On 2/5/20 1:58 AM, Michael Neuling wrote:
Other than the minor things below that I think you need, the patch good with me.

Acked-by: Michael Neuling <mi...@neuling.org>

Subject: Re: [PATCH v2 1/3] powerpc/tm: Clear the current thread's MSR[TS] 
after treclaim

The subject should mention "signals".

How about "powerpc/tm: Clear the current thread's MSR[TS] when transaction is reclaimed on signal delivery" ?


On Mon, 2020-02-03 at 13:09 -0300, Gustavo Luiz Duarte wrote:
After a treclaim, we expect to be in non-transactional state. If we don't
immediately clear the current thread's MSR[TS] and we get preempted, then
tm_recheckpoint_new_task() will recheckpoint and we get rescheduled in
suspended transaction state.

It's not "immediately", it's before re-enabling preemption.

There is a similar comment in the code that needs to be fixed too.

OK.


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 c00000000000de44 (msr 
0x8000000302a03031) tm_scratch=800000010280b033
        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:  c00000000000de44 LR: c000000000034728 CTR: 0000000000000000
        REGS: c00000003fe7bd70 TRAP: 0700   Not tainted  (5.4.0-rc2)
        MSR:  8000000302a03031 <SF,VEC,VSX,FP,ME,IR,DR,LE,TM[SE]>  CR: 44000884 
 XER: 00000000
        CFAR: c00000000000dda4 IRQMASK: 0
        PACATMSCRATCH: 800000010280b033
        GPR00: c000000000034728 c000000f65a17c80 c000000001662800 
00007fffacf3fd78
        GPR04: 0000000000001000 0000000000001000 0000000000000000 
c000000f611f8af0
        GPR08: 0000000000000000 0000000078006001 0000000000000000 
000c000000000000
        GPR12: c000000f611f84b0 c00000003ffcb200 0000000000000000 
0000000000000000
        GPR16: 0000000000000000 0000000000000000 0000000000000000 
0000000000000000
        GPR20: 0000000000000000 0000000000000000 0000000000000000 
c000000f611f8140
        GPR24: 0000000000000000 00007fffacf3fd68 c000000f65a17d90 
c000000f611f7800
        GPR28: c000000f65a17e90 c000000f65a17e90 c000000001685e18 
00007fffacf3f000
        NIP [c00000000000de44] fast_exception_return+0xf4/0x1b0
        LR [c000000000034728] handle_rt_signal64+0x78/0xc50
        Call Trace:
        [c000000f65a17c80] [c000000000034710] handle_rt_signal64+0x60/0xc50 
(unreliable)
        [c000000f65a17d30] [c000000000023640] do_notify_resume+0x330/0x460
        [c000000f65a17e20] [c00000000000dcc4] ret_from_except_lite+0x70/0x74
        Instruction dump:
        7c4ff120 e8410170 7c5a03a6 38400000 f8410060 e8010070 e8410080 e8610088
        60000000 60000000 e8810090 e8210078 <4c000024> 48000000 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 setup_tm_sigcontexts() to inform the
userspace MSR at the moment of the signal delivery.

Found with tm-signal-context-force-tm kernel selftest on P8 KVM.

Why are you mentioning KVM?

That is just what I used... I agree that the issue has nothing to do with KVM. I will remove that on v3.



v2: Fix build failure when tm is disabled.

Fixes: 2b0a576d15e0 ("powerpc: Add new transactional memory state to the signal 
context")
Cc: sta...@vger.kernel.org # v3.9
Signed-off-by: Gustavo Luiz Duarte <gustav...@linux.ibm.com>
---
  arch/powerpc/kernel/signal.c    | 17 +++++++++++++++--
  arch/powerpc/kernel/signal_32.c | 28 ++++++++++++++--------------
  arch/powerpc/kernel/signal_64.c | 22 ++++++++++------------
  3 files changed, 39 insertions(+), 28 deletions(-)

