Re: [PATCH] EXP hashtorture.h: Avoid sporadic SIGSEGV in hash_bkt_rcu

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Tue, Jan 01, 2019 at 09:27:41AM +0900, Akira Yokosawa wrote:
> On 2018/12/31 13:03:07 -0800, Paul E. McKenney wrote:
> > On Tue, Jan 01, 2019 at 12:15:23AM +0900, Akira Yokosawa wrote:
> >> >From 52f5d218442eb64f2798335d56a1838f90d96d5f Mon Sep 17 00:00:00 2001
> >> From: Akira Yokosawa <akiyks@gmail.com>
> >> Date: Mon, 30 Dec 2018 22:54:43 +0900
> >> Subject: [PATCH] EXP hashtorture.h: Avoid sporadic SIGSEGV in hash_bkt_rcu
> >>
> >> Commit 4e22bdc905ff ("Wait at end of test for call_rcu() to finish")
> >> added a couple of synchronize_rcu()s in perftest_update()
> >> and zoo_reader().
> >>
> >> However, there still remains sporadic SIGSEGV in
> >>
> >>     $ ./hash_bkt_rcu --perftest --nupdaters 3
> >>
> >> On the other hand,
> >>
> >>     $ ./hash_bkt_rcu --schroedinger --nupdaters 3
> >>
> >> does not show such issue. Just moving synchronize_rcu()s in
> >> zoo_reader() to zoo_updater() does not resolve the
> >> SIGSEGV.
> >>
> >>
> >> This commit defines rcu_barrier() if not available,
> >> and puts them at both before and after the final loop
> >> of perftest_updater() and zoo_updater().
> >>
> >> It looks like this change can fix the above mentioned
> >> SIGSEGV in "--perftest".
> >>
> >> [Tested on Ubuntu Xenial with liburcu-dev/xenial,now 0.9.1-3 and
> >> liburcu4/xenial,now 0.9.1-3 installed.]
> >>
> >> NOTE:
> >>
> >>     $ ./hash_resize --schroedinger --resizemult 2 --duration 20
> > 
> > I get SIGSEGV and hangs from time to time, so I am looking into this.
> > Thank you for calling it to my attention!
> 
> I've found some suspicious code in hash_resize.c
> 
> hashtab_lock_mod() takes care of ongoing resizing and spin_lock()
> new bucket if necessary. This is good for add, but for delete
> we may still need to lock old bucket.
> 
> And hashtab_unlock_mod() doesn't care ongoing resizing, so
> there can be mismatch of spin_lock() -- spin_unlock().
> 
> Also, htp_master->ht_cur can change during the
> hashtab_lock_mod() -- hashtab_unlock_mod() critical section
> because the update of the pointer by rcu_assign_pointer()
> is ahead of synchronize_rcu().
> 
> Given the resizing is infrequent, the simplest way might be to
> block hashtab_lock_mod while resizing is going on.

I do believe you have found something here, and thank you!  So the
answer to my earlier question as to whether I was smarter when writing
it than now is clearly that I was equally stupid in both cases.  ;-)

Well, it is conference-driven code, but still high time for me to
clean it up.

> There can be a better way to keep concurrent add/del/resize, though.
> Happy hacking! ;-) 

I do believe that I can preserve concurrency between resizing and
deletion, but that is clearly for me to prove.

And thank you again!

