On Fri, Nov 20, 2020 at 7:00 PM Jens Axboe <axboe@xxxxxxxxx> wrote:
>
> Actually, I think we can do even better. How about just having
> do_filp_open() exit after LOOKUP_RCU fails, if LOOKUP_RCU was already
> set in the lookup flags? Then we don't need to change much else, and
> most of it falls out naturally.
So I was thinking doing the RCU lookup unconditionally, and then doing
the nn-RCU lookup if that fails afterwards.
But your patch looks good to me.
Except for the issue you noticed.
> Except it seems that should work, except LOOKUP_RCU does not guarantee
> that we're not going to do IO:
Well, almost nothing guarantees lack of IO, since allocations etc can
still block, but..
> [ 20.463195] schedule+0x5f/0xd0
> [ 20.463444] io_schedule+0x45/0x70
> [ 20.463712] bit_wait_io+0x11/0x50
> [ 20.463981] __wait_on_bit+0x2c/0x90
> [ 20.464264] out_of_line_wait_on_bit+0x86/0x90
> [ 20.464611] ? var_wake_function+0x30/0x30
> [ 20.464932] __ext4_find_entry+0x2b5/0x410
> [ 20.465254] ? d_alloc_parallel+0x241/0x4e0
> [ 20.465581] ext4_lookup+0x51/0x1b0
> [ 20.465855] ? __d_lookup+0x77/0x120
> [ 20.466136] path_openat+0x4e8/0xe40
> [ 20.466417] do_filp_open+0x79/0x100
Hmm. Is this perhaps an O_CREAT case? I think we only do the dcache
lookups under RCU, not the final path component creation.
And there are probably lots of other situations where we finish with
LOOKUP_RCU (with unlazy_walk()), and then continue.
Example: look at "may_lookup()" - if inode_permission() says "I can't
do this without blocking" the logic actually just tries to validate
the current state (that "unlazy_walk()" thing), and then continue
without RCU.
It obviously hasn't been about lockless semantics, it's been about
really being lockless. So LOOKUP_RCU has been a "try to do this
locklessly" rather than "you cannot take any locks".
I guess we would have to add a LOOKUP_NOBLOCK thing to actually then
say "if the RCU lookup fails, return -EAGAIN".
That's probably not a huge undertaking, but yeah, I didn't think of
it. I think this is a "we need to have Al tell us if it's reasonable".
Linus
