Re: Spinlocks waiting with interrupts disabled / preempt disabled.

[Peter Zijlstra]

On Fri, 2008-05-09 at 18:35 +0200, Olaf Weber wrote:
> Ingo Molnar writes:
> > * Christoph Lameter <clameter@xxxxxxx> wrote:
> 
> >> @@ -132,10 +132,14 @@ unsigned long __lockfunc _write_lock_irq
> >> {
> >> unsigned long flags;
> >> 
> >> +retry:
> >> local_irq_save(flags);
> >> -	preempt_disable();
> >> -	_raw_write_lock(lock);
> >> -	return flags;
> >> +	if (_write_trylock(lock))
> >> +		return flags;
> >> +	local_irq_restore(flags);
> >> +	while (!write_can_lock(lock))
> >> +		cpu_relax();
> >> +	goto retry;
> >> }
> >> EXPORT_SYMBOL(_write_lock_irqsave);
> 
> > hm, this is done on a too high level and will turn off some debugging 
> > code. I.e. if we dont just loop long but truly deadlock here we wont 
> > call lib/spinlock_debug.c's _raw_write_lock() code that does some sanity 
> > checks in the debug case.
> 
> > so how about doing this on a deeper level and adding a new 
> > __raw_write_lock_flags() primitive that would look at the flags value 
> > and could enable interrupts in the lowlevel code?
> 
> Thinking of an alternative approach, I'd like to revisit the original
> problem for a moment: on a (big) system running a kernel based on
> 2.6.16 we found that in the following stack trace, pdflush has just
> enabled irqs again on its cpu after running with irqs disabled for
> about 2 minutes.
> 
> >> bt 0xe0000168f4378000
> ================================================================
> STACK TRACE FOR TASK: 0xe0000168f4378000 (pdflush)
> 
>  0 kdba_main_loop+0x14c [0xa0000001004190cc]
>  1 kdb+0x11fc [0xa0000001002a639c]
>  2 kdb_ipi+0x11c [0xa0000001002a12dc]
>  3 handle_IPI+0x27c [0xa00000010005671c]
>  4 handle_IRQ_event+0x8c [0xa0000001000fa40c]
>  5 __do_IRQ+0x12c [0xa0000001000fa5cc]
>  6 ia64_handle_irq+0x15c [0xa00000010001213c]
>  7 ia64_leave_kernel [0xa00000010000cb20]
>  8 _write_unlock_irqrestore+0x40 [0xa000000100558500]
>  9 test_set_page_writeback+0x18c [0xa000000100114b6c]
> 10 xfs_start_page_writeback+0x2c [0xa0000002e4ab914c]
> 11 xfs_page_state_convert+0xa0c [0xa0000002e4abcbac]
> 12 xfs_vm_writepage+0x19c [0xa0000002e4abd13c]
> 13 mpage_writepages+0x3fc [0xa0000001001bcb5c]
> 14 xfs_vm_writepages+0xac [0xa0000002e4ab9eac]
> 15 do_writepages+0xac [0xa0000001001136cc]
> 16 __writeback_single_inode+0x47c [0xa0000001001b8f7c]
> 17 sync_sb_inodes+0x4ac [0xa0000001001b9cec]
> 18 writeback_inodes+0x19c [0xa0000001001baafc]
> 19 wb_kupdate+0x26c [0xa000000100113bcc]
> 20 pdflush+0x38c [0xa00000010011582c]
> 21 kthread+0x22c [0xa0000001000c9dac]
> 22 kernel_thread_helper+0xcc [0xa000000100012f6c]
> 23 start_kernel_thread+0x1c [0xa0000001000094bc]
> ================================================================
> 
> There is, as far as I could tell, no code in the critical region of
> test_set_page_writeback that could take this much time, except for the
> call to _write_lock_irqsave itself.  There the root of the problem
> seems to have been that many threads went after the read lock, and an
> rwlock_t will allow readers in while readers hold the lock, even if a
> writer needs it, thus potentially starving the writer indefinitely (up
> to 2 minutes in the case under discussion).
> 
> Re-enabling irqs when looping in _write_lock_irqsave (and *_irq and
> *_bh) works here because _write_lock_irqsave() wasn't called with irqs
> disabled.  But pdflush will still spend a long time getting the lock.
> 
> The reason we're pursuing this nevertheless is that it is easy to do
> starting from the current code-base, and doesn't impose API or ABI
> changes.

This would be your best option for you kabi constrained .16 kernel.

> An alternative approach would to view this as a bug of the rwlock_t
> code in particular, and fix it by keeping new readers from a acquiring
> an rwlock_t if a writer is also trying to lock it.  For x86 at least,
> I can sketch an implementation that is based on the new ticket-based
> spinlocks:
>  - use the ticket-based lock for writers
>  - add a reader count for readers (16 bits out of 32 are free)
>  - readers get in if the write lock is idle
>  - if the write lock wasn't idle, readers loop until it is
>  - on obtaining the ticket-based lock, the writer waits for active
>    readers to leave (eg reader count to go to zero)
> Contention by writers now keeps readers out, which may be a problem.
> 
> Advantages of such an approach:
>  - Writers are "fair" amongst themselves.
>  - Readers cannot hold off writers.
> 
> Disadvantages:
>  - Contention among writers can hold off readers indefinitely.
>    This scheme only works if the write lock is not "permanently"
>    contended, otherwise it goes from frying pan to fire.
>  - Requires changes to the asm-coded locking primitives, which means
>    it is likely to be done for x86 and ia64 only, unless the platform
>    maintainers step in.
> 
> Given the above, is this worth pursuing for inclusion in mainline?

Keep in mind that rwlock_t must support recusive read locks. Much code
depends on this.

> A third solution would be to look at this problem on a case-by-case
> basis.  In the case under discussion, there may be other kernel bugs
> that aggravate the problem (it is an old kernel after all) and maybe
> this just means the address_space.tree_lock ought to be replaced with
> something else (though I wouldn't claim to know what).

That has been done by Nick Piggin, the lockless pagecache work removes
the read side of the tree_lock.


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