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    Subject[PATCH 0/4][RT] futex: fix tasks blocking on two rt_mutex locks
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    This patch-set address the following problem reported by Mike Galbraith:

    > Stress testing, looking to trigger RCU stalls, I've managed to find a
    > way to repeatably create fireworks. (got RCU stall, see attached)
    >
    > 1. download ltp-full-20100630. Needs to be this version because of
    > testcase bustage in earlier versions, and must be built with gcc > 4.3,
    > else testcases will segfault due to a gcc bug.
    >
    > 2. apply patchlet so you can run testcases/realtime/perf/latency/run.sh
    > at all.
    >
    > --- pthread_cond_many.c.org 2010-07-05 09:05:59.000000000 +0200
    > +++ pthread_cond_many.c 2010-07-04 12:12:25.000000000 +0200
    > @@ -259,7 +259,7 @@ void usage(void)
    >
    > int parse_args(int c, char *v)
    > {
    > - int handled;
    > + int handled = 1;
    > switch (c) {
    > case 'h':
    > usage();
    >
    > 3. add --realtime for no particular reason.
    >
    > --- run.sh.org 2010-07-06 15:54:58.000000000 +0200
    > +++ run.sh 2010-07-06 16:37:34.000000000 +0200
    > @@ -22,7 +22,7 @@ make
    > # process to run realtime. The remainder of the processes (if any)
    > # will run non-realtime in any case.
    >
    > -nthread=5000
    > +nthread=500
    > iter=400
    > nproc=5
    >
    > @@ -39,7 +39,7 @@ i=0
    > i=1
    > while test $i -lt $nproc
    > do
    > - ./pthread_cond_many --broadcast -i $iter -n $nthread > $nthread.$iter.$nproc.$i.out &
    > + ./pthread_cond_many --realtime --broadcast -i $iter -n $nthread > $nthread.$iter.$nproc.$i.out &
    > i=`expr $i + 1`
    > done
    > wait

    The LTP wreckage will be addressed separately <ugh>

    > 4. run it.
    >
    > What happens here is we hit WARN_ON(pendowner->pi_blocked_on != waiter),
    > this does not make it to consoles (poking sysrq-foo doesn't either).
    > Next comes WARN_ON(!pendowner->pi_blocked_on), followed by the NULL
    > explosion, which does make it to consoles.
    >
    > With explosion avoidance, I also see pendowner->pi_blocked_on->task ==
    > NULL at times, but that, as !pendowner->pi_blocked_on, seems to be
    > fallout. The start of bad juju is always pi_blocked_on != waiter.
    >
    > [ 141.609268] BUG: unable to handle kernel NULL pointer dereference at 0000000000000058
    > [ 141.609268] IP: [<ffffffff8106856d>] wakeup_next_waiter+0x12c/0x177
    > [ 141.609268] PGD 20e174067 PUD 20e253067 PMD 0
    > [ 141.609268] Oops: 0000 [#1] PREEMPT SMP
    > [ 141.609268] last sysfs file: /sys/devices/system/cpu/cpu3/cache/index2/shared_cpu_map
    > [ 141.609268] CPU 0
    > [ 141.609268] Pid: 8154, comm: pthread_cond_ma Tainted: G W 2.6.33.6-rt23 #12 MS-7502/MS-7502
    > [ 141.609268] RIP: 0010:[<ffffffff8106856d>] [<ffffffff8106856d>] wakeup_next_waiter+0x12c/0x177
    > [ 141.609268] RSP: 0018:ffff88020e3cdd78 EFLAGS: 00010097
    > [ 141.609268] RAX: 0000000000000000 RBX: ffff8801e8eba5c0 RCX: 0000000000000000
    > [ 141.609268] RDX: ffff880028200000 RSI: 0000000000000046 RDI: 0000000000000009
    > [ 141.609268] RBP: ffff88020e3cdda8 R08: 0000000000000002 R09: 0000000000000000
    > [ 141.609268] R10: 0000000000000005 R11: 0000000000000000 R12: ffffffff81659068
    > [ 141.609268] R13: ffff8801e8ebdb58 R14: 0000000000000000 R15: ffff8801e8ebac08
    > [ 141.609268] FS: 00007f664d539700(0000) GS:ffff880028200000(0000) knlGS:0000000000000000
    > [ 141.609268] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    > [ 141.609268] CR2: 0000000000000058 CR3: 0000000214266000 CR4: 00000000000006f0
    > [ 141.609268] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
    > [ 141.609268] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
    > [ 141.609268] Process pthread_cond_ma (pid: 8154, threadinfo ffff88020e3cc000, task ffff88020e2a4700)
    > [ 141.609268] Stack:
    > [ 141.609268] 0000000000000000 ffffffff81659068 0000000000000202 0000000000000000
    > [ 141.609268] <0> 0000000000000000 0000000080001fda ffff88020e3cddc8 ffffffff812fec48
    > [ 141.609268] <0> ffffffff81659068 0000000000606300 ffff88020e3cddd8 ffffffff812ff1b9
    > [ 141.609268] Call Trace:
    > [ 141.609268] [<ffffffff812fec48>] rt_spin_lock_slowunlock+0x43/0x61
    > [ 141.609268] [<ffffffff812ff1b9>] rt_spin_unlock+0x46/0x48
    > [ 141.609268] [<ffffffff81067d7f>] do_futex+0x83c/0x935
    > [ 141.609268] [<ffffffff810c26ce>] ? handle_mm_fault+0x6de/0x6f1
    > [ 141.609268] [<ffffffff81067e36>] ? do_futex+0x8f3/0x935
    > [ 141.609268] [<ffffffff81067fba>] sys_futex+0x142/0x154
    > [ 141.609268] [<ffffffff81020eb0>] ? do_page_fault+0x23e/0x28e
    > [ 141.609268] [<ffffffff81004aa7>] ? math_state_restore+0x3d/0x3f
    > [ 141.609268] [<ffffffff81004b08>] ? do_device_not_available+0xe/0x12
    > [ 141.609268] [<ffffffff81002c5b>] system_call_fastpath+0x16/0x1b
    > [ 141.609268] Code: c7 09 6d 41 81 e8 ac 34 fd ff 4c 39 ab 70 06 00 00 74 11 be 47 02 00 00 48 c7 c7 09 6d 41 81 e8 92 34 fd ff 48 8b 83 70 06 00 00 <4c> 39 60 58 74 11 be 48 02 00 00 48 c7 c7 09 6d 41 81 e8 74 34
    > [ 141.609268] RIP [<ffffffff8106856d>] wakeup_next_waiter+0x12c/0x177
    > [ 141.609268] RSP <ffff88020e3cdd78>
    > [ 141.609268] CR2: 0000000000000058
    > [ 141.609268] ---[ end trace 58805b944e6f93ce ]---
    > [ 141.609268] note: pthread_cond_ma[8154] exited with preempt_count 2




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