On 07/09/2014 05:50 AM, Vlastimil Babka wrote:
> On 07/09/2014 08:35 AM, Hugh Dickins wrote:
>> On Wed, 9 Jul 2014, Sasha Levin wrote:
>>> [ 363.600969] INFO: task trinity-c327:9203 blocked for more than 120 seconds.
>>> [ 363.605359] Not tainted 3.16.0-rc4-next-20140708-sasha-00022-g94c7290-dirty #772
>>> [ 363.609730] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>>> [ 363.615861] trinity-c327 D 000000000000000b 13496 9203 8559 0x10000004
>>> [ 363.620284] ffff8800b857bce8 0000000000000002 ffffffff9dc11b10 0000000000000001
>>> [ 363.624468] ffff880104860000 ffff8800b857bfd8 00000000001d7740 00000000001d7740
>>> [ 363.629118] ffff880104863000 ffff880104860000 ffff8800b857bcd8 ffff8801eaed8868
>>> [ 363.633879] Call Trace:
>>> [ 363.635442] [<ffffffff9a4dc535>] schedule+0x65/0x70
>>> [ 363.638638] [<ffffffff9a4dc948>] schedule_preempt_disabled+0x18/0x30
>>> [ 363.642833] [<ffffffff9a4df0a5>] mutex_lock_nested+0x2e5/0x550
>>> [ 363.646599] [<ffffffff972a4d7c>] ? shmem_fallocate+0x6c/0x350
>>> [ 363.651319] [<ffffffff9719b721>] ? get_parent_ip+0x11/0x50
>>> [ 363.654683] [<ffffffff972a4d7c>] ? shmem_fallocate+0x6c/0x350
>>> [ 363.658264] [<ffffffff972a4d7c>] shmem_fallocate+0x6c/0x350
>>
>> So it's trying to acquire i_mutex at shmem_fallocate+0x6c...
>>
>>> [ 363.662010] [<ffffffff971bd96e>] ? put_lock_stats.isra.12+0xe/0x30
>>> [ 363.665866] [<ffffffff9730c043>] do_fallocate+0x153/0x1d0
>>> [ 363.669381] [<ffffffff972b472f>] SyS_madvise+0x33f/0x970
>>> [ 363.672906] [<ffffffff9a4e3f13>] tracesys+0xe1/0xe6
>>> [ 363.682900] 2 locks held by trinity-c327/9203:
>>> [ 363.684928] #0: (sb_writers#12){.+.+.+}, at: [<ffffffff9730c02d>] do_fallocate+0x13d/0x1d0
>>> [ 363.715102] #1: (&sb->s_type->i_mutex_key#16){+.+.+.}, at: [<ffffffff972a4d7c>] shmem_fallocate+0x6c/0x350
>>
>> ...but it already holds i_mutex, acquired at shmem_fallocate+0x6c.
>> Am I reading that correctly?
>
> I wonder, why wouldn't lockdep fire here if it was a double lock? I assume lockdep is enabled. It seems to me that the lock #1 is being printed because it's being acquired, not because it already is acquired. __mutex_lock_common() calls mutex_acquire_nest() *before* it actually tries to acquire the mutex. So the output is just confusing.
Nope, it's not a double lock - it's easy to misread lockdep output here.
lockdep marks locks as held even before they are actually acquired:
static __always_inline int __sched
__mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
struct lockdep_map *nest_lock, unsigned long ip,
struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
{
struct task_struct *task = current;
struct mutex_waiter waiter;
unsigned long flags;
int ret;
preempt_disable();
mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip); <=== Lock marked as acquired
This is done to avoid races where the lock is actually acquired but not showing up
in lockdep.
So this trace should be read as: "We acquired sb_writers in do_fallocate() and are
blocked waiting to lock i_mutex in shmem_fallocate".
> So it would again help to see stacks of other tasks, to see who holds the i_mutex and where it's stuck...
The stacks print got garbled due to having large amount of tasks and too low of a
console buffer. I've fixed that and will update when (if) the problem reproduces.
Thanks,
Sasha
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