Обсуждение: possible dsm bug in dsm_attach()
Hi,
dsm_attach() does the following:
nitems = dsm_control->nitems;for (i = 0; i < nitems; ++i){ /* If the reference count is 0, the slot is actually
unused.*/ if (dsm_control->item[i].refcnt == 0) continue;
/* * If the reference count is 1, the slot is still in use, but the * segment is in the process of going
away. Treat that as if we * didn't find a match. */ if (dsm_control->item[i].refcnt == 1) break;
/* Otherwise, if the descriptor matches, we've found a match. */ if (dsm_control->item[i].handle == seg->handle)
{ dsm_control->item[i].refcnt++; seg->control_slot = i; break; }}
The break because of refcnt == 1 doesn't generally seem to be a good
idea. Why are we bailing if there's *any* segment that's in the process
of being removed? I think the check should be there *after* the
dsm_control->item[i].handle == seg->handle check?
Greetings,
Andres Freund
-- Andres Freund http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services
On Tue, May 6, 2014 at 8:43 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> dsm_attach() does the following:
>
> nitems = dsm_control->nitems;
> for (i = 0; i < nitems; ++i)
> {
> /* If the reference count is 0, the slot is actually unused. */
> if (dsm_control->item[i].refcnt == 0)
> continue;
>
> /*
> * If the reference count is 1, the slot is still in use, but the
> * segment is in the process of going away. Treat that as if we
> * didn't find a match.
> */
> if (dsm_control->item[i].refcnt == 1)
> break;
>
> /* Otherwise, if the descriptor matches, we've found a match. */
> if (dsm_control->item[i].handle == seg->handle)
> {
> dsm_control->item[i].refcnt++;
> seg->control_slot = i;
> break;
> }
> }
>
> The break because of refcnt == 1 doesn't generally seem to be a good
> idea. Why are we bailing if there's *any* segment that's in the process
> of being removed? I think the check should be there *after* the
> dsm_control->item[i].handle == seg->handle check?
You are correct. Good catch.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
On 2014-05-06 08:48:57 -0400, Robert Haas wrote: > On Tue, May 6, 2014 at 8:43 AM, Andres Freund <andres@2ndquadrant.com> wrote: > > The break because of refcnt == 1 doesn't generally seem to be a good > > idea. Why are we bailing if there's *any* segment that's in the process > > of being removed? I think the check should be there *after* the > > dsm_control->item[i].handle == seg->handle check? > > You are correct. Good catch. Fix attached. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
Вложения
On Tue, May 6, 2014 at 1:14 PM, Andres Freund <andres@2ndquadrant.com> wrote: > On 2014-05-06 08:48:57 -0400, Robert Haas wrote: >> On Tue, May 6, 2014 at 8:43 AM, Andres Freund <andres@2ndquadrant.com> wrote: >> > The break because of refcnt == 1 doesn't generally seem to be a good >> > idea. Why are we bailing if there's *any* segment that's in the process >> > of being removed? I think the check should be there *after* the >> > dsm_control->item[i].handle == seg->handle check? >> >> You are correct. Good catch. > > Fix attached. Committed, thanks. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
On 2014-05-06 13:45:13 -0400, Robert Haas wrote: > On Tue, May 6, 2014 at 1:14 PM, Andres Freund <andres@2ndquadrant.com> wrote: > > On 2014-05-06 08:48:57 -0400, Robert Haas wrote: > >> On Tue, May 6, 2014 at 8:43 AM, Andres Freund <andres@2ndquadrant.com> wrote: > >> > The break because of refcnt == 1 doesn't generally seem to be a good > >> > idea. Why are we bailing if there's *any* segment that's in the process > >> > of being removed? I think the check should be there *after* the > >> > dsm_control->item[i].handle == seg->handle check? > >> > >> You are correct. Good catch. > > > > Fix attached. > > Committed, thanks. Heh. Not a fan of film references? :) Thanks, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services
On Tue, May 6, 2014 at 1:46 PM, Andres Freund <andres@2ndquadrant.com> wrote: >> > Fix attached. >> >> Committed, thanks. > > Heh. Not a fan of film references? :) I didn't quite put the pieces together there. I just thought the use of "you" was awkward. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
On Tue, May 6, 2014 at 11:15 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Tue, May 6, 2014 at 1:14 PM, Andres Freund <andres@2ndquadrant.com> wrote:
> > On 2014-05-06 08:48:57 -0400, Robert Haas wrote:
> >> On Tue, May 6, 2014 at 8:43 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> >> > The break because of refcnt == 1 doesn't generally seem to be a good
> >> > idea. Why are we bailing if there's *any* segment that's in the process
> >> > of being removed? I think the check should be there *after* the
> >> > dsm_control->item[i].handle == seg->handle check?
> >>
> >> You are correct. Good catch.
> >
> > Fix attached.
>
> Committed, thanks.
>
dsm_create(Size size, int flags)
{
..
/* Lock the control segment so we can register the new segment. */
LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
..
/* Verify that we can support an additional mapping. */
if (nitems >= dsm_control->maxitems)
{
if ((flags & DSM_CREATE_NULL_IF_MAXSEGMENTS) != 0)
{
dsm_impl_op(DSM_OP_DESTROY, seg->handle, 0, &seg->impl_private,
&seg->mapped_address, &seg->mapped_size, WARNING);
if (seg->resowner != NULL)
ResourceOwnerForgetDSM(seg->resowner, seg);
dlist_delete(&seg->node);
pfree(seg);
return NULL;
}
..
}
Is there a reason lock is not released in case we return NULL in above
>
> On Tue, May 6, 2014 at 1:14 PM, Andres Freund <andres@2ndquadrant.com> wrote:
> > On 2014-05-06 08:48:57 -0400, Robert Haas wrote:
> >> On Tue, May 6, 2014 at 8:43 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> >> > The break because of refcnt == 1 doesn't generally seem to be a good
> >> > idea. Why are we bailing if there's *any* segment that's in the process
> >> > of being removed? I think the check should be there *after* the
> >> > dsm_control->item[i].handle == seg->handle check?
> >>
> >> You are correct. Good catch.
> >
> > Fix attached.
>
> Committed, thanks.
>
dsm_create(Size size, int flags)
{
..
/* Lock the control segment so we can register the new segment. */
LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
..
/* Verify that we can support an additional mapping. */
if (nitems >= dsm_control->maxitems)
{
if ((flags & DSM_CREATE_NULL_IF_MAXSEGMENTS) != 0)
{
dsm_impl_op(DSM_OP_DESTROY, seg->handle, 0, &seg->impl_private,
&seg->mapped_address, &seg->mapped_size, WARNING);
if (seg->resowner != NULL)
ResourceOwnerForgetDSM(seg->resowner, seg);
dlist_delete(&seg->node);
pfree(seg);
return NULL;
}
..
}
Is there a reason lock is not released in case we return NULL in above
code?
I am facing an issue in case we need to create many segments for
large inheritance hierarchy. Attached patch fixes the problem for me.
Вложения
On Wed, Mar 25, 2015 at 6:11 AM, Amit Kapila <amit.kapila16@gmail.com> wrote: > I am facing an issue in case we need to create many segments for > large inheritance hierarchy. Attached patch fixes the problem for me. Sigh. You'd think I'd be able to write a 30-line patch without introducing not one but two stupid bugs. But if you did think that, you'd be wrong. Committed. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company