diff --git a/arch/powerpc/kernel/signal.c b/arch/powerpc/kernel/signal.c
index e6c30cee6abf..1660be1061ac 100644
--- a/arch/powerpc/kernel/signal.c
+++ b/arch/powerpc/kernel/signal.c
@@ -200,14 +200,27 @@ unsigned long get_tm_stackpointer(struct task_struct *tsk)
         * normal/non-checkpointed stack pointer.
         */
+ unsigned long ret = tsk->thread.regs->gpr[1];
+
  #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
        BUG_ON(tsk != current);
if (MSR_TM_ACTIVE(tsk->thread.regs->msr)) {
+               preempt_disable();
                tm_reclaim_current(TM_CAUSE_SIGNAL);
                if (MSR_TM_TRANSACTIONAL(tsk->thread.regs->msr))
-                       return tsk->thread.ckpt_regs.gpr[1];
+                       ret = tsk->thread.ckpt_regs.gpr[1];
+
+               /* If we treclaim, we must immediately clear the current
+                * thread's TM bits. Otherwise we might be preempted and have
+                * the live MSR[TS] changed behind our back
+                * (tm_recheckpoint_new_task() would recheckpoint).
+                * Besides, we enter the signal handler in non-transactional
+                * state.
+                */
+               tsk->thread.regs->msr &= ~MSR_TS_MASK;
+               preempt_enable();
        }
  #endif
-       return tsk->thread.regs->gpr[1];
+       return ret;
  }
diff --git a/arch/powerpc/kernel/signal_32.c b/arch/powerpc/kernel/signal_32.c
index 98600b276f76..1b090a76b444 100644
--- a/arch/powerpc/kernel/signal_32.c
+++ b/arch/powerpc/kernel/signal_32.c
@@ -489,19 +489,11 @@ static int save_user_regs(struct pt_regs *regs, struct 
mcontext __user *frame,
   */
  static int save_tm_user_regs(struct pt_regs *regs,
                             struct mcontext __user *frame,
-                            struct mcontext __user *tm_frame, int sigret)
+                            struct mcontext __user *tm_frame, int sigret,
+                            unsigned long msr)
  {
-       unsigned long msr = regs->msr;
-
        WARN_ON(tm_suspend_disabled);
- /* Remove TM bits from thread's MSR. The MSR in the sigcontext
-        * just indicates to userland that we were doing a transaction, but we
-        * don't want to return in transactional state.  This also ensures
-        * that flush_fp_to_thread won't set TIF_RESTORE_TM again.
-        */
-       regs->msr &= ~MSR_TS_MASK;
-
        /* Save both sets of general registers */
        if (save_general_regs(&current->thread.ckpt_regs, frame)
            || save_general_regs(regs, tm_frame))
@@ -912,6 +904,10 @@ int handle_rt_signal32(struct ksignal *ksig, sigset_t 
*oldset,
        int sigret;
        unsigned long tramp;
        struct pt_regs *regs = tsk->thread.regs;
+#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
+       /* Save the thread's msr before get_tm_stackpointer() changes it */
+       unsigned long msr = regs->msr;
+#endif
BUG_ON(tsk != current); @@ -944,13 +940,13 @@ int handle_rt_signal32(struct ksignal *ksig, sigset_t *oldset, #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
        tm_frame = &rt_sf->uc_transact.uc_mcontext;
-       if (MSR_TM_ACTIVE(regs->msr)) {
+       if (MSR_TM_ACTIVE(msr)) {
                if (__put_user((unsigned long)&rt_sf->uc_transact,
                               &rt_sf->uc.uc_link) ||
                    __put_user((unsigned long)tm_frame,
                               &rt_sf->uc_transact.uc_regs))
                        goto badframe;
-               if (save_tm_user_regs(regs, frame, tm_frame, sigret))
+               if (save_tm_user_regs(regs, frame, tm_frame, sigret, msr))
                        goto badframe;
        }
        else
@@ -1369,6 +1365,10 @@ int handle_signal32(struct ksignal *ksig, sigset_t 
*oldset,
        int sigret;
        unsigned long tramp;
        struct pt_regs *regs = tsk->thread.regs;
+#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
