Messages in this thread | | | Date | Tue, 11 Jul 2023 16:17:11 +0800 | Subject | Re: [PATCH v2 1/6] fs: split off vfs_getdents function of getdents64 syscall | From | Hao Xu <> |
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On 5/25/23 19:00, Dominique Martinet wrote: > Christian Brauner wrote on Thu, May 25, 2023 at 11:22:08AM +0200: >>> What was confusing is that default_llseek updates f_pos under the >>> inode_lock (write), and getdents also takes that lock (for read only in >>> shared implem), so I assumed getdents also was just protected by this >>> read lock, but I guess that was a bad assumption (as I kept pointing >>> out, a shared read lock isn't good enough, we definitely agree there) >>> >>> >>> In practice, in the non-registered file case io_uring is also calling >>> fdget, so the lock is held exactly the same as the syscall and I wasn't >> >> No, it really isn't. fdget() doesn't take f_pos_lock at all: >> >> fdget() >> -> __fdget() >> -> __fget_light() >> -> __fget() >> -> __fget_files() >> -> __fget_files_rcu() > > Ugh, I managed to not notice that I was looking at fdget_pos and that > it's not the same as fdget by the time I wrote two paragraphs... These > functions all have too many wrappers and too similar names for a quick > look before work. > >> If that were true then any system call that passes an fd and uses >> fdget() would try to acquire a mutex on f_pos_lock. We'd be serializing >> every *at based system call on f_pos_lock whenever we have multiple fds >> referring to the same file trying to operate on it concurrently. >> >> We do have fdget_pos() and fdput_pos() as a special purpose fdget() for >> a select group of system calls that require this synchronization. > > Right, that makes sense, and invalidates everything I said after that > anyway but it's not like looking stupid ever killed anyone. > > Ok so it would require adding a new wrapper from struct file to struct > fd that'd eventually take the lock and set FDPUT_POS_UNLOCK for... not > fdput_pos but another function for that stopping short of fdput... > Then just call that around both vfs_llseek and vfs_getdents calls; which > is the easy part. > > (Or possibly call mutex_lock directly like Dylan did in [1]...) > [1] https://lore.kernel.org/all/20220222105504.3331010-1-dylany@fb.com/T/#m3609dc8057d0bc8e41ceab643e4d630f7b91bde6 > > > > I'll be honest though I'm thankful for your explanations but I think > I'll just do like Stefan and stop trying for now: the only reason I've > started this was because I wanted to play with io_uring for a new toy > project and it felt awkward without a getdents for crawling a tree; and > I'm long past the point where I should have thrown the towel and just > make that a sequential walk. > There's too many "conditional patches" (NOWAIT, end of dir indicator) > that I don't care about and require additional work to rebase > continuously so I'll just leave it up to someone else who does care. > > So to that someone: feel free to continue from these branches (I've > included the fix for kernfs_fop_readdir that Dan Carpenter reported): > https://github.com/martinetd/linux/commits/io_uring_getdents > https://github.com/martinetd/liburing/commits/getdents > > Or just start over, there's not that much code now hopefully the > baseline requirements have gotten a little bit clearer. > > > Sorry for stirring the mess and leaving halfway, if nobody does continue > I might send a v3 when I have more time/energy in a few months, but it > won't be quick. >
Hi Dominique,
I'd like to take this if you don't mind.
Regards, Hao
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