Messages in this thread | | | Date | Wed, 16 May 2012 07:57:17 -0700 | From | Stephen Hemminger <> | Subject | Re: [PATCH] floppy: convert to delayed work and single-thread wq |
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On Wed, 16 May 2012 09:36:51 +0200 (CEST) Jiri Kosina <jkosina@suse.cz> wrote:
> There are several races in floppy driver between bottom half > (scheduled_work) and timers (fd_timeout, fd_timer). Due to slowness of the > actual floppy devices, those races are never (at least to my knowledge) > triggered on a bare floppy metal. However on virtualized (emulated) floppy > drives, which are of course magnitudes faster than the real ones, these > races trigger reliably. They usually exhibit themselves as NULL pointer > dereferences during DMA setup, such as > > BUG: unable to handle kernel NULL pointer dereference at 0000000a > [ ... snip ... ] > EIP: 0060:[<c02053d5>] EFLAGS: 00010293 CPU: 0 > EAX: ffffe000 EBX: 0000000a ECX: 00000000 EDX: 0000000a > ESI: c05d2718 EDI: 00000000 EBP: 00000000 ESP: f540fe44 > DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068 > Process swapper (pid: 0, ti=f540e000 task=c082d5a0 task.ti=c0826000) > Stack: > ffffe000 00001ffc 00000000 00000000 00000000 c05d2718 c0708b40 f540fe80 > c020470f c05d2718 c0708b40 00000000 f540fe80 0000000a f540fee4 00000000 > c0708b40 f540fee4 00000000 00000000 c020526b 00000000 c05d2718 c0708b40 > Call Trace: > [<c020470f>] dump_trace+0xaf/0x110 > [<c020526b>] show_trace_log_lvl+0x4b/0x60 > [<c0205298>] show_trace+0x18/0x20 > [<c05c5811>] dump_stack+0x6d/0x72 > [<c0248527>] warn_slowpath_common+0x77/0xb0 > [<c02485f3>] warn_slowpath_fmt+0x33/0x40 > [<f7ec593c>] setup_DMA+0x14c/0x210 [floppy] > [<f7ecaa95>] setup_rw_floppy+0x105/0x190 [floppy] > [<c0256d08>] run_timer_softirq+0x168/0x2a0 > [<c024e762>] __do_softirq+0xc2/0x1c0 > [<c02042ed>] do_softirq+0x7d/0xb0 > [<f54d8a00>] 0xf54d89ff > > but other instances can be easily seen as well. This can be observed at > least under VMWare, VirtualBox and KVM. > > This patch converts all the timers and bottom halfs to be processed in a > single workqueue. This aproach has been already discussed back in 2010 and > Acked by Linus [1], but it then never made it to the tree. > > This all is based on original idea and code of Stephen Hemminger. I have > ported original Stepen's code to the current state of the floppy driver, > and performed quite some testing (on real hardware), which didn't reveal > any issues (this includes not only writing and reading data, but also > formatting (unfortunately I didn't find any Double-Density disks any > more)). Ability to handle errors properly (supplying known bad floppies) > has also been verified. > > [1] http://kerneltrap.org/mailarchive/linux-kernel/2010/6/11/4582092 > > Based-on-patch-by: Stephen Hemminger <shemminger@vyatta.com> > Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Thanks, the hold up for me was always finding real working floppy drive to test.
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