Messages in this thread | | | Date | Sat, 27 Oct 2001 13:42:32 -0400 (EDT) | From | Alexander Viro <> | Subject | Re: Poor floppy performance in kernel 2.4.10 |
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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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