+       /* Save the thread's msr before get_tm_stackpointer() changes it */
+       unsigned long msr = regs->msr;
+#endif
BUG_ON(tsk != current); @@ -1402,9 +1402,9 @@ int handle_signal32(struct ksignal *ksig, sigset_t *oldset, #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
        tm_mctx = &frame->mctx_transact;
-       if (MSR_TM_ACTIVE(regs->msr)) {
+       if (MSR_TM_ACTIVE(msr)) {
                if (save_tm_user_regs(regs, &frame->mctx, &frame->mctx_transact,
-                                     sigret))
+                                     sigret, msr))
                        goto badframe;
        }
        else
diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
index 117515564ec7..84ed2e77ef9c 100644
--- a/arch/powerpc/kernel/signal_64.c
+++ b/arch/powerpc/kernel/signal_64.c
@@ -192,7 +192,8 @@ static long setup_sigcontext(struct sigcontext __user *sc,
  static long setup_tm_sigcontexts(struct sigcontext __user *sc,
                                 struct sigcontext __user *tm_sc,
                                 struct task_struct *tsk,
-                                int signr, sigset_t *set, unsigned long 
handler)
+                                int signr, sigset_t *set, unsigned long 
handler,
+                                unsigned long msr)
  {
        /* When CONFIG_ALTIVEC is set, we _always_ setup v_regs even if the
         * process never used altivec yet (MSR_VEC is zero in pt_regs of
@@ -207,12 +208,11 @@ static long setup_tm_sigcontexts(struct sigcontext __user 
*sc,
        elf_vrreg_t __user *tm_v_regs = sigcontext_vmx_regs(tm_sc);
  #endif
        struct pt_regs *regs = tsk->thread.regs;
-       unsigned long msr = tsk->thread.regs->msr;
        long err = 0;
BUG_ON(tsk != current); - BUG_ON(!MSR_TM_ACTIVE(regs->msr));
+       BUG_ON(!MSR_TM_ACTIVE(msr));
WARN_ON(tm_suspend_disabled); @@ -222,13 +222,6 @@ static long setup_tm_sigcontexts(struct sigcontext __user *sc,
         */
        msr |= tsk->thread.ckpt_regs.msr & (MSR_FP | MSR_VEC | MSR_VSX);
- /* Remove TM bits from thread's MSR. The MSR in the sigcontext
-        * just indicates to userland that we were doing a transaction, but we
-        * don't want to return in transactional state.  This also ensures
-        * that flush_fp_to_thread won't set TIF_RESTORE_TM again.
-        */
-       regs->msr &= ~MSR_TS_MASK;
-
  #ifdef CONFIG_ALTIVEC
        err |= __put_user(v_regs, &sc->v_regs);
        err |= __put_user(tm_v_regs, &tm_sc->v_regs);
@@ -824,6 +817,10 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
        unsigned long newsp = 0;
        long err = 0;
        struct pt_regs *regs = tsk->thread.regs;
+#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
+       /* Save the thread's msr before get_tm_stackpointer() changes it */
+       unsigned long msr = regs->msr;
+#endif
BUG_ON(tsk != current); @@ -841,7 +838,7 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
        err |= __put_user(0, &frame->uc.uc_flags);
        err |= __save_altstack(&frame->uc.uc_stack, regs->gpr[1]);
  #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
-       if (MSR_TM_ACTIVE(regs->msr)) {
+       if (MSR_TM_ACTIVE(msr)) {
                /* The ucontext_t passed to userland points to the second
                 * ucontext_t (for transactional state) with its uc_link ptr.
                 */
@@ -849,7 +846,8 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
                err |= setup_tm_sigcontexts(&frame->uc.uc_mcontext,
                                            &frame->uc_transact.uc_mcontext,
                                            tsk, ksig->sig, NULL,
-                                           (unsigned 
long)ksig->ka.sa.sa_handler);
+                                           (unsigned 
long)ksig->ka.sa.sa_handler,
+                                           msr);
        } else
  #endif
        {

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