[Gluster-devel] important change to syncop infra
Pranith Kumar Karampuri
pkarampu at redhat.com
Fri Dec 13 03:39:03 UTC 2013
hi,
If there are no more objections, I will send the final patch on Monday.
Pranith.
----- Original Message -----
> From: "Anand Avati" <avati at gluster.org>
> To: "Anand Subramanian" <anands at redhat.com>
> Cc: "Pranith Kumar Karampuri" <pkarampu at redhat.com>, gluster-devel at nongnu.org
> Sent: Thursday, December 12, 2013 12:09:34 PM
> Subject: Re: [Gluster-devel] important change to syncop infra
>
> The decision of doing away with -O2 is beyond our control, and we shouldn't
> have code which depend on optimization to be disabled to behave properly.
> Representing -errno as return is the cleanest fix (that's how other
> projects which use setcontext/getcontext are behaving too.) If there are
> any new further issues which arise from setcontext/getcontext, I'm tempted
> to change the internal implementation to use a a vanilla pthread pool.
>
> Avati
>
>
> On Wed, Dec 11, 2013 at 10:29 PM, Anand Subramanian
> <ansubram at redhat.com>wrote:
>
> > Is doing away with -O2 an option that was ever considered or is it that we
> > simply must have O2 on?
> >
> > (I understand that turning off O2 can open some so-far-unexposed can of
> > worms and a lot of soaking maybe required, and also that we may have had a
> > good set of perf related reasons to have settled on -O2 in the first place,
> > but wanted to understand nevertheless...)
> >
> > Anand
> >
> >
> >
> > On 12/11/2013 02:21 PM, Pranith Kumar Karampuri wrote:
> >
> >> hi,
> >> We found a day-1 bug when syncop_xxx() infra is used inside a
> >> synctask with compilation optimization (CFLAGS -O2). This bug has been
> >> dormant for at least 2 years.
> >> There are around ~400(rebalance, replace-brick, bd, self-heal-daemon,
> >> quota, fuse lock/fd migration) places where syncop is used in the code
> >> base
> >> all of which are potential candidates which can take the hit.
> >>
> >> I sent first round of patch at http://review.gluster.com/6475 to catch
> >> regressions upstream.
> >> These are the files that are affected by the changes I introduced to fix
> >> this:
> >>
> >> api/src/glfs-fops.c | 36
> >> ++++++++++++++++++++++++++++++++++
> >> api/src/glfs-handleops.c | 15 ++++++++++++++
> >> api/src/glfs-internal.h | 7 +++++++
> >> api/src/glfs-resolve.c | 10 ++++++++++
> >> libglusterfs/src/syncop.c | 117
> >> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >> ++++++++++++-------------------------------------
> >> xlators/cluster/afr/src/afr-self-heald.c | 45
> >> +++++++++++++++++++++++++++++-------------
> >> xlators/cluster/afr/src/pump.c | 12 ++++++++++--
> >> xlators/cluster/dht/src/dht-helper.c | 24
> >> +++++++++++++++--------
> >> xlators/cluster/dht/src/dht-rebalance.c | 168
> >> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >> ++++++++++++++++++++++++++++++++++--------------------------
> >> -------------------------------------
> >> xlators/cluster/dht/src/dht-selfheal.c | 6 ++++--
> >> xlators/features/locks/src/posix.c | 3 ++-
> >> xlators/features/qemu-block/src/bdrv-xlator.c | 15 ++++----------
> >> xlators/features/qemu-block/src/qb-coroutines.c | 14 ++++++++++----
> >> xlators/mount/fuse/src/fuse-bridge.c | 16 ++++++++++-----
> >>
> >> Please review your respective component for these changes in gerrit.
> >>
> >> Thanks
> >> Pranith.
> >>
> >> Detailed explanation of the Root cause:
> >> We found the bug in 'gf_defrag_migrate_data' in rebalance operation:
> >>
> >> Lets look at interesting parts of the function:
> >>
> >> int
> >> gf_defrag_migrate_data (xlator_t *this, gf_defrag_info_t *defrag, loc_t
> >> *loc,
> >> dict_t *migrate_data)
> >> {
> >> .....
