On Tue, 2020-02-11 at 03:38:29 UTC, Gustavo Luiz Duarte wrote:
> After a treclaim, we expect to be in non-transactional state. If we don't
> clear
> the current thread's MSR[TS] before we get preempted, then
> tm_recheckpoint_new_task() will recheckpoint and we get rescheduled in
> suspended transaction state.
>
> When handling a signal caught in transactional state, handle_rt_signal64()
> calls get_tm_stackpointer() that treclaims the transaction using
> tm_reclaim_current() but without clearing the thread's MSR[TS]. This can cause
> the TM Bad Thing exception below if later we pagefault and get preempted
> trying
> to access the user's sigframe, using __put_user(). Afterwards, when we are
> rescheduled back into do_page_fault() (but now in suspended state since the
> thread's MSR[TS] was not cleared), upon executing 'rfid' after completion of
> the page fault handling, the exception is raised because a transition from
> suspended to non-transactional state is invalid.
>
> Unexpected TM Bad Thing exception at c000de44 (msr
> 0x800302a03031) tm_scratch=80010280b033
> Oops: Unrecoverable exception, sig: 6 [#1]
> LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
> Modules linked in: nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6
> nf_defrag_ipv4 ip6_tables ip_tables nft_compat ip_set nf_tables nfnetlink xts
> vmx_crypto sg virtio_balloon
> r_mod cdrom virtio_net net_failover virtio_blk virtio_scsi failover
> dm_mirror dm_region_hash dm_log dm_mod
> CPU: 25 PID: 15547 Comm: a.out Not tainted 5.4.0-rc2 #32
> NIP: c000de44 LR: c0034728 CTR:
> REGS: c0003fe7bd70 TRAP: 0700 Not tainted (5.4.0-rc2)
> MSR: 800302a03031 CR: 44000884
> XER:
> CFAR: c000dda4 IRQMASK: 0
> PACATMSCRATCH: 80010280b033
> GPR00: c0034728 c00f65a17c80 c1662800
> 7fffacf3fd78
> GPR04: 1000 1000
> c00f611f8af0
> GPR08: 78006001
> 000c
> GPR12: c00f611f84b0 c0003ffcb200
>
> GPR16:
>
> GPR20:
> c00f611f8140
> GPR24: 7fffacf3fd68 c00f65a17d90
> c00f611f7800
> GPR28: c00f65a17e90 c00f65a17e90 c1685e18
> 7fffacf3f000
> NIP [c000de44] fast_exception_return+0xf4/0x1b0
> LR [c0034728] handle_rt_signal64+0x78/0xc50
> Call Trace:
> [c00f65a17c80] [c0034710] handle_rt_signal64+0x60/0xc50
> (unreliable)
> [c00f65a17d30] [c0023640] do_notify_resume+0x330/0x460
> [c00f65a17e20] [c000dcc4] ret_from_except_lite+0x70/0x74
> Instruction dump:
> 7c4ff120 e8410170 7c5a03a6 3840 f8410060 e8010070 e8410080 e8610088
> 6000 6000 e8810090 e8210078 <4c24> 4800 e8610178
> 88ed0989
> ---[ end trace 93094aa44b442f87 ]---
>
> The simplified sequence of events that triggers the above exception is:
>
> ... # userspace in NON-TRANSACTIONAL state
> tbegin # userspace in TRANSACTIONAL state
> signal delivery # kernelspace in SUSPENDED state
> handle_rt_signal64()
> get_tm_stackpointer()
> treclaim# kernelspace in NON-TRANSACTIONAL state
> __put_user()
> page fault happens. We will never get back here because of the TM Bad
> Thing exception.
>
> page fault handling kicks in and we voluntarily preempt ourselves
> do_page_fault()
> __schedule()
> __switch_to(other_task)
>
> our task is rescheduled and we recheckpoint because the thread's MSR[TS]
> was not cleared
> __switch_to(our_task)
> switch_to_tm()
> tm_recheckpoint_new_task()
> trechkpt # kernelspace in SUSPENDED state
>
> The page fault handling resumes, but now we are in suspended transaction
> state
> do_page_fault()completes
> rfid <- trying to get back where the page fault happened (we were
> non-transactional back then)
> TM Bad Thing# illegal transition from suspended to
> non-transactional
>
> This patch fixes that issue by clearing the current thread's MSR[TS] just
> after
> treclaim in get_tm_stackpointer() so that we stay in non-transactional state
> in
> case we are preempted. In order to make treclaim and clearing the thread's
> MSR[TS] atomic from a preemption perspective when CONFIG_PREEMPT is set,
> preempt_disable/enable() is used. It's also necessary to save the previous
> value of the thread's MSR before get_tm_stackpointer() is called so that it
> can
> be exposed to the signal handler later in
