Date: Mon, 26 Mar 2007 15:11:03 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-current@freebsd.org Cc: Julian Elischer <julian@elischer.org>, FreeBSD Current <current@freebsd.org> Subject: Re: proc lock might become an sx lock? Message-ID: <200703261511.04364.jhb@freebsd.org> In-Reply-To: <45F0D1F5.9010200@elischer.org> References: <45F0D1F5.9010200@elischer.org>
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On Thursday 08 March 2007 10:18:13 pm Julian Elischer wrote:
> currently the thread list in the process is protected by the sched lock.
> for a process with a lot of threads this is probably not a good idea.
> I experimented with making it protected by the proc loc, but the following
> sort of thing happens a lot:
>
> sx_slock(&allproc_lock);
> FOREACH_PROC_IN_SYSTEM(p) {
> mtx_lock_spin(&sched_lock);
> FOREACH_THREAD_IN_PROC(p, td) {
> ...
> }
> mtx_unlock_spin(&sched_lock);
>
> Changing the protection of the thread list to use the proc lock would
> replace the sched_lock with the proc lock, but.....
> this has a problem.. the proc lock is a mutex and can therefore not be
inside the
> allproc_lock.
>
> and in fact you get:
>
> Trying to mount root from ufs:/dev/aacd0s1d
> panic: blockable sleep lock (sleep mutex) process lock @
kern/sched_4bsd.c:383
> cpuid = 2
> KDB: enter: panic
> [thread pid 48 tid 100054 ]
> Stopped at kdb_enter+0x2b: nop
> db> bt
> Tracing pid 48 tid 100054 td 0xc5ff4a20
> kdb_enter(c06ce300) at kdb_enter+0x2b
> panic(c06d3506,c06e7061,c06cd73b,c06cff47,17f,...) at panic+0x11c
> witness_checkorder(c60a12c8,9,c06cff47,17f) at witness_checkorder+0xb8
> _mtx_lock_flags(c60a12c8,0,c06cff3e,17f,85,...) at _mtx_lock_flags+0x87
> schedcpu(e65a9d24,c0516f50,0,e65a9d38,c6259000,...) at schedcpu+0x80
> schedcpu_thread(0,e65a9d38) at schedcpu_thread+0x9
>
> My reading of the man page is that making it an sx lock and locking it in
> shared mode would be sufficient for this sort of thing (assuming we are not
changing
> the thread list) and would be just fine.
>
> I'm not very familiar with the implementation of sx locks in freeBSD so I'm
just learning
> about them.
>
> am I reading this right? and does anyone else have any thoughts on this?
Use rwlocks to make a mutex have reader/writer semantics but still fit into
the current lock order. However, you likely should coordinate sched_lock
changes like this with Attilio and Jeff R first as they are removing
sched_lock and already have substantial diffs.
--
John Baldwin
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