Messages in this thread | | | Date | Sat, 01 Jun 2002 15:25:56 -0700 | From | Andrew Morton <> | Subject | Re: [patch 12/16] fix race between writeback and unlink |
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William Lee Irwin III wrote: > > Linus Torvalds wrote: > >> The general VFS layer really shouldn't have assigned that strogn a meaning > >> to "i_nlink" anyway, it's not for the VFS layer to decide (and it only > >> causes problems for any non-UNIX-on-a-disk filesystems). > > On Sat, Jun 01, 2002 at 12:19:36PM -0700, Andrew Morton wrote: > > Yes, I suspect all the inode refcounting, locking, I_FREEING, I_LOCK, etc > > could do with a spring clean. Make it a bit more conventional. I'll > > discuss with Al when he resurfaces. > > I'm somewhat concerned about the protection of ->i_size, since that > appears to be accessed in generic_file_read() without any protection > against writers to the field. From a quick glance at current 2.5 (it > looks like 2.4 has this too) it looks like it's written to by > vmtruncate() through notify_change() with the ->i_sem and BKL held at > the moment, but generic_file_read() doesn't take either before reading > it, and there may be still other writers.
truncate and write change i_size, under i_sem. The i_size test on the read path doesn't really need to be there, I suspect. It handles the window where i_size has been decreased by truncate but the filesystem hasn't finished truncating the blocks yet. It also optimises reads outside the end of file - no point in calling into the filesystem to try to map blocks which aren't there.
> I also don't see the anything > like read_barrier_depends() for lockless algorithms or any atomic reads. > Even on machines with extremely strong memory consistency models like > i386, as loff_t is long long, it would seem possible to catch a partial > update and see an entirely bogus ->i_size value.
That's true. sys_stat() also could see a confusing intermediate value. A while back Ingo and Linus were tossing around possible solutions to this based on x86 compare-and-exchange operations, but nothing conclusive came out of it. It's a "known bug".
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