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@xxxxxxxxx>
> 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!
> 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@xxxxxxxxx>
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
>
>
