Re: thread concurrent file operation

[Jimmy Pan <dspjmt@xxxxxxxxx>]

actually i dont see why there is a need to ptovide a lock by the kernel.
the locking should be at userspace.
you can test it by massive write to a file, that would demonstrat the kernel
didnt confer a lock

sent from my samsung

Hello,

I was looking at how a syscall read/write was done, and i found this : 

   ....

   loff_t pos = file_pos_read(f.file);

   ret = vfs_read(f.file, buf, count, &pos);

   file_pos_write(f.file, pos);

   fdput(f);

   ...

My questions are :

Where did the locking go? I would have imaginated something like :

   ....

   lock(f);

   loff_t pos = file_pos_read(f.file);

   ret = vfs_read(f.file, buf, count, &pos);

   file_pos_write(f.file, pos);

   fdput(f);

   unlock(f);

   ...

If multiple threads try to read/write at the same time, they could read/write at the same offset ?

If my understanding are correct, is this POSIX compliant ?

thanks.

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