							Thanx, Paul

>         Thanks, Akira
> > 
> >> still fails with SIGSEGV frequently in zoo_del(). GDB says:
> >>
> >>     (gdb) where
> >>     #0  0x0000000000402b27 in cds_list_del_rcu (elem=0x7ff8fc0138f0)
> >>         at /usr/include/urcu/rculist.h:71
> >>     #1  hashtab_del (htep=0x7ff8fc0138d0, htp_master=<optimized out>)
> >>         at hash_resize.c:261
> >>     #2  zoo_del (zhep=0x7ff8fc0138d0) at hashtorture.h:1007
> >>     #3  zoo_updater (arg=0x1e8b298) at hashtorture.h:1153
> >>     #4  0x00007ff9057d16ba in start_thread (arg=0x7ff903fed700)
> >>         at pthread_create.c:333
> >>     #5  0x00007ff9050f741d in clone ()
> >>         at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
> >>
> >> Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
> > 
> > Good catch, queue and pushed, thank you!
> > 
> > With one small modification -- given that liburcu has had rcu_barrier()
> > for some years now, I removed the "training wheels" (and unreliable)
> > use of the wait and pair of synchronize_rcu() calls.
> > 
> >> ---
> >> Hi Paul,
> >>
> >> This is a partial fix, but it resolves SIGSEGV in "--perftest" of
> >> hash_bkt_rcu and hash_resize.
> >>
> >> "--schroedinger" of hash_resize with resizing enabled still seg faults
> >> as mentioned in the commit log.
> >>
> >> By the way, what version of liburcu are you using?
> > 
> > It is about two years old, but it does have rcu_barrier().
> > 
> > 								Thanx, Paul
> > 
> >>         Thanks, Akira
> >> --
> >>  CodeSamples/datastruct/hash/hashtorture.h | 24 ++++++++++++++++--------
> >>  1 file changed, 16 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/CodeSamples/datastruct/hash/hashtorture.h b/CodeSamples/datastruct/hash/hashtorture.h
> >> index 0e90220..9ae3dfa 100644
> >> --- a/CodeSamples/datastruct/hash/hashtorture.h
> >> +++ b/CodeSamples/datastruct/hash/hashtorture.h
> >> @@ -55,6 +55,15 @@ void (*defer_del_done)(struct ht_elem *htep) = NULL;
> >>  #ifndef quiescent_state
> >>  #define quiescent_state() do ; while (0)
> >>  #define synchronize_rcu() do ; while (0)
> >> +#define rcu_barrier() do ; while (0)
> >> +#else
> >> +#ifndef rcu_barrier
> >> +#define rcu_barrier() do { \
> >> +		synchronize_rcu(); \
> >> +		poll(NULL, 0, 100); \
> >> +		synchronize_rcu(); \
> >> +	} while (0)
> >> +#endif /* #ifndef rcu_barrier */
> >>  #endif /* #ifndef quiescent_state */
> >>  
> >>  /*
> >> @@ -765,6 +774,7 @@ void *perftest_reader(void *arg)
> >>  		if (i >= ne)
> >>  			i = i % ne + offset;
> >>  	}
> >> +
> >>  	pap->nlookups = nlookups;
> >>  	pap->nlookupfails = nlookupfails;
> >>  	hash_unregister_thread();
> >> @@ -839,6 +849,7 @@ void *perftest_updater(void *arg)
> >>  			quiescent_state();
> >>  	}
> >>  
> >> +	rcu_barrier();
> >>  	/* Test over, so remove all our elements from the hash table. */
> >>  	for (i = 0; i < elperupdater; i++) {
> >>  		if (thep[i].in_table != 1)
> >> @@ -846,10 +857,7 @@ void *perftest_updater(void *arg)
> >>  		BUG_ON(!perftest_lookup(thep[i].data));
> >>  		perftest_del(&thep[i]);
> >>  	}
> >> -	/* Really want rcu_barrier(), but missing from old liburcu versions. */
> >> -	synchronize_rcu();
> >> -	poll(NULL, 0, 100);
> >> -	synchronize_rcu();
> >> +	rcu_barrier();
> >>  
> >>  	hash_unregister_thread();
> >>  	free(thep);
> >> @@ -1048,10 +1056,6 @@ void *zoo_reader(void *arg)
> >>  		if (i >= ne)
> >>  			i = i % ne + offset;
> >>  	}
> >> -	/* Really want rcu_barrier(), but missing from old liburcu versions. */
> >> -	synchronize_rcu();
> >> -	poll(NULL, 0, 100);
> >> -	synchronize_rcu();
> >>  
> >>  	pap->nlookups = nlookups;
> >>  	pap->nlookupfails = nlookupfails;
> >> @@ -1136,15 +1140,19 @@ void *zoo_updater(void *arg)
> >>  			quiescent_state();
> >>  	}
> >>  
> >> +	rcu_barrier();
> >>  	/* Test over, so remove all our elements from the hash table. */
> >>  	for (i = 0; i < elperupdater; i++) {
> >>  		if (!zheplist[i])
> >>  			continue;
> >>  		zoo_del(zheplist[i]);
> >>  	}
> >> +	rcu_barrier();
> >> +
> >>  	hash_unregister_thread();
> >>  	pap->nadds = nadds;
> >>  	pap->ndels = ndels;
> >> +	free(zheplist);
> >>  	return NULL;
> >>  }
> >>  
> >> -- 
> >> 2.7.4
> >>
> >>
> > 
>