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SubjectRe: Poor floppy performance in kernel 2.4.10


On Sat, 27 Oct 2001, Alain Knaff wrote:

> Cursory examination of floppy.c (as an example of a block device
> driver) showed that bdops are also registered using devfs_register and
> register_disk (what's THAT for?!? Floppies don't have partitions...)

Actually, _that_ is Right Thing(tm) - it should allocate a structure that
would contain pointer to methods table and would be controlled by
driver. devfs_register() would get that + prefered name, etc. so that
we had a common object. Then driver would have a point where it could
tell the rest of kernel that disk is gone.

> Apparently, devfs_register allows a direct mapping from the device's
> name to its driver, without going through its major/minor number.
>
> Thus, a possible solution would be to equip all possible paths leading
> to the driver's block_device_operations with correct "teardown"
> function. Thus, not only unregister_blkdev would dump the cache, but
> also devfs_unregister (maybe near the place in unregister() where
> de->u.fcb.ops = NULL is done?). Best make this call generic, such as

No go. We can have situations where some of uses come from devfs and
some - from normal device nodes. struct block_device will be the same.

> All this begs of course the following question: what kind of
> idnetifier does the buffer cache code actually use to refer to the
> block device, if there is no longer a major?

Right now - major:minor, in 2.5 - struct block_device *.

> We could either use bdev->bd_sem (awkward, as many drivers implement
> multiple bdev's), or a new per-major device lock to protect that
> section.

I'd rather have refcount raised by get_blkfops(). Again, that code path
is not a problem. devfs_get_ops() is.

> unregister_blkdev would need to acquire the same lock while zero-ing
> blkdevs[major].bdops.

We could put bdev on per-major cyclic list and have it killed on
unregister_blkdev(). _That_ is easy. The trouble being, with devfs
we don't have a single removal point. Sometimes it's still
unregister_blkdev(), sometimes - crapload of devfs_unregister() for
each minor, sometimes - both. Worse yet, we have one more place that
holds pointer to block_device_operations - gendisk. Also used by
devfs (and nothing else) and logics is, to put it mildly, fuzzy.

Frankly, at that point I would prefer to remove the code in devfs that
tries to provide bdev methods by devfs entry. Rationale:
a) it's fucked up beyond any repair
b) it will be useless until we switch buffer cache to block_device *
c) we will need to change that logics anyway - as it is the thing is
inherently racy
d) right now it stands in the way of long-living cache stuff _and_
introduces an oopsable race between mount and rmmod.


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