Re: Bug about sched_4bsd?

On Mon, Jan 18, 2010 at 3:58 AM, Attilio Rao <attilio_at_freebsd.org> wrote:
> 2010/1/17 Kohji Okuno <okuno.kohji_at_jp.panasonic.com>:
>> Hello,
>>
>> Could you check sched_4bsd.patch, please?
>
> I think, instead, that what needs to happen is to have sched_switch()
> to do a lock handover from sleepq/turnstile spinlock to schedlock.
> That way, if threads are willing to contest on td_lock they will be
> still inhibited.
> I'm not sure if this patch breaks any invariant, if you may test I
> would appreciate:
> http://www.freebsd.org/~attilio/sched_4bsd_schedlock.diff

I stressed an 8-core machine with pho's stress2 kernel stress suite and
your patch seems to break the invariant THREAD_LOCKPTR_ASSERT in
turnstile_claim:subr_turnstile.c

The relevant stack trace are:

Tracing command creat pid 79098 tid 100624 td 0xc8c59af0
kdb_enter(c0c9b0d1,c0c9b0d1,c0c9f546,e978a85c,1,...) at kdb_enter+0x3a
panic(c0c9f546,c9633af0,c0dfc024,c9d3e280,c0f5bd3c,...) at panic+0x136
turnstile_claim(c9d3e280,2,c0ca7376,1f0,4,...) at turnstile_claim+0x148
_rw_try_upgrade(c0f5bd3c,c0ca7376,1f0,e978a990,e978ab88,...) at
_rw_try_upgrade+0xe6
cache_lookup(c99e9bb0,e978ab74,e978ab88,0,0,...) at cache_lookup+0x362
nfs_lookup(e978aa50,e978aa50,e978ab5c,200000,e978ab5c,...) at nfs_lookup+0xf6
VOP_LOOKUP_APV(c0da5fe0,e978aa50,e978ab88,1f1,e978ab74,...) at
VOP_LOOKUP_APV+0xa5
lookup(e978ab5c,c0ca7aea,ea,c5,cc093d48,...) at lookup+0x66b
namei(e978ab5c,c08d3aab,c0c94c96,c0c92df9,3,...) at namei+0x55f
kern_statat_vnhook(c8c59af0,0,ffffff9c,bfbfdf78,0,...) at
kern_statat_vnhook+0x72
kern_statat(c8c59af0,0,ffffff9c,bfbfdf78,0,...) at kern_statat+0x3c
kern_stat(c8c59af0,bfbfdf78,0,e978ac18,2,...) at kern_stat+0x36
stat(c8c59af0,e978acf8,8,c0ca19f2,c0d88230,...) at stat+0x2f
syscall(e978ad38) at syscall+0x2a3
Xint0x80_syscall() at Xint0x80_syscall+0x20
--- syscall (188, FreeBSD ELF32, stat), eip = 0x2817d1a3, esp =
0xbfbfdecc, ebp = 0xbfbfe388 ---

Tracing command creat pid 79122 tid 100202 td 0xc9633af0
sched_switch(c9633af0,0,103,18c,48fb36ba,...) at sched_switch+0x1c5
mi_switch(103,0,c0c9fc0e,2e2,c9d3e280,...) at mi_switch+0x200
turnstile_wait(c9d3e280,0,0,c0f5bd3c,c0f2cb10,...) at turnstile_wait+0x495
_rw_wlock_hard(c0f5bd3c,c9633af0,c0ca7376,209,0,...) at _rw_wlock_hard+0x20c
_rw_wlock(c0f5bd3c,c0ca7376,209,e9147990,e9147b88,...) at _rw_wlock+0xae
cache_lookup(c99e9aa0,e9147b74,e9147b88,0,0,...) at cache_lookup+0x46f
nfs_lookup(e9147a50,e9147a50,e9147b5c,200000,e9147b5c,...) at nfs_lookup+0xf6
VOP_LOOKUP_APV(c0da5fe0,e9147a50,e9147b88,1f1,e9147b74,...) at
VOP_LOOKUP_APV+0xa5
lookup(e9147b5c,c0ca7aea,ea,c5,c962caa0,...) at lookup+0x66b
namei(e9147b5c,c08d3aab,c0c94c96,c0c92df9,3,...) at namei+0x55f
kern_statat_vnhook(c9633af0,0,ffffff9c,bfbfdf78,0,...) at
kern_statat_vnhook+0x72
kern_statat(c9633af0,0,ffffff9c,bfbfdf78,0,...) at kern_statat+0x3c
kern_stat(c9633af0,bfbfdf78,0,e9147c18,2,...) at kern_stat+0x36
stat(c9633af0,e9147cf8,8,c0c828f4,c0d88230,...) at stat+0x2f
syscall(e9147d38) at syscall+0x2a3
Xint0x80_syscall() at Xint0x80_syscall+0x20
--- syscall (188, FreeBSD ELF32, stat), eip = 0x2817d1a3, esp =
0xbfbfdecc, ebp = 0xbfbfe388 ---

What seems to happen to me is:

The thread 0xc8c59af0 sleeps on the turnstile queue by a previous call
to turnstile_wait.
The thread 0xc9633af0 call turnstile_wait and does a voluntary switch.
The call to thread_lock_set
added from your patch to sched_switch, wakes up the thread 0xc8c59af0
and while thread 0xc9633af0
is in the middle of sched_switch (so before cpu_switch), thread
0xc8c59af0 is running turnstile_claim
that discover the thread 0xc9633af0 not hold a turnstile lock by
THREAD_LOCKPTR_ASSERT invariant assertion.

I attached the entire stack trace.
If needed I have the coredump.

Hope this help.

--
Gianni