> >> code section - [ Loop ]
> >> while ((ret = syncop_readdirp (this, fd, 131072, offset, NULL,
> >> &entries)) != 0) {
> >> .....
> >> code section - [ ERRNO-1 ] (errno of readdirp is stored in
> >> readdir_operrno by a thread)
> >> /* Need to keep track of ENOENT errno, that means, there
> >> is no
> >> need to send more readdirp() */
> >> readdir_operrno = errno;
> >> .....
> >> code section - [ SYNCOP-1 ] (syncop_getxattr is called by a thread)
> >> ret = syncop_getxattr (this, &entry_loc, &dict,
> >> GF_XATTR_LINKINFO_KEY);
> >> code section - [ ERRNO-2] (checking for failures of syncop_getxattr().
> >> This may not always be executed in same thread which executed [SYNCOP-1])
> >> if (ret < 0) {
> >> if (errno != ENODATA) {
> >> loglevel = GF_LOG_ERROR;
> >> defrag->total_failures += 1;
> >> .....
> >> }
> >>
> >> the function above could be executed by thread(t1) till [SYNCOP-1] and
> >> code from [ERRNO-2] can be executed by a different thread(t2) because of
> >> the way syncop-infra schedules the tasks.
> >>
> >> when the code is compiled with -O2 optimization this is the assembly code
> >> that is generated:
> >> [ERRNO-1]
> >> 1165 readdir_operrno = errno; <<---- errno gets
> >> expanded as *(__errno_location())
> >> 0x00007fd149d48b60 <+496>: callq 0x7fd149d410c0
> >> <__errno_location at plt>
> >> 0x00007fd149d48b72 <+514>: mov %rax,0x50(%rsp) <<------
> >> Address returned by __errno_location() is stored in a special location in
> >> stack for later use.
> >> 0x00007fd149d48b77 <+519>: mov (%rax),%eax
> >> 0x00007fd149d48b79 <+521>: mov %eax,0x78(%rsp)
> >> ....
> >> [ERRNO-2]
> >> 1281 if (errno != ENODATA) {
> >> 0x00007fd149d492ae <+2366>: mov 0x50(%rsp),%rax <<-----
> >> Because it already stored the address returned by __errno_location(), it
> >> just dereferences the address to get the errno value. BUT THIS CODE NEED
> >> NOT BE EXECUTED BY SAME THREAD!!!
> >> 0x00007fd149d492b3 <+2371>: mov $0x9,%ebp
> >> 0x00007fd149d492b8 <+2376>: mov (%rax),%edi
> >> 0x00007fd149d492ba <+2378>: cmp $0x3d,%edi
> >>
> >> The problem is that __errno_location() value of t1 and t2 are different.
> >> So [ERRNO-2] ends up reading errno of t1 instead of errno of t2 even
> >> though
> >> t2 is executing [ERRNO-2] code section.
> >>
> >> When code is compiled without any optimization for [ERRNO-2]:
> >> 1281 if (errno != ENODATA) {
> >> 0x00007fd58e7a326f <+2237>: callq 0x7fd58e797300
> >> <__errno_location at plt><<--- As it is calling __errno_location() again it
> >> gets the location from t2 so it works as intended.
> >> 0x00007fd58e7a3274 <+2242>: mov (%rax),%eax
> >> 0x00007fd58e7a3276 <+2244>: cmp $0x3d,%eax
> >> 0x00007fd58e7a3279 <+2247>: je 0x7fd58e7a32a1
> >> <gf_defrag_migrate_data+2287>
> >>
> >> Fix:
> >> We decided to make syncop_xxx() return (-errno) value as the return value
> >> in case of errors and all the functions which make syncop_xxx() will need
> >> to use (-ret) to figure out the reason for failure in case of syncop_xxx()
> >> failures.
> >>
> >> Pranith
> >>
> >> _______________________________________________
> >> Gluster-devel mailing list
> >> Gluster-devel at nongnu.org
> >> https://lists.nongnu.org/mailman/listinfo/gluster-devel
> >>
> >
> >
> > _______________________________________________
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> > Gluster-devel at nongnu.org
> > https://lists.nongnu.org/mailman/listinfo/gluster-devel
> >
>
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