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SubjectRe: [PATCH] sched: Avoid that __wait_on_bit_lock() hangs
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On 08/13/2016 09:32 AM, Oleg Nesterov wrote:
> On 08/12, Bart Van Assche wrote:
>> before I started testing. It took some time
>> before I could reproduce the hang in truncate_inode_pages_range().
>
> all I can say this contradicts with the previous testing results with
> my previous patch or with your change in abort_exclusive_wait().

Hello Oleg,

My opinion is that all this means is that we do not yet have a full
understanding of what is going on.

BTW, I have improved my page lock owner instrumentation patch such that
it prints a call stack of the lock owner if lock_page() takes too long.
The following call stack was reported:

__lock_page / pid 8549 / m 0x2: timeout - continuing to wait for 8549
[<ffffffff8102b316>] save_stack_trace+0x26/0x50
[<ffffffff81152bee>] add_to_page_cache_lru+0x7e/0x170
[<ffffffff8121bfc5>] mpage_readpages+0xc5/0x170
[<ffffffff81215548>] blkdev_readpages+0x18/0x20
[<ffffffff81163a68>] __do_page_cache_readahead+0x268/0x310
[<ffffffff811640a8>] force_page_cache_readahead+0xa8/0x100
[<ffffffff81164139>] page_cache_sync_readahead+0x39/0x40
[<ffffffff81153967>] generic_file_read_iter+0x707/0x920
[<ffffffff81215920>] blkdev_read_iter+0x30/0x40
[<ffffffff811d4b4b>] __vfs_read+0xbb/0x130
[<ffffffff811d4f31>] vfs_read+0x91/0x130
[<ffffffff811d62b4>] SyS_read+0x44/0xa0
[<ffffffff816281e5>] entry_SYSCALL_64_fastpath+0x18/0xa8

My understanding of mpage_readpages() is that the page unlock happens
after readahead I/O completed (see also page_endio()). So this probably
means that an I/O request submitted because of readahead code did not
get completed. I will see whether I can find anything that's wrong in
the block layer.

Bart.

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