On Wed, 2006-01-25 at 19:00 +0100, Ingo Molnar wrote:
> clocksource_lock is used in softirq-context via e.g.
> timeofday_periodic_hook() -> get_next_clocksource(), but the lock is not
> acquired in a softirq-safe manner - which could lead to deadlocks.
Nice catch. Andrew also noticed this issue just a few hours before you
sent this. Great minds...
> this bug was found via the lock validator i'm working on:
>
> ============================
> [ BUG: illegal lock usage! ]
> ----------------------------
> illegal {used-in-softirq} -> {enabled-softirqs} usage.
> swapper/1 [HC0[0]:SC0[0]:HE1:SE1] takes {clocksource_lock [u:21]}, at:
> [<c013f526>] register_clocksource+0x16/0xf0
> {used-in-softirq} state was registered at:
> [<c013f22d>] get_next_clocksource+0xd/0x40
> hardirqs last enabled at: [<c011d31a>] vprintk+0x28a/0x3e0
> softirqs last enabled at: [<c0122999>] irq_exit+0x39/0x50
>
> other info that might help in debugging this:
> ------------------------------
> | showing all locks held by: | (swapper/1 [c30d7790, 125]): <none>
> ------------------------------
>
> [<c010432d>] show_trace+0xd/0x10
> [<c0104347>] dump_stack+0x17/0x20
> [<c01379b2>] print_usage_bug+0x1e2/0x200
> [<c013817d>] mark_lock+0x28d/0x2a0
> [<c0138835>] debug_lock_chain+0x6a5/0x10d0
> [<c013929d>] debug_lock_chain_spin+0x3d/0x60
> [<c026570d>] _raw_spin_lock+0x2d/0x90
> [<c04d88d8>] _spin_lock+0x8/0x10
> [<c013f526>] register_clocksource+0x16/0xf0
> [<c0681137>] init_pit_clocksource+0x57/0x90
> [<c01003fa>] init+0xfa/0x3e0
> [<c0100ef5>] kernel_thread_helper+0x5/0x10
>
> the fix is to lock clocksource_lock in a softirq-safe way.
>
> (with this patch applied, the timeofday code validates fine.)
I'm getting lots of local_bh_disable warnings with this patch on my
timeofday tree (interrupts are disabled in timefoday_periodic_hook which
calls get_next_clocksource). Wouldn't using spin_lock_irqsave/restore be
better here (seems to work for me)?
thanks
-john
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