_count, 1);rwlock_init(&f->f_owner.lock);spin_lock_init(&f->f_lock);eventpoll_init_file(f);/* f->f_version: 0 */On Thu, Feb 7, 2013 at 4:44 PM, Karaoui mohamed lamine <moharaka@xxxxxxxxx> wrote:Tahnks guys!2013/1/30 Karaoui mohamed lamine <moharaka@xxxxxxxxx>thanks, i think i get it.2013/1/30 <Valdis.Kletnieks@xxxxxx>On Tue, 29 Jan 2013 20:16:26 +0100, you said:If you have non-overlapping writes, the kernel will eventually sort it out
> Actually my question is :
> Does POSIX specifies the fact that we need to use "lockf" to be able to do
> read/write operation in different offset ? Is'n the kernel supposed to
> ensure this ?
for you. If your writes overlap, you'll have to provide your own locking
via lockf() or similar, and synchronization via other methods.
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Regards,
Peter Teoh
Regards,
Peter Teoh
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