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Re: [PATCH 0/1] trace2: fix tracing when NO_PTHREADS is defined






On 5/28/2019 2:28 AM, Jeff King wrote:
On Sat, May 25, 2019 at 05:43:55PM +0700, Duy Nguyen wrote:

+typedef struct {
+       void *data;
+       /* extra indirection because setspecific is passed key by value */
+       void **vdata;

Ha! I was thinking a separate key->value mapping which is complicated
in C. But this works pretty well for a single thread, and it even
supports multiple keys.

I really wish that all of the functions passed the pthread_key_t by
reference. That would make it possible to define the key as a single
pointer.

I'm not sure if pthread_key_t's are meant to be shallow-copyable. I.e.,
should this work:

   void foo(pthread_key_t *out)
   {
	pthread_key_t tmp;
	pthread_key_create(&tmp, NULL);
	*out = tmp;
   }
   ...
   pthread_key_t k;
   foo(&k);
   pthread_setspecific(k, some_ptr);

It does not with my proposed plan, because the pointer in tmp.data went
out of scope, leaving tmp.vdata (and thus k.vdata) as a dangling
pointer.

The code above seems like a vaguely crazy thing to do. But if we want to
be absolutely paranoid, we'd have to malloc() an extra pointer in the
create() function, instead of carrying it inside the key. Or just make a
global "void *thread_specific_data[PTHREAD_KEYS_MAX]" and make each key
an integer index into it.

It's pretty clear that they expect one of those two implementations,
given that POSIX says key creation can report either ENOMEM, or EAGAIN
if we exceed PTHREAD_KEYS_MAX. :)

-Peff



Yes, a fixed global void* array should be fine.
Besides there aren't that many calls to pthread_key_create()  (My use in
Trace2 is the second, right?), so just create an arbitrary value for
PTHREAD_KEYS_MAX and be done.


Jeff