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Date: Tue, 21 Jun 2022 14:28:23 -0700
From: Tyler Retzlaff <roretzla@linux.microsoft.com>
To: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Cc: dev@dpdk.org, thomas@monjalon.net, anatoly.burakov@intel.com,
Narcisa Vasile <navasile@microsoft.com>
Subject: Re: [PATCH v2 2/6] eal: add thread lifetime management
Message-ID: <20220621212823.GF18214@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
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On Tue, Jun 21, 2022 at 10:44:21PM +0300, Dmitry Kozlyuk wrote:
> 2022-06-21 11:51 (UTC-0700), Tyler Retzlaff:
> > > > +int
> > > > +rte_thread_join(rte_thread_t thread_id, unsigned long *value_ptr)
> > > > +{
> > > > + int ret = 0;
> > > > + void *res = NULL;
> > > > + void **pres = NULL;
> > > > +
> > > > + if (value_ptr != NULL)
> > > > + pres = &res;
> > > > +
> > > > + ret = pthread_join((pthread_t)thread_id.opaque_id, pres);
> > > > + if (ret != 0) {
> > > > + RTE_LOG(DEBUG, EAL, "pthread_join failed\n");
> > > > + return ret;
> > > > + }
> > > > +
> > > > + if (value_ptr != NULL && *pres != NULL)
> > > > + *value_ptr = *(unsigned long *)(*pres);
> > > > +
> > > > + return 0;
> > > > +}
> > >
> > > What makes *pres == NULL special?
> >
> > it's not clear what you mean, can you explain? maybe there is some
> > context i am missing from the original patch series?
>
> There's no previous context.
> After ptread_join(), *pres holds the return value of the thread routine.
> You only assign *value_ptr if value_ptr is not NULL (obviously correct)
> and if *pres != NULL, that is, if the thread returned a non-NULL value.
> But this value is opaque, why do you filter NULL?
i don't think it is opaque here? unsigned long * value_ptr says we have
to store an integer. which leads to a discussion of what should get
stored at the value_ptr location if pthread_join() itself returns no
result but the caller of rte_thread_join() requests the result.
> Perhaps you meant if (pres != NULL), no dereference?
that i think is just a repeat of a test checking if the caller of
rte_thread_join is interested in the result?
i.e. value_ptr != NULL -> pres != NULL
both pres and *pres are dereferenced so it seems to track that prior to
those dereferences they have to be validated as being non-NULL.
i don't see how we could avoid dereferencing **pres to satisfy the
calling contract when the result is requested.
now if value_ptr was unsigned long ** i guess i'd understand. i could
always be reading the code wrong. but thinking about further there is
another problem with this in that we really don't know what is being
aliased in *pres when using the pthread implementation, since pthread
could be returning a pointer to something narrow or with unknown layout
where later dereferencing it as something wider or in this case
specifically as unsigned long * would have horrible consequences.
i think this ends up semi-related to your other comment about what the
result type from rte_thread_func is, we can discuss offline and post
details back to the list.
>
> > > > +int
> > > > +rte_thread_create(rte_thread_t *thread_id,
> > > > + const rte_thread_attr_t *thread_attr,
> > > > + rte_thread_func thread_func, void *args)
> > > > +{
> > > > + int ret = 0;
> > > > + DWORD tid;
> > > > + HANDLE thread_handle = NULL;
> > > > + GROUP_AFFINITY thread_affinity;
> > > > + struct thread_routine_ctx *ctx = NULL;
> > > > +
> > > > + ctx = calloc(1, sizeof(*ctx));
> > > > + if (ctx == NULL) {
> > > > + RTE_LOG(DEBUG, EAL, "Insufficient memory for thread context allocations\n");
> > > > + ret = ENOMEM;
> > > > + goto cleanup;
> > > > + }
> > > > + ctx->routine_args = args;
> > > > + ctx->thread_func = thread_func;
> > > > +
> > > > + thread_handle = CreateThread(NULL, 0, thread_func_wrapper, ctx,
> > > > + CREATE_SUSPENDED, &tid);
> > > > + if (thread_handle == NULL) {
> > > > + ret = thread_log_last_error("CreateThread()");
> > > > + free(ctx);
> > > > + goto cleanup;
> > >
> > > Missing `free(ctx)` from other error paths below.
> >
> > beyond this point free(ctx) will happen in thread_func_wrapper. i will
> > add a comment to make it clear.
>
> Not if you exit before ResumeThread()
> and thread_func_wrapper() will never execute to call free().
yes, you are right i forgot that this thread is created suspended.