__do_softirq() uses __local_bh_disable_ip(SOFTIRQ_OFFSET) to disable SOFTIRQs and signaling that softirqs are served.
When done, __do_softirq() calls __local_bh_enable(SOFTIRQ_OFFSET) to enable them again. The reason it does not call __local_bh_enable_ip(SOFTIRQ_OFFSET) is because __local_bh_enable_ip() would process still-pending softirqs. Now, we face the same problem as in __local_bh_disable_ip() with the preempt tracer hooking into preempt_count_sub(). This will case lockdep warning, when for example the kernel is configured to run all IRQs as threads and preemptoff tracer is activated: [ 42.209684] WARNING: CPU: 2 PID: 16 at kernel/locking/lockdep.c:3574 check_flags.part.36+0x1bc/0x210() [ 42.210053] DEBUG_LOCKS_WARN_ON(current->softirqs_enabled) [ 42.210053] Kernel panic - not syncing: panic_on_warn set ... [ 42.210053] [ 42.210053] CPU: 2 PID: 16 Comm: ksoftirqd/2 Not tainted 4.1.0-rc4 #666 [ 42.210053] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140709_153950- 04/01/2014 [ 42.210053] ffff88007ca47ab8 ffff88007ca47ab8 ffffffff81b5a06a 0000000000000007 [ 42.210053] ffffffff81ef08b8 ffff88007ca47b38 ffffffff81b567ee 0000000000000102 [ 42.210053] ffffffff00000008 ffff88007ca47b48 ffff88007ca47ae8 ffffffff81b56733 [ 42.210053] Call Trace: [ 42.210053] [<ffffffff81b5a06a>] dump_stack+0x4f/0x7b [ 42.210053] [<ffffffff81b567ee>] panic+0xc0/0x1e9 [ 42.210053] [<ffffffff81b56733>] ? panic+0x5/0x1e9 [ 42.210053] [<ffffffff81b66608>] ? _raw_spin_unlock_irqrestore+0x38/0x80 [ 42.210053] [<ffffffff81057fe0>] warn_slowpath_common+0xc0/0xc0 [ 42.210053] [<ffffffff81058026>] warn_slowpath_fmt+0x46/0x50 [ 42.210053] [<ffffffff81057fe5>] ? warn_slowpath_fmt+0x5/0x50 [ 42.210053] [<ffffffff810ac90c>] check_flags.part.36+0x1bc/0x210 [ 42.210053] [<ffffffff810ad5a9>] lock_acquire+0x119/0x2b0 [ 42.210053] [<ffffffff81b663ae>] ? _raw_spin_lock_irqsave+0x1e/0x90 [ 42.210053] [<ffffffff810886a5>] ? preempt_count_add+0x5/0xc0 [ 42.210053] [<ffffffff81b663dc>] _raw_spin_lock_irqsave+0x4c/0x90 [ 42.210053] [<ffffffff8113f1b6>] ? check_critical_timing+0xa6/0x160 [ 42.210053] [<ffffffff81b66395>] ? _raw_spin_lock_irqsave+0x5/0x90 [ 42.210053] [<ffffffff8113f1b6>] check_critical_timing+0xa6/0x160 [ 42.210053] [<ffffffff8105e40b>] ? __do_softirq+0x3fb/0x670 [ 42.210053] [<ffffffff8105c7f6>] ? __local_bh_enable+0x36/0x70 [ 42.210053] [<ffffffff8105e40b>] ? __do_softirq+0x3fb/0x670 [ 42.210053] [<ffffffff8105c7f6>] ? __local_bh_enable+0x36/0x70 [ 42.210053] [<ffffffff8113fde1>] trace_preempt_on+0xf1/0x110 [ 42.210053] [<ffffffff81088765>] ? preempt_count_sub+0x5/0xf0 [ 42.210053] [<ffffffff8108880b>] preempt_count_sub+0xab/0xf0 [ 42.210053] [<ffffffff8105c7f6>] ? __local_bh_enable+0x36/0x70 [ 42.210053] [<ffffffff8105c7f6>] __local_bh_enable+0x36/0x70 [ 42.210053] [<ffffffff8105e40b>] ? __do_softirq+0x3fb/0x670 [ 42.210053] [<ffffffff8105e40b>] __do_softirq+0x3fb/0x670 [ 42.210053] [<ffffffff8105e69f>] run_ksoftirqd+0x1f/0x60 [ 42.210053] [<ffffffff8107f8f3>] smpboot_thread_fn+0x193/0x2a0 [ 42.210053] [<ffffffff8107f760>] ? sort_range+0x30/0x30 [ 42.210053] [<ffffffff8107bac2>] kthread+0xf2/0x110 [ 42.210053] [<ffffffff81b61ac3>] ? wait_for_completion+0xc3/0x120 [ 42.210053] [<ffffffff8108880b>] ? preempt_count_sub+0xab/0xf0 [ 42.210053] [<ffffffff8107b9d0>] ? kthread_create_on_node+0x240/0x240 [ 42.210053] [<ffffffff81b674c2>] ret_from_fork+0x42/0x70 [ 42.210053] [<ffffffff8107b9d0>] ? kthread_create_on_node+0x240/0x240 [ 42.210053] Kernel Offset: disabled [ 42.210053] ---[ end Kernel panic - not syncing: panic_on_warn set ... To mitigate this, call the preempt_tracer manually and then update the preempt_count. While at we are at it also update the comment on _local_bh_enable. cond_resched_softirq() used to call _local_bh_enable(), now it uses local_bh_enable(). This was changed by: commit 98d8256739f2c6c636fa2da359f5949c739ae839 Author: Thomas Gleixner <[email protected]> Date: Wed May 23 13:58:18 2007 -0700 Prevent going idle with softirq pending Signed-off-by: Daniel Wagner <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Steven Rostedt <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: [email protected] --- kernel/softirq.c | 22 ++++++++++++++++++---- 1 file changed, 18 insertions(+), 4 deletions(-) diff --git a/kernel/softirq.c b/kernel/softirq.c index 479e443..9ec59f2 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -124,19 +124,33 @@ void __local_bh_disable_ip(unsigned long ip, unsigned int cnt) EXPORT_SYMBOL(__local_bh_disable_ip); #endif /* CONFIG_TRACE_IRQFLAGS */ +/* + * Special-case - softirqs can safely be enabled in + * __do_softirq(), without processing still-pending softirqs. + */ static void __local_bh_enable(unsigned int cnt) { WARN_ON_ONCE(!irqs_disabled()); + /* + * The preempt tracer hooks into preempt_count_sub and will break + * lockdep because it calls back into lockdep before SOFTIRQ_OFFSET + * is cleared while current->softirq_enabled is set. + * We must call the trace_preempt_off now, and decrement preempt_count + * afterwards manually. + */ + if (preempt_count() == cnt) + trace_preempt_on(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1)); + if (softirq_count() == (cnt & SOFTIRQ_MASK)) trace_softirqs_on(_RET_IP_); - preempt_count_sub(cnt); + + __preempt_count_sub(cnt); } /* - * Special-case - softirqs can safely be enabled in - * cond_resched_softirq(), or by __do_softirq(), - * without processing still-pending softirqs: + * Special-case - enable softirqs without processing + * still pending softirqs from IRQ context. */ void _local_bh_enable(void) { -- 2.1.0 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
